[PATCH] sched/cache: Refresh LLC capacity across CPU hotplug

[PATCH] sched/cache: Refresh LLC capacity across CPU hotplug

[Davi Chaves Azevedo]

The scheduler scales LLC capacity by the fraction of cache-sharing CPUs
covered by a domain:

  llc_bytes = cache_size * span_weight / shared_weight

During CPU teardown, sched_cpu_deactivate() rebuilds scheduler domains
before cacheinfo_cpu_pre_down() removes the CPU from shared_cpu_map. The
new domains therefore use the old sharing weight. The later call to
sched_update_llc_bytes() looks up the departing CPU's sd_llc, which has
already been detached, and returns without correcting the surviving CPUs.

On a Ryzen 5 7535U with twelve logical CPUs sharing a 16 MiB LLC,
offlining one SMT sibling left the remaining CPUs with:

  llc_bytes = floor(16777216 * 11 / 12) = 15379114 bytes

The correct capacity is still 16777216 bytes. On systems with active
cache-aware scheduling, an underestimated capacity can cause
exceed_llc_capacity() to reject aggregation for a process whose footprint
would fit. Unchanged cpuset partitions sharing the physical cache can
also retain stale capacity when a CPU comes online in another partition.

Pass the cache-sharing mask already retained by cacheinfo to the
scheduler update. Refresh every surviving CPU using its own LLC domain
so that each partition receives the correct share. This also preserves
the correction needed as cache-sharing maps grow during boot.

Keep the existing CPU-hotplug and scheduler-domain synchronization. The
update remains on the hotplug path; no steady-state scheduling operation
or persistent allocation is added.

Fixes: 7030513a0877 ("sched/cache: Calculate the LLC size and store it in sched_domain")
Signed-off-by: Davi Chaves Azevedo <davichazbh@gmail.com>
---
The issue was identified by tracing the scheduler/cacheinfo teardown
ordering, then checking live llc_bytes values using the running kernel's
BTF layout and /proc/kcore.

Validation:
  - Reproduced the stale value on 7.2.3-arch1-3 on the Ryzen system above.
    The patched kernel retained 16777216 bytes on every surviving CPU.
  - Ten SMT-thread and ten whole-core hotplug cycles passed on the patched
    kernel, including capacity checks after each removal and restoration.
    The existing limited CPU-hotplug selftest also passed.
  - Source-level state fixtures: five failures in eight scenarios before
    the fix, eight passes after it. These cover partition changes, unequal
    spans, sparse CPU IDs and boot-time map growth, but not concurrency.
  - Full x86-64 baseline and patched bzImage/modules builds passed with
    matched configs apart from LOCALVERSION. Focused ARM64, x86 without
    CONFIG_SCHED_CACHE, and x86 UP builds also passed.

This host has only one LLC, so the live checks establish the accounting
correction, not an aggregation speedup. No controlled same-version
performance comparison or multi-LLC hardware result is claimed.

 drivers/base/cacheinfo.c       | 11 ++++++-----
 include/linux/sched/topology.h |  4 ++--
 kernel/sched/topology.c        | 27 ++++++++++++---------------
 3 files changed, 20 insertions(+), 22 deletions(-)

diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
index 9f9c72727a05..7a47a392568a 100644
--- a/drivers/base/cacheinfo.c
+++ b/drivers/base/cacheinfo.c
@@ -1040,9 +1040,10 @@ static int cacheinfo_cpu_online(unsigned int cpu)
 	rc = cache_add_dev(cpu);
 	if (rc)
 		goto err;
-	if (cpu_map_shared_cache(true, cpu, &cpu_map))
+	if (cpu_map_shared_cache(true, cpu, &cpu_map)) {
 		update_per_cpu_data_slice_size(true, cpu, cpu_map);
-	sched_update_llc_bytes(cpu);
+		sched_update_llc_bytes(cpu_map);
+	}
 	return 0;
 err:
 	free_cache_attributes(cpu);
@@ -1059,10 +1060,10 @@ static int cacheinfo_cpu_pre_down(unsigned int cpu)
 		cpu_cache_sysfs_exit(cpu);
 
 	free_cache_attributes(cpu);
-	if (nr_shared > 1)
+	if (nr_shared > 1) {
 		update_per_cpu_data_slice_size(false, cpu, cpu_map);
-
-	sched_update_llc_bytes(cpu);
+		sched_update_llc_bytes(cpu_map);
+	}
 
 	return 0;
 }
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index b5d9d7c2b8ad..f96812d71c51 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -281,9 +281,9 @@ static inline int task_node(const struct task_struct *p)
 }
 
 #ifdef CONFIG_SCHED_CACHE
-extern void sched_update_llc_bytes(unsigned int cpu);
+extern void sched_update_llc_bytes(const struct cpumask *cpus);
 #else
-static inline void sched_update_llc_bytes(unsigned int cpu) { }
+static inline void sched_update_llc_bytes(const struct cpumask *cpus) { }
 #endif
 
 #endif /* _LINUX_SCHED_TOPOLOGY_H */
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 0248227d983a..a10eecc49d51 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -985,8 +985,8 @@ void sched_cache_active_set(void)
 }
 
 /*
- * Update the bottom sched_domain's llc_bytes for @cpu and all its
- * LLC siblings. Called from cacheinfo_cpu_online() or
+ * Update the bottom sched_domain's llc_bytes for @cpus sharing a physical
+ * LLC. Called from cacheinfo_cpu_online() or
  * cacheinfo_cpu_pre_down() with cpu hotplug lock held.
  *
  * Note: get_effective_llc_bytes() returns 0 on PowerPC.
@@ -996,32 +996,29 @@ void sched_cache_active_set(void)
  * and does not populates the per-CPU struct cpu_cacheinfo array
  * that get_cpu_cacheinfo_llc() reads.
  */
-void sched_update_llc_bytes(unsigned int cpu)
+void sched_update_llc_bytes(const struct cpumask *cpus)
 {
 	struct sched_domain *sd, *sdp;
 	unsigned int i;
 
 	sched_domains_mutex_lock();
 
-	sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, cpu));
-	if (!sdp)
-		goto unlock;
-
 	/*
-	 * ci->shared_cpu_map is built incrementally as CPUs come
-	 * online, so the first CPU in an LLC initially sees
-	 * hw_weight == 1 and computes an inflated llc_bytes in
-	 * get_effective_llc_bytes().  Re-evaluating every LLC
-	 * sibling on each online event corrects this once the full
-	 * shared_cpu_map is known.
+	 * The departing CPU's domains have already been detached when
+	 * cacheinfo removes it. Use the surviving cache siblings instead.
+	 * They may belong to different cpuset partitions, so use each CPU's
+	 * own LLC domain to scale its share of the physical cache.
 	 */
-	for_each_cpu(i, sched_domain_span(sdp)) {
+	for_each_cpu(i, cpus) {
+		sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, i));
+		if (!sdp)
+			continue;
+
 		sd = rcu_dereference_sched_domain(cpu_rq(i)->sd);
 		if (sd)
 			sd->llc_bytes = get_effective_llc_bytes(i, sdp);
 	}
 
-unlock:
 	sched_domains_mutex_unlock();
 }
 
base-commit: 50d05c7c76c96b90462f24debacca971d2e86713

Re: [PATCH] sched/cache: Refresh LLC capacity across CPU hotplug

[Chen, Yu C]

Hi Davi,

On 9/11/2026 9:48 PM, Davi Chaves Azevedo wrote:
> The scheduler scales LLC capacity by the fraction of cache-sharing CPUs
> covered by a domain:
> 
>    llc_bytes = cache_size * span_weight / shared_weight
> 
> During CPU teardown, sched_cpu_deactivate() rebuilds scheduler domains
> before cacheinfo_cpu_pre_down() removes the CPU from shared_cpu_map. The
> new domains therefore use the old sharing weight. The later call to
> sched_update_llc_bytes() looks up the departing CPU's sd_llc, which has
> already been detached, and returns without correcting the surviving CPUs.
> 
> On a Ryzen 5 7535U with twelve logical CPUs sharing a 16 MiB LLC,
> offlining one SMT sibling left the remaining CPUs with:
> 
>    llc_bytes = floor(16777216 * 11 / 12) = 15379114 bytes
> 
> The correct capacity is still 16777216 bytes. On systems with active
> cache-aware scheduling, an underestimated capacity can cause
> exceed_llc_capacity() to reject aggregation for a process whose footprint
> would fit. Unchanged cpuset partitions sharing the physical cache can
> also retain stale capacity when a CPU comes online in another partition.
> 

Ah right, thanks very much for catching this. The current 
sched_update_llc_bytes()
only considers the bootup sequence for build_sched_domains() and 
cacheinfo_cpu_online(),
but misses the runtime CPU hotplug scenario, where it incorrectly 
ignores all the surviving
CPUs due to an offline CPU.

> Pass the cache-sharing mask already retained by cacheinfo to the
> scheduler update. Refresh every surviving CPU using its own LLC domain
> so that each partition receives the correct share. This also preserves
> the correction needed as cache-sharing maps grow during boot.
> 
> Keep the existing CPU-hotplug and scheduler-domain synchronization. The
> update remains on the hotplug path; no steady-state scheduling operation
> or persistent allocation is added.
> 
> Fixes: 7030513a0877 ("sched/cache: Calculate the LLC size and store it in sched_domain")
> Signed-off-by: Davi Chaves Azevedo <davichazbh@gmail.com>
> ---
> The issue was identified by tracing the scheduler/cacheinfo teardown
> ordering, then checking live llc_bytes values using the running kernel's
> BTF layout and /proc/kcore.
> 
> Validation:
>    - Reproduced the stale value on 7.2.3-arch1-3 on the Ryzen system above.
>      The patched kernel retained 16777216 bytes on every surviving CPU.
>    - Ten SMT-thread and ten whole-core hotplug cycles passed on the patched
>      kernel, including capacity checks after each removal and restoration.
>      The existing limited CPU-hotplug selftest also passed.
>    - Source-level state fixtures: five failures in eight scenarios before
>      the fix, eight passes after it. These cover partition changes, unequal
>      spans, sparse CPU IDs and boot-time map growth, but not concurrency.
>    - Full x86-64 baseline and patched bzImage/modules builds passed with
>      matched configs apart from LOCALVERSION. Focused ARM64, x86 without
>      CONFIG_SCHED_CACHE, and x86 UP builds also passed.
> 
> This host has only one LLC, so the live checks establish the accounting
> correction, not an aggregation speedup. No controlled same-version
> performance comparison or multi-LLC hardware result is claimed.
> 
>   drivers/base/cacheinfo.c       | 11 ++++++-----
>   include/linux/sched/topology.h |  4 ++--
>   kernel/sched/topology.c        | 27 ++++++++++++---------------
>   3 files changed, 20 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> index 9f9c72727a05..7a47a392568a 100644
> --- a/drivers/base/cacheinfo.c
> +++ b/drivers/base/cacheinfo.c
> @@ -1040,9 +1040,10 @@ static int cacheinfo_cpu_online(unsigned int cpu)
>   	rc = cache_add_dev(cpu);
>   	if (rc)
>   		goto err;
> -	if (cpu_map_shared_cache(true, cpu, &cpu_map))
> +	if (cpu_map_shared_cache(true, cpu, &cpu_map)) {
>   		update_per_cpu_data_slice_size(true, cpu, cpu_map);
> -	sched_update_llc_bytes(cpu);
> +		sched_update_llc_bytes(cpu_map);
> +	}
>   	return 0;
>   err:
>   	free_cache_attributes(cpu);
> @@ -1059,10 +1060,10 @@ static int cacheinfo_cpu_pre_down(unsigned int cpu)
>   		cpu_cache_sysfs_exit(cpu);
>   
>   	free_cache_attributes(cpu);
> -	if (nr_shared > 1)
> +	if (nr_shared > 1) {
>   		update_per_cpu_data_slice_size(false, cpu, cpu_map);
> -
> -	sched_update_llc_bytes(cpu);
> +		sched_update_llc_bytes(cpu_map);
> +	}
>   
>   	return 0;
>   }
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index b5d9d7c2b8ad..f96812d71c51 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -281,9 +281,9 @@ static inline int task_node(const struct task_struct *p)
>   }
>   
>   #ifdef CONFIG_SCHED_CACHE
> -extern void sched_update_llc_bytes(unsigned int cpu);
> +extern void sched_update_llc_bytes(const struct cpumask *cpus);
>   #else
> -static inline void sched_update_llc_bytes(unsigned int cpu) { }
> +static inline void sched_update_llc_bytes(const struct cpumask *cpus) { }
>   #endif
>   
>   #endif /* _LINUX_SCHED_TOPOLOGY_H */
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 0248227d983a..a10eecc49d51 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -985,8 +985,8 @@ void sched_cache_active_set(void)
>   }
>   
>   /*
> - * Update the bottom sched_domain's llc_bytes for @cpu and all its
> - * LLC siblings. Called from cacheinfo_cpu_online() or
> + * Update the bottom sched_domain's llc_bytes for @cpus sharing a physical
> + * LLC. Called from cacheinfo_cpu_online() or
>    * cacheinfo_cpu_pre_down() with cpu hotplug lock held.
>    *
>    * Note: get_effective_llc_bytes() returns 0 on PowerPC.
> @@ -996,32 +996,29 @@ void sched_cache_active_set(void)
>    * and does not populates the per-CPU struct cpu_cacheinfo array
>    * that get_cpu_cacheinfo_llc() reads.
>    */
> -void sched_update_llc_bytes(unsigned int cpu)
> +void sched_update_llc_bytes(const struct cpumask *cpus)
>   {
>   	struct sched_domain *sd, *sdp;
>   	unsigned int i;
>   
>   	sched_domains_mutex_lock();
>   
> -	sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, cpu));
> -	if (!sdp)
> -		goto unlock;
> -
>   	/*
> -	 * ci->shared_cpu_map is built incrementally as CPUs come
> -	 * online, so the first CPU in an LLC initially sees
> -	 * hw_weight == 1 and computes an inflated llc_bytes in
> -	 * get_effective_llc_bytes().  Re-evaluating every LLC
> -	 * sibling on each online event corrects this once the full
> -	 * shared_cpu_map is known.
Maybe the above comments can be kept, because they describe the bootup
scenario, with your offline case/domain partition added — just in case
in the future the reader might wonder why we iterate every online CPU
again and again during cacheinfo_cpu_online()/offline().
> +	 * The departing CPU's domains have already been detached when
> +	 * cacheinfo removes it. Use the surviving cache siblings instead.
> +	 * They may belong to different cpuset partitions, so use each CPU's
> +	 * own LLC domain to scale its share of the physical cache.
>   	 */
> -	for_each_cpu(i, sched_domain_span(sdp)) {
> +	for_each_cpu(i, cpus) {
> +		sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, i));
> +		if (!sdp)
> +			continue;
> +
>   		sd = rcu_dereference_sched_domain(cpu_rq(i)->sd);
>   		if (sd)
>   			sd->llc_bytes = get_effective_llc_bytes(i, sdp);
>   	}
>   
> -unlock:
>   	sched_domains_mutex_unlock();
>   }
>   
> base-commit: 50d05c7c76c96b90462f24debacca971d2e86713

I have reproduced this issue on an AMD Ryzen 8945HX machine, which has 2 
LLCs
and 8 cores/LLC. It is also reproduced on a Xeon platform with 4 LLCs 
per node.
With the patch applied, sd->llc_bytes is back to normal. This patch 
looks reasonable
to me.

Reviewed-by: Chen Yu <yu.c.chen@intel.com>

thanks,
Chenyu

[PATCH v2] sched/cache: Refresh LLC capacity across CPU hotplug

[Davi Chaves Azevedo]

The scheduler scales LLC capacity by the fraction of cache-sharing CPUs
covered by a domain:

  llc_bytes = cache_size * span_weight / shared_weight

During CPU teardown, sched_cpu_deactivate() rebuilds scheduler domains
before cacheinfo_cpu_pre_down() removes the CPU from shared_cpu_map. The
new domains therefore use the old sharing weight. The later call to
sched_update_llc_bytes() looks up the departing CPU's sd_llc, which has
already been detached, and returns without correcting the surviving CPUs.

On a Ryzen 5 7535U with twelve logical CPUs sharing a 16 MiB LLC,
offlining one SMT sibling left the remaining CPUs with:

  llc_bytes = floor(16777216 * 11 / 12) = 15379114 bytes

The correct capacity is still 16777216 bytes. On systems with active
cache-aware scheduling, an underestimated capacity can cause
exceed_llc_capacity() to reject aggregation for a process whose footprint
would fit. Unchanged cpuset partitions sharing the physical cache can
also retain stale capacity when a CPU comes online in another partition.

Pass the cache-sharing mask already retained by cacheinfo to the
scheduler update. Refresh every surviving CPU using its own LLC domain
so that each partition receives the correct share. This also preserves
the correction needed as cache-sharing maps grow during boot.

Keep the existing CPU-hotplug and scheduler-domain synchronization. The
update remains on the hotplug path; no steady-state scheduling operation
or persistent allocation is added.

Fixes: 7030513a0877 ("sched/cache: Calculate the LLC size and store it in sched_domain")
Signed-off-by: Davi Chaves Azevedo <davichazbh@gmail.com>
Reviewed-by: Chen Yu <yu.c.chen@intel.com>
---
Changes in v2:
  - Restore the original boot-time shared_cpu_map explanation, as Chen Yu
    suggested, alongside the CPU-offline and cpuset-partition rationale.
    No functional changes from v1.
  - Add Chen Yu's Reviewed-by tag and document his multi-LLC testing.

v1:
https://lore.kernel.org/r/20260911134825.420748-1-davichazbh@gmail.com
Review:
https://lore.kernel.org/r/a3433e6a-0d1f-44a8-99bd-bc63d1a15913@intel.com

The issue was identified by tracing the scheduler/cacheinfo teardown
ordering, then checking live llc_bytes values using the running kernel's
BTF layout and /proc/kcore.

Local validation performed for v1 (no functional changes in v2):
  - Reproduced the stale value on 7.2.3-arch1-3 on the Ryzen system above.
    The patched kernel retained 16777216 bytes on every surviving CPU.
  - Ten SMT-thread and ten whole-core hotplug cycles passed on the patched
    kernel, including capacity checks after each removal and restoration.
    The existing limited CPU-hotplug selftest also passed.
  - Source-level state fixtures: five failures in eight scenarios before
    the fix, eight passes after it. These cover partition changes, unequal
    spans, sparse CPU IDs and boot-time map growth, but not concurrency.
  - Full x86-64 baseline and patched bzImage/modules builds passed with
    matched configs apart from LOCALVERSION. Focused ARM64, x86 without
    CONFIG_SCHED_CACHE, and x86 UP builds also passed.

Thanks to Chen Yu <yu.c.chen@intel.com> for the additional verification
and review. He reproduced the issue and confirmed that v1 restored the
expected sd->llc_bytes on:
  - AMD Ryzen 8945HX: two LLCs, eight cores per LLC.
  - Xeon: four LLCs per node.

The local tests used a single-LLC host; Chen Yu reported the multi-LLC
results above. These checks validate LLC accounting.
Builds and hotplug tests were not rerun for this comment revision.

 drivers/base/cacheinfo.c       | 11 ++++++-----
 include/linux/sched/topology.h |  4 ++--
 kernel/sched/topology.c        | 22 +++++++++++++---------
 3 files changed, 21 insertions(+), 16 deletions(-)

diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
index 9f9c72727a05..7a47a392568a 100644
--- a/drivers/base/cacheinfo.c
+++ b/drivers/base/cacheinfo.c
@@ -1040,9 +1040,10 @@ static int cacheinfo_cpu_online(unsigned int cpu)
 	rc = cache_add_dev(cpu);
 	if (rc)
 		goto err;
-	if (cpu_map_shared_cache(true, cpu, &cpu_map))
+	if (cpu_map_shared_cache(true, cpu, &cpu_map)) {
 		update_per_cpu_data_slice_size(true, cpu, cpu_map);
-	sched_update_llc_bytes(cpu);
+		sched_update_llc_bytes(cpu_map);
+	}
 	return 0;
 err:
 	free_cache_attributes(cpu);
@@ -1059,10 +1060,10 @@ static int cacheinfo_cpu_pre_down(unsigned int cpu)
 		cpu_cache_sysfs_exit(cpu);
 
 	free_cache_attributes(cpu);
-	if (nr_shared > 1)
+	if (nr_shared > 1) {
 		update_per_cpu_data_slice_size(false, cpu, cpu_map);
-
-	sched_update_llc_bytes(cpu);
+		sched_update_llc_bytes(cpu_map);
+	}
 
 	return 0;
 }
diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index b5d9d7c2b8ad..f96812d71c51 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -281,9 +281,9 @@ static inline int task_node(const struct task_struct *p)
 }
 
 #ifdef CONFIG_SCHED_CACHE
-extern void sched_update_llc_bytes(unsigned int cpu);
+extern void sched_update_llc_bytes(const struct cpumask *cpus);
 #else
-static inline void sched_update_llc_bytes(unsigned int cpu) { }
+static inline void sched_update_llc_bytes(const struct cpumask *cpus) { }
 #endif
 
 #endif /* _LINUX_SCHED_TOPOLOGY_H */
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 0248227d983a..3dab0253976f 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -985,8 +985,8 @@ void sched_cache_active_set(void)
 }
 
 /*
- * Update the bottom sched_domain's llc_bytes for @cpu and all its
- * LLC siblings. Called from cacheinfo_cpu_online() or
+ * Update the bottom sched_domain's llc_bytes for @cpus sharing a physical
+ * LLC. Called from cacheinfo_cpu_online() or
  * cacheinfo_cpu_pre_down() with cpu hotplug lock held.
  *
  * Note: get_effective_llc_bytes() returns 0 on PowerPC.
@@ -996,17 +996,13 @@ void sched_cache_active_set(void)
  * and does not populates the per-CPU struct cpu_cacheinfo array
  * that get_cpu_cacheinfo_llc() reads.
  */
-void sched_update_llc_bytes(unsigned int cpu)
+void sched_update_llc_bytes(const struct cpumask *cpus)
 {
 	struct sched_domain *sd, *sdp;
 	unsigned int i;
 
 	sched_domains_mutex_lock();
 
-	sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, cpu));
-	if (!sdp)
-		goto unlock;
-
 	/*
 	 * ci->shared_cpu_map is built incrementally as CPUs come
 	 * online, so the first CPU in an LLC initially sees
@@ -1014,14 +1010,22 @@ void sched_update_llc_bytes(unsigned int cpu)
 	 * get_effective_llc_bytes().  Re-evaluating every LLC
 	 * sibling on each online event corrects this once the full
 	 * shared_cpu_map is known.
+	 *
+	 * The departing CPU's domains have already been detached when
+	 * cacheinfo removes it. Use the surviving cache siblings instead.
+	 * They may belong to different cpuset partitions, so use each CPU's
+	 * own LLC domain to scale its share of the physical cache.
 	 */
-	for_each_cpu(i, sched_domain_span(sdp)) {
+	for_each_cpu(i, cpus) {
+		sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, i));
+		if (!sdp)
+			continue;
+
 		sd = rcu_dereference_sched_domain(cpu_rq(i)->sd);
 		if (sd)
 			sd->llc_bytes = get_effective_llc_bytes(i, sdp);
 	}
 
-unlock:
 	sched_domains_mutex_unlock();
 }
 
base-commit: 50d05c7c76c96b90462f24debacca971d2e86713

Re: [PATCH] sched/cache: Refresh LLC capacity across CPU hotplug

[Davi Chaves Azevedo]

Hi Chenyu,

Agreed. I've restored the original boot-time shared_cpu_map explanation
and kept the CPU-offline and cpuset-partition rationale alongside it.

Thanks for the review and for testing the fix on both the Ryzen 8945HX
and Xeon systems. I've included your Reviewed-by tag and noted those
results in v2. There are no functional changes from v1.

Thanks,
Davi

Re: [PATCH v2] sched/cache: Refresh LLC capacity across CPU hotplug

[Tim Chen]

On Fri, 2026-09-11 at 19:02 -0300, Davi Chaves Azevedo wrote:
> The scheduler scales LLC capacity by the fraction of cache-sharing CPUs
> covered by a domain:
> 
>   llc_bytes = cache_size * span_weight / shared_weight
> 
> During CPU teardown, sched_cpu_deactivate() rebuilds scheduler domains
> before cacheinfo_cpu_pre_down() removes the CPU from shared_cpu_map. The
> new domains therefore use the old sharing weight. The later call to
> sched_update_llc_bytes() looks up the departing CPU's sd_llc, which has
> already been detached, and returns without correcting the surviving CPUs.
> 
> On a Ryzen 5 7535U with twelve logical CPUs sharing a 16 MiB LLC,
> offlining one SMT sibling left the remaining CPUs with:
> 
>   llc_bytes = floor(16777216 * 11 / 12) = 15379114 bytes
> 
> The correct capacity is still 16777216 bytes. On systems with active
> cache-aware scheduling, an underestimated capacity can cause
> exceed_llc_capacity() to reject aggregation for a process whose footprint
> would fit. Unchanged cpuset partitions sharing the physical cache can
> also retain stale capacity when a CPU comes online in another partition.
> 
> Pass the cache-sharing mask already retained by cacheinfo to the
> scheduler update. Refresh every surviving CPU using its own LLC domain
> so that each partition receives the correct share. This also preserves
> the correction needed as cache-sharing maps grow during boot.
> 
> Keep the existing CPU-hotplug and scheduler-domain synchronization. The
> update remains on the hotplug path; no steady-state scheduling operation
> or persistent allocation is added.
> 


Thanks.  The patch looks good to me.

Tim

> Fixes: 7030513a0877 ("sched/cache: Calculate the LLC size and store it in sched_domain")
> Signed-off-by: Davi Chaves Azevedo <davichazbh@gmail.com>
> Reviewed-by: Chen Yu <yu.c.chen@intel.com>
> ---
> Changes in v2:
>   - Restore the original boot-time shared_cpu_map explanation, as Chen Yu
>     suggested, alongside the CPU-offline and cpuset-partition rationale.
>     No functional changes from v1.
>   - Add Chen Yu's Reviewed-by tag and document his multi-LLC testing.
> 
> v1:
> https://lore.kernel.org/r/20260911134825.420748-1-davichazbh@gmail.com
> Review:
> https://lore.kernel.org/r/a3433e6a-0d1f-44a8-99bd-bc63d1a15913@intel.com
> 
> The issue was identified by tracing the scheduler/cacheinfo teardown
> ordering, then checking live llc_bytes values using the running kernel's
> BTF layout and /proc/kcore.
> 
> Local validation performed for v1 (no functional changes in v2):
>   - Reproduced the stale value on 7.2.3-arch1-3 on the Ryzen system above.
>     The patched kernel retained 16777216 bytes on every surviving CPU.
>   - Ten SMT-thread and ten whole-core hotplug cycles passed on the patched
>     kernel, including capacity checks after each removal and restoration.
>     The existing limited CPU-hotplug selftest also passed.
>   - Source-level state fixtures: five failures in eight scenarios before
>     the fix, eight passes after it. These cover partition changes, unequal
>     spans, sparse CPU IDs and boot-time map growth, but not concurrency.
>   - Full x86-64 baseline and patched bzImage/modules builds passed with
>     matched configs apart from LOCALVERSION. Focused ARM64, x86 without
>     CONFIG_SCHED_CACHE, and x86 UP builds also passed.
> 
> Thanks to Chen Yu <yu.c.chen@intel.com> for the additional verification
> and review. He reproduced the issue and confirmed that v1 restored the
> expected sd->llc_bytes on:
>   - AMD Ryzen 8945HX: two LLCs, eight cores per LLC.
>   - Xeon: four LLCs per node.
> 
> The local tests used a single-LLC host; Chen Yu reported the multi-LLC
> results above. These checks validate LLC accounting.
> Builds and hotplug tests were not rerun for this comment revision.
> 
>  drivers/base/cacheinfo.c       | 11 ++++++-----
>  include/linux/sched/topology.h |  4 ++--
>  kernel/sched/topology.c        | 22 +++++++++++++---------
>  3 files changed, 21 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> index 9f9c72727a05..7a47a392568a 100644
> --- a/drivers/base/cacheinfo.c
> +++ b/drivers/base/cacheinfo.c
> @@ -1040,9 +1040,10 @@ static int cacheinfo_cpu_online(unsigned int cpu)
>  	rc = cache_add_dev(cpu);
>  	if (rc)
>  		goto err;
> -	if (cpu_map_shared_cache(true, cpu, &cpu_map))
> +	if (cpu_map_shared_cache(true, cpu, &cpu_map)) {
>  		update_per_cpu_data_slice_size(true, cpu, cpu_map);
> -	sched_update_llc_bytes(cpu);
> +		sched_update_llc_bytes(cpu_map);
> +	}
>  	return 0;
>  err:
>  	free_cache_attributes(cpu);
> @@ -1059,10 +1060,10 @@ static int cacheinfo_cpu_pre_down(unsigned int cpu)
>  		cpu_cache_sysfs_exit(cpu);
>  
>  	free_cache_attributes(cpu);
> -	if (nr_shared > 1)
> +	if (nr_shared > 1) {
>  		update_per_cpu_data_slice_size(false, cpu, cpu_map);
> -
> -	sched_update_llc_bytes(cpu);
> +		sched_update_llc_bytes(cpu_map);
> +	}
>  
>  	return 0;
>  }
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index b5d9d7c2b8ad..f96812d71c51 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -281,9 +281,9 @@ static inline int task_node(const struct task_struct *p)
>  }
>  
>  #ifdef CONFIG_SCHED_CACHE
> -extern void sched_update_llc_bytes(unsigned int cpu);
> +extern void sched_update_llc_bytes(const struct cpumask *cpus);
>  #else
> -static inline void sched_update_llc_bytes(unsigned int cpu) { }
> +static inline void sched_update_llc_bytes(const struct cpumask *cpus) { }
>  #endif
>  
>  #endif /* _LINUX_SCHED_TOPOLOGY_H */
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 0248227d983a..3dab0253976f 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -985,8 +985,8 @@ void sched_cache_active_set(void)
>  }
>  
>  /*
> - * Update the bottom sched_domain's llc_bytes for @cpu and all its
> - * LLC siblings. Called from cacheinfo_cpu_online() or
> + * Update the bottom sched_domain's llc_bytes for @cpus sharing a physical
> + * LLC. Called from cacheinfo_cpu_online() or
>   * cacheinfo_cpu_pre_down() with cpu hotplug lock held.
>   *
>   * Note: get_effective_llc_bytes() returns 0 on PowerPC.
> @@ -996,17 +996,13 @@ void sched_cache_active_set(void)
>   * and does not populates the per-CPU struct cpu_cacheinfo array
>   * that get_cpu_cacheinfo_llc() reads.
>   */
> -void sched_update_llc_bytes(unsigned int cpu)
> +void sched_update_llc_bytes(const struct cpumask *cpus)
>  {
>  	struct sched_domain *sd, *sdp;
>  	unsigned int i;
>  
>  	sched_domains_mutex_lock();
>  
> -	sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, cpu));
> -	if (!sdp)
> -		goto unlock;
> -
>  	/*
>  	 * ci->shared_cpu_map is built incrementally as CPUs come
>  	 * online, so the first CPU in an LLC initially sees
> @@ -1014,14 +1010,22 @@ void sched_update_llc_bytes(unsigned int cpu)
>  	 * get_effective_llc_bytes().  Re-evaluating every LLC
>  	 * sibling on each online event corrects this once the full
>  	 * shared_cpu_map is known.
> +	 *
> +	 * The departing CPU's domains have already been detached when
> +	 * cacheinfo removes it. Use the surviving cache siblings instead.
> +	 * They may belong to different cpuset partitions, so use each CPU's
> +	 * own LLC domain to scale its share of the physical cache.
>  	 */
> -	for_each_cpu(i, sched_domain_span(sdp)) {
> +	for_each_cpu(i, cpus) {
> +		sdp = rcu_dereference_sched_domain(per_cpu(sd_llc, i));
> +		if (!sdp)
> +			continue;
> +
>  		sd = rcu_dereference_sched_domain(cpu_rq(i)->sd);
>  		if (sd)
>  			sd->llc_bytes = get_effective_llc_bytes(i, sdp);
>  	}
>  
> -unlock:
>  	sched_domains_mutex_unlock();
>  }
>  
> base-commit: 50d05c7c76c96b90462f24debacca971d2e86713