[PATCH] sched/topology: Assert non-NUMA topology masks don't (partially) overlap

[PATCH] sched/topology: Assert non-NUMA topology masks don't (partially) overlap

[Valentin Schneider]

topology.c::get_group() relies on the assumption that non-NUMA domains do
not partially overlap. Zeng Tao pointed out in [1] that such topology
descriptions, while completely bogus, can end up being exposed to the
scheduler.

In his example (8 CPUs, 2-node system), we end up with:
  MC span for CPU3 == 3-7
  MC span for CPU4 == 4-7

The first pass through get_group(3, sdd@MC) will result in the following
sched_group list:

  3 -> 4 -> 5 -> 6 -> 7
  ^                  /
   `----------------'

And a later pass through get_group(4, sdd@MC) will "corrupt" that to:

  3 -> 4 -> 5 -> 6 -> 7
       ^             /
	`-----------'

which will completely break things like 'while (sg != sd->groups)' when
using CPU3's base sched_domain.


There already are some architecture-specific checks in place such as
x86/kernel/smpboot.c::topology.sane(), but this is something we can detect
in the core scheduler, so it seems worthwhile to do so.

Warn and abort the construction of the sched domains if such a broken
topology description is detected. Note that this is somewhat
expensive (O(t.c²), 't' non-NUMA topology levels and 'c' CPUs) and could be
gated under SCHED_DEBUG if deemed necessary.

Testing
=======

Dietmar managed to reproduce this using the following qemu incantation:

  $ qemu-system-aarch64 -kernel ./Image -hda ./qemu-image-aarch64.img \
  -append 'root=/dev/vda console=ttyAMA0 loglevel=8 sched_debug' -smp \
  cores=8 --nographic -m 512 -cpu cortex-a53 -machine virt -numa \
  node,cpus=0-2,nodeid=0 -numa node,cpus=3-7,nodeid=1

alongside the following drivers/base/arch_topology.c hack (AIUI wouldn't be
needed if '-smp cores=X, sockets=Y' would work with qemu):

8<---
@@ -465,6 +465,9 @@ void update_siblings_masks(unsigned int cpuid)
 		if (cpuid_topo->package_id != cpu_topo->package_id)
 			continue;

+		if ((cpu < 4 && cpuid > 3) || (cpu > 3 && cpuid < 4))
+			continue;
+
 		cpumask_set_cpu(cpuid, &cpu_topo->core_sibling);
 		cpumask_set_cpu(cpu, &cpuid_topo->core_sibling);

8<---

[1]: https://lkml.kernel.org/r/1577088979-8545-1-git-send-email-prime.zeng@hisilicon.com

Reported-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
A "less intrusive" alternative is to assert the sd->groups list doesn't get
re-written, which is a symptom of such bogus topologies. I've briefly
tested this, you can have a look at it here:

  http://www.linux-arm.org/git?p=linux-vs.git;a=commit;h=e0ead72137332cbd3d69c9055ab29e6ffae5b37b

fetchable via:
  git fetch git://linux-arm.org/linux-vs mainline/topology/non_numa_overlap_sg
---
 kernel/sched/topology.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6ec1e595b1d4..dfb64c08a407 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1879,6 +1879,42 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
 	return sd;
 }
 
+/*
+ * Ensure topology masks are sane, i.e. there are no conflicts (overlaps) for
+ * any two given CPUs at this (non-NUMA) topology level.
+ */
+static bool topology_span_sane(struct sched_domain_topology_level *tl,
+			      const struct cpumask *cpu_map, int cpu)
+{
+	int i;
+
+	/* NUMA levels are allowed to overlap */
+	if (tl->flags & SDTL_OVERLAP)
+		return true;
+
+	/*
+	 * Non-NUMA levels cannot partially overlap - they must be either
+	 * completely equal or completely disjoint. Otherwise we can end up
+	 * breaking the sched_group lists - i.e. a later get_group() pass
+	 * breaks the linking done for an earlier span.
+	 */
+	for_each_cpu(i, cpu_map) {
+		if (i == cpu)
+			continue;
+		/*
+		 * We should 'and' all those masks with 'cpu_map' to exactly
+		 * match the topology we're about to build, but that can only
+		 * remove CPUs, which only lessens our ability to detect
+		 * overlaps
+		 */
+		if (!cpumask_equal(tl->mask(cpu), tl->mask(i)) &&
+		    cpumask_intersects(tl->mask(cpu), tl->mask(i)))
+			return false;
+	}
+
+	return true;
+}
+
 /*
  * Find the sched_domain_topology_level where all CPU capacities are visible
  * for all CPUs.
@@ -1975,6 +2011,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 				has_asym = true;
 			}
 
+			if (WARN_ON(!topology_span_sane(tl, cpu_map, i)))
+				goto error;
+
 			sd = build_sched_domain(tl, cpu_map, attr, sd, dflags, i);
 
 			if (tl == sched_domain_topology)
-- 
2.24.0

Re: [PATCH] sched/topology: Assert non-NUMA topology masks don't (partially) overlap

[Peter Zijlstra]

On Wed, Jan 15, 2020 at 04:09:15PM +0000, Valentin Schneider wrote:
> A "less intrusive" alternative is to assert the sd->groups list doesn't get
> re-written, which is a symptom of such bogus topologies. I've briefly
> tested this, you can have a look at it here:
> 
>   http://www.linux-arm.org/git?p=linux-vs.git;a=commit;h=e0ead72137332cbd3d69c9055ab29e6ffae5b37b

Something like that might still make sense. Can't never be too careful,
right ;-)

>  kernel/sched/topology.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 6ec1e595b1d4..dfb64c08a407 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1879,6 +1879,42 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
>  	return sd;
>  }
>  
> +/*
> + * Ensure topology masks are sane, i.e. there are no conflicts (overlaps) for
> + * any two given CPUs at this (non-NUMA) topology level.
> + */
> +static bool topology_span_sane(struct sched_domain_topology_level *tl,
> +			      const struct cpumask *cpu_map, int cpu)
> +{
> +	int i;
> +
> +	/* NUMA levels are allowed to overlap */
> +	if (tl->flags & SDTL_OVERLAP)
> +		return true;
> +
> +	/*
> +	 * Non-NUMA levels cannot partially overlap - they must be either
> +	 * completely equal or completely disjoint. Otherwise we can end up
> +	 * breaking the sched_group lists - i.e. a later get_group() pass
> +	 * breaks the linking done for an earlier span.
> +	 */
> +	for_each_cpu(i, cpu_map) {
> +		if (i == cpu)
> +			continue;
> +		/*
> +		 * We should 'and' all those masks with 'cpu_map' to exactly
> +		 * match the topology we're about to build, but that can only
> +		 * remove CPUs, which only lessens our ability to detect
> +		 * overlaps
> +		 */
> +		if (!cpumask_equal(tl->mask(cpu), tl->mask(i)) &&
> +		    cpumask_intersects(tl->mask(cpu), tl->mask(i)))
> +			return false;
> +	}
> +
> +	return true;
> +}
> +
>  /*
>   * Find the sched_domain_topology_level where all CPU capacities are visible
>   * for all CPUs.
> @@ -1975,6 +2011,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>  				has_asym = true;
>  			}
>  
> +			if (WARN_ON(!topology_span_sane(tl, cpu_map, i)))
> +				goto error;
> +
>  			sd = build_sched_domain(tl, cpu_map, attr, sd, dflags, i);
>  
>  			if (tl == sched_domain_topology)

This is O(nr_cpus), but then, that function already is, so I don't see a
problem with this.

I'll take it, thanks!

Re: [PATCH] sched/topology: Assert non-NUMA topology masks don't (partially) overlap

[Valentin Schneider]

On 16/01/2020 10:44, Peter Zijlstra wrote:
> On Wed, Jan 15, 2020 at 04:09:15PM +0000, Valentin Schneider wrote:
>> A "less intrusive" alternative is to assert the sd->groups list doesn't get
>> re-written, which is a symptom of such bogus topologies. I've briefly
>> tested this, you can have a look at it here:
>>
>>   http://www.linux-arm.org/git?p=linux-vs.git;a=commit;h=e0ead72137332cbd3d69c9055ab29e6ffae5b37b
> 
> Something like that might still make sense. Can't never be too careful,
> right ;-)
> 

True. I'll ponder about it and see if I can't make it a bit neater

Re: [PATCH] sched/topology: Assert non-NUMA topology masks don't (partially) overlap

[Peter Zijlstra]

On Thu, Jan 16, 2020 at 11:44:28AM +0100, Peter Zijlstra wrote:
> On Wed, Jan 15, 2020 at 04:09:15PM +0000, Valentin Schneider wrote:
> > @@ -1975,6 +2011,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> >  				has_asym = true;
> >  			}
> >  
> > +			if (WARN_ON(!topology_span_sane(tl, cpu_map, i)))
> > +				goto error;
> > +
> >  			sd = build_sched_domain(tl, cpu_map, attr, sd, dflags, i);
> >  
> >  			if (tl == sched_domain_topology)
> 
> This is O(nr_cpus), but then, that function already is, so I don't see a
> problem with this.

Clearly I meant to write O(nr_cpus^2), there's a bunch of nested
for_each_cpu() in there.

> I'll take it, thanks!

Re: [PATCH] sched/topology: Assert non-NUMA topology masks don't (partially) overlap

[Valentin Schneider]

On 16/01/2020 15:19, Peter Zijlstra wrote:
> On Thu, Jan 16, 2020 at 11:44:28AM +0100, Peter Zijlstra wrote:
>> On Wed, Jan 15, 2020 at 04:09:15PM +0000, Valentin Schneider wrote:
>>> @@ -1975,6 +2011,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
>>>  				has_asym = true;
>>>  			}
>>>  
>>> +			if (WARN_ON(!topology_span_sane(tl, cpu_map, i)))
>>> +				goto error;
>>> +
>>>  			sd = build_sched_domain(tl, cpu_map, attr, sd, dflags, i);
>>>  
>>>  			if (tl == sched_domain_topology)
>>
>> This is O(nr_cpus), but then, that function already is, so I don't see a
>> problem with this.
> 
> Clearly I meant to write O(nr_cpus^2), there's a bunch of nested
> for_each_cpu() in there.
> 

Hm so the sanity check is O(topo_levels * nr_cpus²), and I think that
roughly matches the complexity of building the sched_groups (I say
roughly because you go over less CPUs as you go up the topo levels).

As you said the important bit is that we don't make build_sched_domains()
noticeably worse, which I think is the case...

>> I'll take it, thanks!

[tip: sched/core] sched/topology: Assert non-NUMA topology masks don't (partially) overlap

[tip-bot2 for Valentin Schneider]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     ccf74128d66ce937876184ad55db2e0276af08d3
Gitweb:        https://git.kernel.org/tip/ccf74128d66ce937876184ad55db2e0276af08d3
Author:        Valentin Schneider <valentin.schneider@arm.com>
AuthorDate:    Wed, 15 Jan 2020 16:09:15 
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Fri, 17 Jan 2020 10:19:23 +01:00

sched/topology: Assert non-NUMA topology masks don't (partially) overlap

topology.c::get_group() relies on the assumption that non-NUMA domains do
not partially overlap. Zeng Tao pointed out in [1] that such topology
descriptions, while completely bogus, can end up being exposed to the
scheduler.

In his example (8 CPUs, 2-node system), we end up with:
  MC span for CPU3 == 3-7
  MC span for CPU4 == 4-7

The first pass through get_group(3, sdd@MC) will result in the following
sched_group list:

  3 -> 4 -> 5 -> 6 -> 7
  ^                  /
   `----------------'

And a later pass through get_group(4, sdd@MC) will "corrupt" that to:

  3 -> 4 -> 5 -> 6 -> 7
       ^             /
	`-----------'

which will completely break things like 'while (sg != sd->groups)' when
using CPU3's base sched_domain.

There already are some architecture-specific checks in place such as
x86/kernel/smpboot.c::topology.sane(), but this is something we can detect
in the core scheduler, so it seems worthwhile to do so.

Warn and abort the construction of the sched domains if such a broken
topology description is detected. Note that this is somewhat
expensive (O(t.c²), 't' non-NUMA topology levels and 'c' CPUs) and could be
gated under SCHED_DEBUG if deemed necessary.

Testing
=======

Dietmar managed to reproduce this using the following qemu incantation:

  $ qemu-system-aarch64 -kernel ./Image -hda ./qemu-image-aarch64.img \
  -append 'root=/dev/vda console=ttyAMA0 loglevel=8 sched_debug' -smp \
  cores=8 --nographic -m 512 -cpu cortex-a53 -machine virt -numa \
  node,cpus=0-2,nodeid=0 -numa node,cpus=3-7,nodeid=1

alongside the following drivers/base/arch_topology.c hack (AIUI wouldn't be
needed if '-smp cores=X, sockets=Y' would work with qemu):

8<---
@@ -465,6 +465,9 @@ void update_siblings_masks(unsigned int cpuid)
 		if (cpuid_topo->package_id != cpu_topo->package_id)
 			continue;

+		if ((cpu < 4 && cpuid > 3) || (cpu > 3 && cpuid < 4))
+			continue;
+
 		cpumask_set_cpu(cpuid, &cpu_topo->core_sibling);
 		cpumask_set_cpu(cpu, &cpuid_topo->core_sibling);

8<---

[1]: https://lkml.kernel.org/r/1577088979-8545-1-git-send-email-prime.zeng@hisilicon.com

Reported-by: Zeng Tao <prime.zeng@hisilicon.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200115160915.22575-1-valentin.schneider@arm.com
---
 kernel/sched/topology.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 6ec1e59..dfb64c0 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1880,6 +1880,42 @@ static struct sched_domain *build_sched_domain(struct sched_domain_topology_leve
 }
 
 /*
+ * Ensure topology masks are sane, i.e. there are no conflicts (overlaps) for
+ * any two given CPUs at this (non-NUMA) topology level.
+ */
+static bool topology_span_sane(struct sched_domain_topology_level *tl,
+			      const struct cpumask *cpu_map, int cpu)
+{
+	int i;
+
+	/* NUMA levels are allowed to overlap */
+	if (tl->flags & SDTL_OVERLAP)
+		return true;
+
+	/*
+	 * Non-NUMA levels cannot partially overlap - they must be either
+	 * completely equal or completely disjoint. Otherwise we can end up
+	 * breaking the sched_group lists - i.e. a later get_group() pass
+	 * breaks the linking done for an earlier span.
+	 */
+	for_each_cpu(i, cpu_map) {
+		if (i == cpu)
+			continue;
+		/*
+		 * We should 'and' all those masks with 'cpu_map' to exactly
+		 * match the topology we're about to build, but that can only
+		 * remove CPUs, which only lessens our ability to detect
+		 * overlaps
+		 */
+		if (!cpumask_equal(tl->mask(cpu), tl->mask(i)) &&
+		    cpumask_intersects(tl->mask(cpu), tl->mask(i)))
+			return false;
+	}
+
+	return true;
+}
+
+/*
  * Find the sched_domain_topology_level where all CPU capacities are visible
  * for all CPUs.
  */
@@ -1975,6 +2011,9 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 				has_asym = true;
 			}
 
+			if (WARN_ON(!topology_span_sane(tl, cpu_map, i)))
+				goto error;
+
 			sd = build_sched_domain(tl, cpu_map, attr, sd, dflags, i);
 
 			if (tl == sched_domain_topology)