[PATCH] sched/clock: Avoid false sharing for sched_clock_irqtime

[PATCH] sched/clock: Avoid false sharing for sched_clock_irqtime

[Wangyang Guo]

Read-mostly sched_clock_irqtime may share the same cacheline with
frequently updated nohz struct. Make it as cache aligned to avoid
false sharing issue.

Details:
We observed ~3% cycles hotspots in irqtime_account_irq when running
SPECjbb2015 in a 2-sockets system. Most of cycles spent in reading
sched_clock_irqtime, which is a read-mostly var.

perf c2c (cachelien view) shows it has false sharing with nohz struct:
     Num RmtHitm LclHitm  Offset records             Symbol
   6.25%   0.00%   0.00%   0x0       4   [k] _nohz_idle_balance.isra.0
  18.75% 100.00%   0.00%   0x8      14   [k] nohz_balance_exit_idle
   6.25%   0.00%   0.00%   0x8       8   [k] nohz_balance_enter_idle
   6.25%   0.00%   0.00%   0xc       8   [k] sched_balance_newidle
   6.25%   0.00%   0.00%  0x10      31   [k] nohz_balancer_kick
   6.25%   0.00%   0.00%  0x20      16   [k] sched_balance_newidle
  37.50%   0.00%   0.00%  0x38      50   [k] irqtime_account_irq
   6.25%   0.00%   0.00%  0x38      47   [k] account_process_tick
   6.25%   0.00%   0.00%  0x38      12   [k] account_idle_ticks

Offsets:
*  0x0 -- nohz.idle_cpu_mask (r)
*  0x8 -- nohz.nr_cpus (w)
* 0x38 -- sched_clock_irqtime (r), not in nohz, but share cacheline

The layout in /proc/kallsyms can also confirm that:
ffffffff88600d40 b nohz
ffffffff88600d68 B arch_needs_tick_broadcast
ffffffff88600d6c b __key.264
ffffffff88600d6c b __key.265
ffffffff88600d70 b dl_generation
ffffffff88600d78 b sched_clock_irqtime

With the patch applied, irqtime_account_irq hotspot disappear.

Previous Discussion:
https://lore.kernel.org/all/20251211055612.4071266-1-wangyang.guo@intel.com/T/#u

Suggested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Reported-by: Benjamin Lei <benjamin.lei@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Reviewed-by: Tianyou Li <tianyou.li@intel.com>
Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
---
 kernel/sched/cputime.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 7097de2c8cda..34d15972f5fb 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -25,7 +25,7 @@
  */
 DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
 
-int sched_clock_irqtime;
+int sched_clock_irqtime __cacheline_aligned;
 
 void enable_sched_clock_irqtime(void)
 {
-- 
2.47.3

Re: [PATCH] sched/clock: Avoid false sharing for sched_clock_irqtime

[K Prateek Nayak]

Hello Wangyang,

On 1/13/2026 8:36 AM, Wangyang Guo wrote:
> Read-mostly sched_clock_irqtime may share the same cacheline with
> frequently updated nohz struct. Make it as cache aligned to avoid
> false sharing issue.

nit.

Since you mention this is a "Read-mostly" data ...

[..snip..]

> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 7097de2c8cda..34d15972f5fb 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -25,7 +25,7 @@
>   */
>  DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
>  
> -int sched_clock_irqtime;
> +int sched_clock_irqtime __cacheline_aligned;

Have you tried annotating it with "__ro_after_init" or "__read_mostly"?

That should also ensure "sched_clock_irqtime" doesn't have false-sharing
with the nohz struct without using an entire cacheline worth of space.

-- 
Thanks and Regards,
Prateek

Re: [PATCH] sched/clock: Avoid false sharing for sched_clock_irqtime

[Guo, Wangyang]

On 1/13/2026 12:26 PM, K Prateek Nayak wrote:
> Hello Wangyang,
> 
> On 1/13/2026 8:36 AM, Wangyang Guo wrote:
>> Read-mostly sched_clock_irqtime may share the same cacheline with
>> frequently updated nohz struct. Make it as cache aligned to avoid
>> false sharing issue.
> 
> nit.
> 
> Since you mention this is a "Read-mostly" data ...
> 
> [..snip..]
> 
>> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
>> index 7097de2c8cda..34d15972f5fb 100644
>> --- a/kernel/sched/cputime.c
>> +++ b/kernel/sched/cputime.c
>> @@ -25,7 +25,7 @@
>>    */
>>   DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
>>   
>> -int sched_clock_irqtime;
>> +int sched_clock_irqtime __cacheline_aligned;
> 
> Have you tried annotating it with "__ro_after_init" or "__read_mostly"?
> 
> That should also ensure "sched_clock_irqtime" doesn't have false-sharing
> with the nohz struct without using an entire cacheline worth of space.

__read_mostly works and avoids wasting spaces. It's better than 
__cacheline_aligned, patch v2 is sent out as:
https://lore.kernel.org/all/20260113074807.3404180-1-wangyang.guo@intel.com/

BR
Wangyang

Re: [PATCH] sched/clock: Avoid false sharing for sched_clock_irqtime

[Shrikanth Hegde]

Hi Wangyang.

On 1/13/26 8:36 AM, Wangyang Guo wrote:
> Read-mostly sched_clock_irqtime may share the same cacheline with
> frequently updated nohz struct. Make it as cache aligned to avoid
> false sharing issue.
> 
> Details:
> We observed ~3% cycles hotspots in irqtime_account_irq when running
> SPECjbb2015 in a 2-sockets system. Most of cycles spent in reading
> sched_clock_irqtime, which is a read-mostly var.
> 
> perf c2c (cachelien view) shows it has false sharing with nohz struct:
>       Num RmtHitm LclHitm  Offset records             Symbol
>     6.25%   0.00%   0.00%   0x0       4   [k] _nohz_idle_balance.isra.0
>    18.75% 100.00%   0.00%   0x8      14   [k] nohz_balance_exit_idle
>     6.25%   0.00%   0.00%   0x8       8   [k] nohz_balance_enter_idle
>     6.25%   0.00%   0.00%   0xc       8   [k] sched_balance_newidle
>     6.25%   0.00%   0.00%  0x10      31   [k] nohz_balancer_kick
>     6.25%   0.00%   0.00%  0x20      16   [k] sched_balance_newidle
>    37.50%   0.00%   0.00%  0x38      50   [k] irqtime_account_irq
>     6.25%   0.00%   0.00%  0x38      47   [k] account_process_tick
>     6.25%   0.00%   0.00%  0x38      12   [k] account_idle_ticks
> 
> Offsets:
> *  0x0 -- nohz.idle_cpu_mask (r)
> *  0x8 -- nohz.nr_cpus (w)
> * 0x38 -- sched_clock_irqtime (r), not in nohz, but share cacheline
> 
> The layout in /proc/kallsyms can also confirm that:
> ffffffff88600d40 b nohz
> ffffffff88600d68 B arch_needs_tick_broadcast
> ffffffff88600d6c b __key.264
> ffffffff88600d6c b __key.265
> ffffffff88600d70 b dl_generation
> ffffffff88600d78 b sched_clock_irqtime
> 
> With the patch applied, irqtime_account_irq hotspot disappear.
> 
> Previous Discussion:
> https://lore.kernel.org/all/20251211055612.4071266-1-wangyang.guo@intel.com/T/#u
> 
> Suggested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
> Reported-by: Benjamin Lei <benjamin.lei@intel.com>
> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> Reviewed-by: Tianyou Li <tianyou.li@intel.com>
> Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
> ---
>   kernel/sched/cputime.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 7097de2c8cda..34d15972f5fb 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -25,7 +25,7 @@
>    */
>   DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
>   
> -int sched_clock_irqtime;
> +int sched_clock_irqtime __cacheline_aligned;
>   
>   void enable_sched_clock_irqtime(void)
>   {

Could you please apply this and see if you observe the issue still.
https://lore.kernel.org/all/20260112050442.138446-1-sshegde@linux.ibm.com/

You have CONFIG_CPUMASK_OFFSTACK=y. You still might see occasional cache
misses with the patch applied, but it would be quite less.