Re: [PATCH v2] sched/clock: Avoid false sharing for sched_clock_irqtime

[Shrikanth Hegde]

On 1/13/26 1:18 PM, Wangyang Guo wrote:
> Read-mostly sched_clock_irqtime may share the same cacheline with
> frequently updated nohz struct. Mark it as __read_mostly 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.
>
> Changes since V1:
> - Use __read_mostly instead of __cacheline_aligned to avoid wasting
>    spaces.
>
> History:
>    v1: https://lore.kernel.org/all/20260113022958.3379650-1-wangyang.guo@xxxxxxxxx/
>    prev discussions: https://lore.kernel.org/all/20251211055612.4071266-1-wangyang.guo@xxxxxxxxx/T/#u
>
> Suggested-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>

I think it would appropriate to take prateek's tag here.

> Reported-by: Benjamin Lei <benjamin.lei@xxxxxxxxx>
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Reviewed-by: Tianyou Li <tianyou.li@xxxxxxxxx>
> Signed-off-by: Wangyang Guo <wangyang.guo@xxxxxxxxx>
> ---
>   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..fc8f8081c48f 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 __read_mostly;
>   
>   void enable_sched_clock_irqtime(void)
>   {

Patch should be good on its own.

Reviewed-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>