Re: [PATCH v5 0/3] sched/fair: Improve nohz fields for large systems

[Vincent Guittot]

On Thu, 15 Jan 2026 at 08:36, Shrikanth Hegde <sshegde@xxxxxxxxxxxxx> wrote:
>
> Running on large systems nohz.nr_cpus cacheline was seen as contended.
> There is atomic inc/dec and read happening on many
> CPUs at a time and it is possible for this line to bounce often.
>
> 1st and 2nd patch are minor ones. Looks like correct things to do.
> Not very important ones.
>
> 3rd patch: Main patch which is to get rid of nr_cpus.Instead, use the cpumask
> which is always updated alongside with it. Functionally it should serve
> the same purpose. Rest of the fields aren't updated that often. So this
> line shouldn't bounce that often.
>
> Contention issue with nohz.idle_cpus_mask still remains. Mostly it is in
> separate cacheline than nohz. There are ongoing efforts to mitigate it. It
> is not addressed by this series.
>
> v4 -> v5:
> - Collected tags (Thanks to K Prateek Nayak, Valentin Schneider)
> - Added comment for patch 1, making note of a narrow window where
>   kick_ilb will be called un-necessarily. (Vincent Guittot)

Looks good to me
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

>
> v3 -> v4:
> - Added to changelog on one less cacheline being dirtied on idle
>   entry/exit (Valentin Schneider)
>
> v2 -> v3:
> - Converted out to return when there are no CPU is in tickless mode
>   since find_ilb_cpu returns anyway (K Prateek Nayak)
>
> v1 -> v2:
> - Dropped patch to check has_blocked based on time.
> - Detailed changelog for removing nr_cpus (Thanks to Ingo Molnar)
>
> v1: https://lore.kernel.org/all/20251201183146.74443-1-sshegde@xxxxxxxxxxxxx/
> v2: https://lore.kernel.org/all/20260102124744.360872-1-sshegde@xxxxxxxxxxxxx/
> v3: https://lore.kernel.org/all/20260107065125.669668-1-sshegde@xxxxxxxxxxxxx/
> v4: https://lore.kernel.org/all/20260112050442.138446-1-sshegde@xxxxxxxxxxxxx/
>
> Shrikanth Hegde (3):
>   sched/fair: Move checking for nohz cpus after time check
>   sched/fair: Change likelyhood of nohz.nr_cpus
>   sched/fair: Remove nohz.nr_cpus and use weight of cpumask instead
>
>  kernel/sched/fair.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
>
> --
> 2.51.0
>