linux-kernel - [PATCH v3 0/3] sched/fair: Improve nohz fields for large systems

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Message-ID: <20260107065125.669668-1-sshegde@linux.ibm.com>
Date: Wed,  7 Jan 2026 12:21:22 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org
Cc: sshegde@...ux.ibm.com, kprateek.nayak@....com, juri.lelli@...hat.com,
        vschneid@...hat.com, tglx@...utronix.de, dietmar.eggemann@....com,
        anna-maria@...utronix.de, frederic@...nel.org, wangyang.guo@...el.com
Subject: [PATCH v3 0/3] sched/fair: Improve nohz fields for large systems

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.

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@linux.ibm.com/
v2: https://lore.kernel.org/all/20260102124744.360872-1-sshegde@linux.ibm.com/

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 | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

-- 
2.47.3