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sched/fair: Move checking for nohz cpus after time check

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Current code does.
- Read nohz.nr_cpus
- Check if the time has passed to do NOHZ idle balance

Instead do this.
- Check if the time has passed to do NOHZ idle balance
- Read nohz.nr_cpus

This will skip the read most of the time in normal system usage.
i.e when there are nohz.nr_cpus (system is not 100% busy).

Note that when there are no idle CPUs(100% busy), even if the flag gets
set to NOHZ_STATS_KICK | NOHZ_NEXT_KICK, find_new_ilb will fail and
there will be no NOHZ idle balance. In such cases there will be a very
narrow window where, kick_ilb will be called un-necessarily.
However current functionality is still retained.

Note: This patch doesn't solve any cacheline overheads. No improvement
in performance apart from saving a few cycles of reading nohz.nr_cpus

Reviewed-and-tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://patch.msgid.link/20260115073524.376643-2-sshegde@linux.ibm.com
This commit is contained in:
Shrikanth Hegde
2026-01-15 13:05:22 +05:30
committed by Peter Zijlstra
parent 553255cc85
commit 6b67c8a72e
1 changed files with 16 additions and 7 deletions
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23
kernel/sched/fair.c
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@@ -12441,6 +12441,22 @@ static void nohz_balancer_kick(struct rq *rq)
*/
nohz_balance_exit_idle(rq);
if (READ_ONCE(nohz.has_blocked_load) &&
time_after(now, READ_ONCE(nohz.next_blocked)))
flags = NOHZ_STATS_KICK;
/*
* Most of the time system is not 100% busy. i.e nohz.nr_cpus > 0
* Skip the read if time is not due.
*
* If none are in tickless mode, there maybe a narrow window
* (28 jiffies, HZ=1000) where flags maybe set and kick_ilb called.
* But idle load balancing is not done as find_new_ilb fails.
* That's very rare. So read nohz.nr_cpus only if time is due.
*/
if (time_before(now, nohz.next_balance))
goto out;
/*
* None are in tickless mode and hence no need for NOHZ idle load
* balancing:
@@ -12448,13 +12464,6 @@ static void nohz_balancer_kick(struct rq *rq)
if (likely(!atomic_read(&nohz.nr_cpus)))
return;
if (READ_ONCE(nohz.has_blocked_load) &&
time_after(now, READ_ONCE(nohz.next_blocked)))
flags = NOHZ_STATS_KICK;
if (time_before(now, nohz.next_balance))
goto out;
if (rq->nr_running >= 2) {
flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
goto out;