From: Shrikanth Hegde <sshegde@linux.ibm.com>
To: K Prateek Nayak <kprateek.nayak@amd.com>,
Peter Zijlstra <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
Valentin Schneider <vschneid@redhat.com>,
"Gautham R. Shenoy" <gautham.shenoy@amd.com>,
Swapnil Sapkal <swapnil.sapkal@amd.com>,
Ingo Molnar <mingo@redhat.com>,
Juri Lelli <juri.lelli@redhat.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Anna-Maria Behnsen <anna-maria@linutronix.de>,
Frederic Weisbecker <frederic@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 18/19] sched/fair: Optimize global "nohz.nr_cpus" tracking
Date: Thu, 25 Sep 2025 01:32:36 +0530 [thread overview]
Message-ID: <f9546172-95e7-498d-9fca-18ff142f0bbb@linux.ibm.com> (raw)
In-Reply-To: <20250904041516.3046-19-kprateek.nayak@amd.com>
On 9/4/25 9:45 AM, K Prateek Nayak wrote:
> Optimize "nohz.nr_cpus" by tracking number of "sd_nohz->shared" with
> non-zero "nr_idle_cpus" count via "nohz.nr_doms" and only updating at
> the boundary of "sd_nohz->shared->nr_idle_cpus" going from 0 -> 1 and
> back from 1 -> 0.
>
> This also introduces a chance of double accounting when a nohz idle
> entry or the tick races with hotplug or cpuset as described in
> __nohz_exit_idle_tracking().
>
> __nohz_exit_idle_tracking() called when the sched_domain_shared nodes
> tracking idle CPUs are freed is used to correct any potential double
> accounting which can unnecessarily trigger nohz idle balances even when
> all the CPUs have tick enabled.
>
Is it possible to get rid of this nr_cpus or nr_doms altogether?
The reason being, with current code, one updates the nohz.idle_cpus_mask and
then inc/dec nr_cpus.
The only use it to decide to do periodic idle balancing or not.
If instead, could use cpumask_empty(nohz.idle_cpus_mask) check no?
It may not be every tick accurate, but that may be ok.
I haven't gone through your series in detail yet, but similar thing is doable,
check if the list is empty or not.
next prev parent reply other threads:[~2025-09-24 20:03 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-09-04 4:14 [RFC PATCH 00/19] sched/fair: Distributed nohz idle CPU tracking for idle load balancing K Prateek Nayak
2025-09-04 4:14 ` [RFC PATCH 01/19] sched/fair: Simplify set_cpu_sd_state_*() with guards K Prateek Nayak
2025-09-24 20:26 ` Shrikanth Hegde
2025-09-25 2:11 ` K Prateek Nayak
2025-09-04 4:14 ` [RFC PATCH 02/19] sched/topology: Optimize sd->shared allocation and assignment K Prateek Nayak
2025-09-04 4:14 ` [RFC PATCH 03/19] sched/fair: Use rq->nohz_tick_stopped in update_nohz_stats() K Prateek Nayak
2025-09-24 20:17 ` Shrikanth Hegde
2025-09-25 1:48 ` K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 04/19] sched/fair: Use xchg() to set sd->nohz_idle state K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 05/19] sched/topology: Attach new hierarchy in rq_attach_root() K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 06/19] sched/fair: Fixup sd->nohz_idle state during hotplug / cpuset K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 07/19] sched/fair: Account idle cpus instead of busy cpus in sd->shared K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 08/19] sched/topology: Introduce fallback sd->shared assignment K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 09/19] sched/topology: Introduce percpu sd_nohz for nohz state tracking K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 10/19] sched/topology: Introduce "idle_cpus_mask" in sd->shared K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 11/19] sched/topology: Introduce "nohz_shared_list" to keep track of sd->shared K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 12/19] sched/fair: Reorder the barrier in nohz_balance_enter_idle() K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 13/19] sched/fair: Extract the main _nohz_idle_balance() loop into a helper K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 14/19] sched/fair: Convert find_new_ilb() to use nohz_shared_list K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 15/19] sched/fair: Introduce sched_asym_prefer_idle() for ILB kick K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 16/19] sched/fair: Convert sched_balance_nohz_idle() to use nohz_shared_list K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 17/19] sched/fair: Remove "nohz.idle_cpus_mask" K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 18/19] sched/fair: Optimize global "nohz.nr_cpus" tracking K Prateek Nayak
2025-09-24 20:02 ` Shrikanth Hegde [this message]
2025-09-25 2:37 ` K Prateek Nayak
2025-09-04 4:15 ` [RFC PATCH 19/19] sched/topology: Add basic debug information for "nohz_shared_list" K Prateek Nayak
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