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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=f9546172-95e7-498d-9fca-18ff142f0bbb@linux.ibm.com \
--to=sshegde@linux.ibm.com \
--cc=anna-maria@linutronix.de \
--cc=bsegall@google.com \
--cc=dietmar.eggemann@arm.com \
--cc=frederic@kernel.org \
--cc=gautham.shenoy@amd.com \
--cc=juri.lelli@redhat.com \
--cc=kprateek.nayak@amd.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mgorman@suse.de \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
--cc=swapnil.sapkal@amd.com \
--cc=tglx@linutronix.de \
--cc=vincent.guittot@linaro.org \
--cc=vschneid@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.