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Thu, 10 Nov 2022 15:18:18 -0500 X-MC-Unique: vSnFnYSJPpiHS_qT8FiEKg-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 646C8101A52A; Thu, 10 Nov 2022 20:18:17 +0000 (UTC) Received: from fuller.cnet (ovpn-112-3.gru2.redhat.com [10.97.112.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EFBCB492B1B; Thu, 10 Nov 2022 20:18:16 +0000 (UTC) Received: by fuller.cnet (Postfix, from userid 1000) id 0432F416D897; Thu, 10 Nov 2022 16:15:56 -0300 (-03) Date: Thu, 10 Nov 2022 16:15:56 -0300 From: Marcelo Tosatti To: Frederic Weisbecker Cc: Aaron Tomlin , cl@linux.com, tglx@linutronix.de, mingo@kernel.org, peterz@infradead.org, pauld@redhat.com, neelx@redhat.com, oleksandr@natalenko.name, atomlin@atomlin.com, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v8 3/5] mm/vmstat: Do not queue vmstat_update if tick is stopped Message-ID: References: <20220924152227.819815-1-atomlin@redhat.com> <20220924152227.819815-4-atomlin@redhat.com> <20221024110311.GA1285913@lothringen> MIME-Version: 1.0 Content-Type: text/plain; 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s=arc-20220608; d=hostedemail.com; t=1668111502; a=rsa-sha256; cv=none; b=bERVoV7gnNmt7gFJq02hGkaIefVb1u/PNI/m2w01ACwnRVLoqXOO0jNIEBNB0GFZ3J8PMi StJ5Aj9usdQjAu/rEwO+Y0CPa0fs9wXyUQs8HTEEMAICFWXuHcE7boayo8V67gPI682wGy jVVemtwXdkfV/HXdcYM1f88URdS6j8s= X-Rspam-User: X-Stat-Signature: nmhh8infdzwo67r4pwdx3b453c3eftsm X-Rspamd-Queue-Id: 1BC28C0005 Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=c3hUwxzT; spf=pass (imf10.hostedemail.com: domain of mtosatti@redhat.com designates 170.10.129.124 as permitted sender) smtp.mailfrom=mtosatti@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam02 X-HE-Tag: 1668111501-326327 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Nov 09, 2022 at 04:40:26PM -0300, Marcelo Tosatti wrote: > On Mon, Oct 24, 2022 at 01:03:11PM +0200, Frederic Weisbecker wrote: > > Lines: 94 > > > > On Sat, Sep 24, 2022 at 04:22:25PM +0100, Aaron Tomlin wrote: > > > From: Marcelo Tosatti > > > > > > From the vmstat shepherd, for CPUs that have the tick stopped, do not > > > queue local work to flush the per-CPU vmstats, since in that case the > > > flush is performed on return to userspace or when entering idle. Also > > > cancel any delayed work on the local CPU, when entering idle on nohz > > > full CPUs. Per-CPU pages can be freed remotely from housekeeping CPUs. > > > > > > Signed-off-by: Marcelo Tosatti > > > --- > > > mm/vmstat.c | 18 +++++++++++++----- > > > 1 file changed, 13 insertions(+), 5 deletions(-) > > > > > > diff --git a/mm/vmstat.c b/mm/vmstat.c > > > index 472175642bd9..3b9a497965b4 100644 > > > --- a/mm/vmstat.c > > > +++ b/mm/vmstat.c > > > @@ -29,6 +29,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > > > > #include "internal.h" > > > > > > @@ -1990,19 +1991,23 @@ static void vmstat_update(struct work_struct *w) > > > */ > > > void quiet_vmstat(void) > > > { > > > + struct delayed_work *dw; > > > + > > > if (system_state != SYSTEM_RUNNING) > > > return; > > > > > > if (!is_vmstat_dirty()) > > > return; > > > > > > + refresh_cpu_vm_stats(false); > > > + > > > /* > > > - * Just refresh counters and do not care about the pending delayed > > > - * vmstat_update. It doesn't fire that often to matter and canceling > > > - * it would be too expensive from this path. > > > - * vmstat_shepherd will take care about that for us. > > > + * If the tick is stopped, cancel any delayed work to avoid > > > + * interruptions to this CPU in the future. > > > */ > > > - refresh_cpu_vm_stats(false); > > > + dw = &per_cpu(vmstat_work, smp_processor_id()); > > > + if (delayed_work_pending(dw) && tick_nohz_tick_stopped()) > > > + cancel_delayed_work(dw); > > > } > > > > > > /* > > > @@ -2024,6 +2029,9 @@ static void vmstat_shepherd(struct work_struct *w) > > > for_each_online_cpu(cpu) { > > > struct delayed_work *dw = &per_cpu(vmstat_work, cpu); > > > > > > + if (tick_nohz_tick_stopped_cpu(cpu)) > > > + continue; > > > + > > > if (!delayed_work_pending(dw) && per_cpu(vmstat_dirty, cpu)) > > > queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0); > > > > All these checks are racy though. You may well eventually: > > > > 1) Arm the timer after the CPU has entered in userspace > > 2) Not arm the timer when the CPU has entered the kernel > > > > How about converting that to an IPI instead? This should be a good candidate > > for the future IPI deferment. > > > > Another possible way to go is this: > > > > 1) vmstat_shepherd completely ignores nohz_full CPUs > > 2) vmstat_work is only ever armed locally > > 3) A nohz_full CPU turning its local vmstat as dirty checks if vmstat_work is > > pending. If not, queue it, possibly through a self IPI (IRQ_WORK) to get > > away with current locking context. > > I'm afraid there might be workloads where local vmstat touch is a > hot-path. > > > 3) Fold on idle if dirty > > 4) Fold on user enter and disarm vmstat_work if pending > > > > Does that sound possible? > > > > Thanks. > > I guess so, but proper barriers would also work. > > Do you have any particular reason for the 1-4 sequence above > instead of barriers? I think a per-CPU atomic variable might be necessary, not just barriers. Thanks.