linux-kernel - Re: [PATCH] sched/fair: reduce false sharing on sched_balance

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Message-ID: <CANn89iL1nEjveUUBz5Kyr4rPaUhy+WO4FViAa_5fv0LvvKFjmg@mail.gmail.com>
Date: Fri, 25 Apr 2025 09:22:00 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Yafang Shao <laoar.shao@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, linux-kernel <linux-kernel@...r.kernel.org>, 
	Eric Dumazet <eric.dumazet@...il.com>, Sean Christopherson <seanjc@...gle.com>, 
	Josh Don <joshdon@...gle.com>
Subject: Re: [PATCH] sched/fair: reduce false sharing on sched_balance_running

On Thu, Apr 24, 2025 at 7:29 PM Yafang Shao <laoar.shao@...il.com> wrote:
>
> On Thu, Apr 24, 2025 at 11:50 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Thu, Apr 24, 2025 at 7:46 AM Yafang Shao <laoar.shao@...il.com> wrote:
> > >
> > > On Thu, Apr 24, 2025 at 1:46 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > rebalance_domains() can attempt to change sched_balance_running
> > > > more than 350,000 times per second on our servers.
> > > >
> > > > If sched_clock_irqtime and sched_balance_running share the
> > > > same cache line, we see a very high cost on hosts with 480 threads
> > > > dealing with many interrupts.
> > >
> > > CONFIG_IRQ_TIME_ACCOUNTING is enabled on your systems, right?
> > > Have you observed any impact on task CPU utilization measurements due
> > > to this configuration? [0]
> > >
> > > If cache misses on sched_clock_irqtime are indeed the bottleneck,  why
> > > not align it to improve performance?
> >
> > "Align it" meaning what exactly ?
>
> Such as :
>
> static __cacheline_aligned_in_smp int sched_clock_irqtime;

Please look at __read_mostly attribute instead.

Thanks.


>
> > Once sched_clock_irqtime is in a
> > read-mostly location everything is fine.
> >
> > The main bottleneck is the false sharing on these Intel 6980P cpus...
> >
> > On a dual socket system, this false sharing is using something like 4%
> > of the total memory bandwidth,
> > and causes apparent high costs on other parts of the kernel.
> >
> > >
> > > [0]. https://lore.kernel.org/all/20250103022409.2544-1-laoar.shao@gmail.com/
> >
> > What part should I look at, and how is this related to my patch ?
>
> Unrelated to your patch. Please ignore it if you haven't seen this issue.
>
> --
> Regards
> Yafang