Re: [PATCH] sched/fair: reduce false sharing on sched_balance_running

[Eric Dumazet]

On Thu, Apr 24, 2025 at 7:29 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> On Thu, Apr 24, 2025 at 11:50 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Thu, Apr 24, 2025 at 7:46 AM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
> > >
> > > On Thu, Apr 24, 2025 at 1:46 AM Eric Dumazet <edumazet@xxxxxxxxxx> 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@xxxxxxxxx/
> >
> > 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