linux-kernel - Re: [RFC PATCH 3/3] sched: Implement shared wakequeue in CFS

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Message-ID: <20230621024300.GA15990@maniforge>
Date:   Tue, 20 Jun 2023 21:43:00 -0500
From:   David Vernet <void@...ifault.com>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        rostedt@...dmis.org, dietmar.eggemann@....com, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        joshdon@...gle.com, roman.gushchin@...ux.dev, tj@...nel.org,
        kernel-team@...a.com
Subject: Re: [RFC PATCH 3/3] sched: Implement shared wakequeue in CFS

On Wed, Jun 21, 2023 at 10:35:34AM +0800, Aaron Lu wrote:
> On Tue, Jun 20, 2023 at 12:36:26PM -0500, David Vernet wrote:
> > On Fri, Jun 16, 2023 at 08:53:38AM +0800, Aaron Lu wrote:
> > > I also tried that on the 18cores/36threads/LLC Skylake and the contention
> > > is indeed much smaller than UDP_RR:
> > > 
> > >      7.30%     7.29%  [kernel.vmlinux]      [k]      native_queued_spin_lock_slowpath
> > > 
> > > But I wouldn't say it's entirely gone. Also consider Skylake has a lot
> > > fewer cores per LLC than later Intel servers like Icelake and Sapphire
> > > Rapids and I expect things would be worse on those two machines.
> > 
> > I cannot reproduce this contention locally, even on a slightly larger
> 
> With netperf client number equal to nr_cpu?

No, that confusion was only the first time around. See below though, I'm
not sure what insights are to be gained by continuing to tinker with
netperf runs.

> > Skylake. Not really sure what to make of the difference here. Perhaps
> > it's because you're running with CONFIG_SCHED_CORE=y? What is the
> 
> Yes I had that config on but I didn't tag any tasks or groups.
> 
> > change in throughput when you run the default workload on your SKL?
> 
> The throughput dropped a little with SWQUEUE:
> 
>                  avg_throughput    native_queued_spin_lock_slowpath%
> NO_SWQUEUE:      9528.061111111108      0.09%
> SWQUEUE:         8984.369722222222      8.05%
> 
> avg_throughput: average throughput of all netperf client's throughput,
> higher is better.
> 
> I run this workload like this:
> "
> netserver
> 
> for i in `seq 72`; do
>         netperf -l 60 -n 72 -6 &
> done
> 
> sleep 30
> perf record -ag -e cycles:pp -- sleep 5 &
> 
> wait
> "
> (the '-n 72' should be redundant but I just keep it there)

At this point I'd say we've spent quite a bit of time discussing netperf
results. We understand where the contention is coming from, and yes,
we've established that there are going to be some configurations where
swqueue is not well suited. We've also established that there are
configurations where it will and does perform well, including on
netperf.

I'm not sure what we're hoping to gain by continuing to run various
netperf workloads with your specific parameters?

Thanks,
David