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SubjectRe: [PATCH] sched/fair: check for idle core
On Tue, Oct 20, 2020 at 06:37:59PM +0200, Julia Lawall wrote:
> On a thread wakeup, the change [1] from runnable load average to load
> average for comparing candidate cores means that recent short-running
> daemons on the core where a thread ran previously can be considered to
> have a higher load than the core performing the wakeup, even when the
> core where the thread ran previously is currently idle. This can
> cause a thread to migrate, taking the place of some other thread that
> is about to wake up, and so on. To avoid unnecessary migrations,
> extend wake_affine_idle to check whether the core where the thread
> previously ran is currently idle, and if so return that core as the
> target.
>
> [1] commit 11f10e5420f6ce ("sched/fair: Use load instead of runnable
> load in wakeup path")
>
> This particularly has an impact when using passive (intel_cpufreq)
> power management, where kworkers run every 0.004 seconds on all cores,
> increasing the likelihood that an idle core will be considered to have
> a load.
>
> The following numbers were obtained with the benchmarking tool
> hyperfine (https://github.com/sharkdp/hyperfine) on the NAS parallel
> benchmarks (https://www.nas.nasa.gov/publications/npb.html). The
> tests were run on an 80-core Intel(R) Xeon(R) CPU E7-8870 v4 @
> 2.10GHz. Active (intel_pstate) and passive (intel_cpufreq) power
> management were used. Times are in seconds. All experiments use all
> 160 hardware threads.
>
> v5.9/active v5.9+patch/active
> bt.C.c 24.725724+-0.962340 23.349608+-1.607214
> lu.C.x 29.105952+-4.804203 25.249052+-5.561617
> sp.C.x 31.220696+-1.831335 30.227760+-2.429792
> ua.C.x 26.606118+-1.767384 25.778367+-1.263850
>
> v5.9/passive v5.9+patch/passive
> bt.C.c 25.330360+-1.028316 23.544036+-1.020189
> lu.C.x 35.872659+-4.872090 23.719295+-3.883848
> sp.C.x 32.141310+-2.289541 29.125363+-0.872300
> ua.C.x 29.024597+-1.667049 25.728888+-1.539772
>
> On the smaller data sets (A and B) and on the other NAS benchmarks
> there is no impact on performance.
>
> Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>

I suspect that the benefit of this patch is due to avoiding the overhead
of wake_affine_weight() check because the following check exists in
select_idle_sibling

/*
* If the previous CPU is cache affine and idle, don't be stupid:
*/
if (prev != target && cpus_share_cache(prev, target) &&
(available_idle_cpu(prev) || sched_idle_cpu(prev)))
return prev;

Still, the concept makes some sense to avoid wake_affine_weight but look
at the earlier part of wake_affine_idle()

if (available_idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu))
return available_idle_cpu(prev_cpu) ? prev_cpu : this_cpu;

This thing is almost completely useless because this_cpu is only going to
be idle if it's a wakeup from interrupt context when the CPU was otherwise
idle *but* it takes care to only use the CPU if this and prev share LLC.

The patch as it stands may leave a task on a remote node when it should
have been pulled local to the waker because prev happened to be idle. This
is not guaranteed because a node could have multiple LLCs and prev is
still appropriate but that's a different problem entirely and requires
much deeper surgery. Still, not pulling a task from a remote node is
a change in expected behaviour. While it's possible that NUMA domains
will not even reach this path, it depends on the NUMA distance as can
be seen in sd_init() for the setting of SD_WAKE_AFFINE so I think the
cpus_share_cache check is necessary.

I think it would be more appropriate to rework that block that checks
this_cpu to instead check if the CPUs share cache first and then return one
of them (preference to prev based on the comment above it about avoiding
a migration) if either one is idle.

I see Vincent already agreed with the patch so I could be wrong. Vincent,
did I miss something stupid?

--
Mel Gorman
SUSE Labs

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