Re: [PATCH v2] sched/fair: check for idle core

[Mel Gorman <mgorman@suse.de>]

On Fri, Oct 23, 2020 at 11:21:50AM +0200, Julia Lawall wrote:
> 
> 
> On Fri, 23 Oct 2020, Mel Gorman wrote:
> 
> > On Thu, Oct 22, 2020 at 03:15:50PM +0200, Julia Lawall wrote:
> > > In the case of a thread wakeup, wake_affine determines whether a core
> > > will be chosen for the thread on the socket where the thread ran
> > > previously or on the socket of the waker.  This is done primarily by
> > > comparing the load of the core where th thread ran previously (prev)
> > > and the load of the waker (this).
> > >
> > > commit 11f10e5420f6 ("sched/fair: Use load instead of runnable load
> > > in wakeup path") changed the load computation from the runnable load
> > > to the load average, where the latter includes the load of threads
> > > that have already blocked on the core.
> > >
> > > When a short-running daemon processes happens to run on prev, this
> > > change raised the situation that prev could appear to have a greater
> > > load than this, even when prev is actually idle.  When prev and this
> > > are on the same socket, the idle prev is detected later, in
> > > select_idle_sibling.  But if that does not hold, prev is completely
> > > ignored, causing the waking thread to move to the socket of the waker.
> > > In the case of N mostly active threads on N cores, this triggers other
> > > migrations and hurts performance.
> > >
> > > In contrast, before commit 11f10e5420f6, the load on an idle core
> > > was 0, and in the case of a non-idle waker core, the effect of
> > > wake_affine was to select prev as the target for searching for a core
> > > for the waking thread.
> > >
> > > To avoid unnecessary migrations, extend wake_affine_idle to check
> > > whether the core where the thread previously ran is currently idle,
> > > and if so simply return that core as the target.
> > > target
> > > [1] commit 11f10e5420f6ce ("sched/fair: Use load instead of runnable
> > > load in wakeup path")
> > >
> > > This particularly has an impact when using the ondemand power manager,
> > > 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/intel-pstate	v5.9+patch/intel-pstate
> > > 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/ondemand		v5.9+patch/ondemand
> > > 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.
> > >
> > > This also has a major impact on the splash2x.volrend benchmark of the
> > > parsec benchmark suite that goes from 1m25 without this patch to 0m45,
> > > in active (intel_pstate) mode.
> > >
> > > Fixes: 11f10e5420f6 ("sched/fair: Use load instead of runnable load in wakeup path")
> > > Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
> > > Reviewed-by Vincent Guittot <vincent.guittot@linaro.org>
> > >
> >
> > In principal, I think the patch is ok after the recent discussion. I'm
> > holding off an ack until a battery of tests on loads with different
> > levels of utilisation and wakeup patterns makes its way through a test
> > grid. It's based on Linus's tree mid-merge window that includes what is
> > in the scheduler pipeline
> 
> OK, if it doesn't work out, it would be interesting to know what goes
> badly.
> 

Yep, if something goes wrong, I'll make the full logs available, details
on reproducing it etc.

-- 
Mel Gorman
SUSE Labs