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

[Vincent Guittot]

On Mon, 25 Jan 2021 at 10:20, Julia Lawall <julia.lawall@xxxxxxxx> wrote:
>
>
>
> On Mon, 25 Jan 2021, Mel Gorman wrote:
>
> > On Sun, Jan 24, 2021 at 09:38:14PM +0100, Julia Lawall wrote:
> > >
> > >
> > > On Tue, 27 Oct 2020, Mel Gorman wrote:
> > >
> > > > On Thu, Oct 22, 2020 at 03:15:50PM +0200, Julia Lawall wrote:
> > > > > Fixes: 11f10e5420f6 ("sched/fair: Use load instead of runnable load in wakeup path")
> > > > > Signed-off-by: Julia Lawall <Julia.Lawall@xxxxxxxx>
> > > > > Reviewed-by Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > > > >
> > > >
> > > > While not a universal win, it was mostly a win or neutral. In few cases
> > > > where there was a problem, one benchmark I'm a bit suspicious of generally
> > > > as occasionally it generates bad results for unknown and unpredictable
> > > > reasons. In another, it was very machine specific and the differences
> > > > were small in absolte time rather than relative time. Other tests on the
> > > > same machine were fine so overall;
> > > >
> > > > Acked-by: Mel Gorman <mgorman@xxxxxxx>
> > >
> > > Recently, we have been testing the phoronix multicore benchmarks.  On v5.9
> > > with this patch, the preparation time of phoronix slows down, from ~23
> > > seconds to ~28 seconds.  In v5.11-rc4, we see 29 seconds.  It's not yet
> > > clear what causes the problem.  But perhaps the patch should be removed
> > > from v5.11, until the problem is understood.
> > >
> > > commit d8fcb81f1acf651a0e50eacecca43d0524984f87
> > >
> >
> > I'm not 100% convinved given that it was a mix of wins and losses. In
> > the wakup path in general, universal wins almost never happen. It's not
> > 100% clear from your mail what happens during the preparation patch. If
> > it included time to download the benchmarks and install then it would be
> > inherently variable due to network time (if download) or cache hotness
> > (if installing/compiling). While preparation time can be interesting --
> > for example, if preparation involves reading a lot of files from disk,
> > it's not universally interesting when it's not the critical phase of a
> > benchmark.
>
> The benchmark is completely downloaded prior to the runs.  There seems to
> be some perturbation to the activation of containerd.  Normally it is
> even:  *   *   *   *

Does it impact the benchmark results too or only the preparation prior
to running the benchmark ?

>
> and with the patch it becomes more like: *     **     **
>
> That is every other one is on time, and every other one is late.
>
> But I don't know why this happens.
>
> julia
>
> >
> > I think it would be better to wait until the problem is fully understood
> > to see if it's a timing artifact (e.g. a race between when prev_cpu is
> > observed to be idle and when it is busy).

I agree that a better understanding of what is happening is necessary
before any changes

> >
> > --
> > Mel Gorman
> > SUSE Labs
> >