From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vincent Guittot Date: Wed, 21 Oct 2020 12:27:34 +0000 Subject: Re: [PATCH] sched/fair: check for idle core Message-Id: List-Id: References: <1603211879-1064-1-git-send-email-Julia.Lawall@inria.fr> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Julia Lawall Cc: Ingo Molnar , kernel-janitors@vger.kernel.org, Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , linux-kernel , Valentin Schneider , Gilles Muller On Wed, 21 Oct 2020 at 09:29, Vincent Guittot wrote: > > Hi Julia, > > On Tue, 20 Oct 2020 at 19:21, 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. > > A fix tag is missing Fixes: 11f10e5420f6 ("sched/fair: Use load instead of runnable load in wakeup path") > > Signed-off-by: Julia Lawall > > Reviewed-by Vincent Guittot > > > > > --- > > kernel/sched/fair.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index aa4c6227cd6d..9b23dad883ee 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -5804,6 +5804,9 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync) > > if (sync && cpu_rq(this_cpu)->nr_running = 1) > > return this_cpu; > > > > + if (available_idle_cpu(prev_cpu)) > > + return prev_cpu; > > + > > return nr_cpumask_bits; > > } > > > >