linux-kernel - Re: [PATCH v2 6/8] sched/idle: Move busy

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Date:   Tue, 18 May 2021 12:48:43 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Aubrey Li <aubrey.li@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Parth Shah <parth@...ux.ibm.com>
Subject: Re: [PATCH v2 6/8] sched/idle: Move busy_cpu accounting to idle
 callback

* Aubrey Li <aubrey.li@...ux.intel.com> [2021-05-18 14:05:56]:

> On 5/18/21 12:00 PM, Srikar Dronamraju wrote:
> > * Aubrey Li <aubrey.li@...ux.intel.com> [2021-05-18 08:59:00]:
> > 
> >> On 5/17/21 8:57 PM, Srikar Dronamraju wrote:
> >>> * Aubrey Li <aubrey.li@...ux.intel.com> [2021-05-17 20:48:46]:
> >>>
> >>>> On 5/17/21 6:40 PM, Srikar Dronamraju wrote:
> >>>>> * Aubrey Li <aubrey.li@...ux.intel.com> [2021-05-14 12:11:50]:
> >>>>>
> >>>>>> On 5/13/21 3:31 PM, Srikar Dronamraju wrote:
> >>>>>>> * Aubrey Li <aubrey.li@...ux.intel.com> [2021-05-12 16:08:24]:
> >>>>>>>> On 5/7/21 12:45 AM, Srikar Dronamraju wrote:
> >>>>>

<snip>
> >>
> >> This is v3. It looks like hackbench gets better. And netperf still has
> >> some notable changes under 2 x overcommit cases.
> >>
> > 
> > Thanks Aubrey for the results. netperf (2X) case does seem to regress.
> > I was actually expecting the results to get better with overcommit.
> > Can you confirm if this was just v3 or with v3 + set_next_idle_core
> > disabled?
> 
> Do you mean set_idle_cores(not set_next_idle_core) actually? Gautham's patch
> changed "this" to "target" in set_idle_cores, and I removed it to apply
> v3-2-8-sched-fair-Maintain-the-identity-of-idle-core.patch for tip/sched/core
> commit-id 915a2bc3c6b7.

Thats correct,

In the 3rd patch, I had introduced set_next_idle_core
which is suppose to set idle_cores in the LLC.
What I suspected was is this one is causing issues in your 48 CPU LLC.

I am expecting set_next_idle_core to be spending much time in your scenario.
I was planning for something like the below on top of my patch.
With this we dont look for an idle-core if we already know that we dont find one.
But in the mean while I had asked if you could have dropped the call to
set_next_idle_core.

-- 
Thanks and Regards
Srikar Dronamraju

------------>8-----------------8<--------------------------

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bee8e5225d99..2e2113262647 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6207,6 +6207,9 @@ static void set_next_idle_core(int target)
 	if (!sd)
 		return;
 
+	if (atomic_read(&sd->shared->nr_busy_cpus) * 2 >=  per_cpu(sd_llc_size, target))
+		goto out;
+
 	cpumask_andnot(cpus, sched_domain_span(sd), cpu_smt_mask(target));
 	for_each_cpu_wrap(core, cpus, target) {
 		bool idle = true;
@@ -6225,6 +6228,7 @@ static void set_next_idle_core(int target)
 
 		cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
 	}
+out:
 	set_idle_core(target, -2);
 }