linux-kernel - Re: [RFC PATCH 6/6] cpuset: Add cpuset.isolation

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Date:   Thu, 15 Jul 2021 02:07:06 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Nitesh Lal <nilal@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicolas Saenz <nsaenzju@...hat.com>,
        Christoph Lameter <cl@...two.de>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>, cgroups@...r.kernel.org
Subject: Re: [RFC PATCH 6/6] cpuset: Add cpuset.isolation_mask file

On Thu, Jul 15, 2021 at 12:44:08AM +0100, Valentin Schneider wrote:
> On 15/07/21 01:13, Frederic Weisbecker wrote:
> > On Wed, Jul 14, 2021 at 06:52:43PM +0200, Peter Zijlstra wrote:
> >>
> >> cpusets already has means to create paritions; why are you creating
> >> something else?
> >
> > I was about to answer that the semantics of isolcpus, which reference
> > a NULL domain, are different from SD_LOAD_BALANCE implied by
> > cpuset.sched_load_balance. But then I realize that SD_LOAD_BALANCE has
> > been removed.
> >
> > How cpuset.sched_load_balance is implemented then? Commit
> > e669ac8ab952df2f07dee1e1efbf40647d6de332 ("sched: Remove checks against
> > SD_LOAD_BALANCE") advertize that setting cpuset.sched_load_balance to 0
> > ends up creating NULL domain but that's not what I get. For example if I
> > mount a single cpuset root (no other cpuset mountpoints):
> >
> > $ mount -t cgroup none ./cpuset -o cpuset
> > $ cd cpuset
> > $ cat cpuset.cpus
> > 0-7
> > $ cat cpuset.sched_load_balance
> > 1
> > $ echo 0 > cpuset.sched_load_balance
> > $ ls /sys/kernel/debug/domains/cpu1/
> > domain0  domain1
> >
> > I still get the domains on all CPUs...
> 
> Huh. That's on v5.14-rc1 with an automounted cpuset:
> 
> $ cat /sys/fs/cgroup/cpuset/cpuset.cpus
> 0-5
> $ cat /sys/fs/cgroup/cpuset/cpuset.sched_load_balance
> 1
> 
> $ ls /sys/kernel/debug/sched/domains/cpu*
> /sys/kernel/debug/sched/domains/cpu0:
> domain0  domain1
> 
> /sys/kernel/debug/sched/domains/cpu1:
> domain0  domain1
> 
> /sys/kernel/debug/sched/domains/cpu2:
> domain0  domain1
> 
> /sys/kernel/debug/sched/domains/cpu3:
> domain0  domain1
> 
> /sys/kernel/debug/sched/domains/cpu4:
> domain0  domain1
> 
> /sys/kernel/debug/sched/domains/cpu5:
> domain0  domain1
> 
> $ echo 0 > /sys/fs/cgroup/cpuset/cpuset.sched_load_balance
> $ ls /sys/kernel/debug/sched/domains/cpu*
> /sys/kernel/debug/sched/domains/cpu0:
> 
> /sys/kernel/debug/sched/domains/cpu1:
> 
> /sys/kernel/debug/sched/domains/cpu2:
> 
> /sys/kernel/debug/sched/domains/cpu3:
> 
> /sys/kernel/debug/sched/domains/cpu4:
> 
> /sys/kernel/debug/sched/domains/cpu5:
> 
> 
> I also checked that you can keep cpuset.sched_load_balance=0 at the root
> and create exclusive child cpusets with different values of
> sched_load_balance, giving you some CPUs attached to the NULL domain and
> some others with a sched_domain hierarchy that stays within the cpuset span.

Ok I must have done something wrong somewhere, I'll check further tomorrow.

Thanks!