linux-kernel - Re: [PATCH RFC 1/2] sched: Minimize the idle cpu selection race window.

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Message-ID: <20171123160006.tik2loyzzlavf5ub@destiny>
Date:   Thu, 23 Nov 2017 11:00:07 -0500
From:   Josef Bacik <josef@...icpanda.com>
To:     Mike Galbraith <efault@....de>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Atish Patra <atish.patra@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joelaf@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Brendan Jackman <brendan.jackman@....com>,
        Josef Bacik <jbacik@...com>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC 1/2] sched: Minimize the idle cpu selection race
 window.

On Thu, Nov 23, 2017 at 02:13:01PM +0100, Mike Galbraith wrote:
> On Thu, 2017-11-23 at 11:52 +0100, Uladzislau Rezki wrote:
> > Hello, Atish, Peter, all.
> > 
> > I have a question about if a task's nr_cpus_allowed is 1.
> > In that scenario we do not call select_task_rq. Therefore
> > even thought a task "p" is placed on idle CPU that CPU
> > will not be marked as claimed for wake-up.
> > 
> > What do you think about adding per_cpu(claim_wakeup, cpu) = 1;
> > to select_task_rq() instead and possibly get rid of them from
> > other places (increases a race window a bit)?
> 
> My thoughts on all of this is that we need less SIS, not more.  Rather
> than trying so hard for the absolute lowest wakeup latency, which
> induces throughput/efficiency robbing bouncing, I think we'd be better
> of considering leaving an already llc affine task where it is if the
> average cycle time is sufficiently low that it will likely hit the CPU
> RSN.  Completely ignoring low utilization kernel threads would go a
> long way to getting rid of bouncing userspace (which tends to have a
> meaningful footprint), all over hell and creation.
> 
> You could also periodically send mobile kthreads down the slow path to
> try to keep them the hell away from partially busy CPUs, as well as
> anything else that hasn't run for a while, to keep background cruft
> from continually injecting itself into the middle of a cross core
> cyber-sex.
>

And on this thanksgiving I'm thankful for Mike, and his entertaining early
morning emails.

Josef