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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Steven Rostedt' <rostedt@goodmis.org>
CC:     'Vincent Guittot' <vincent.guittot@linaro.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: RE: sched/fair: scheduler not running high priority process on idle
 cpu
Thread-Topic: sched/fair: scheduler not running high priority process on idle
 cpu
Thread-Index: AdXK8cUFXa7JpPXmQNq7oQ32S9fYHAACik4AAADJLkAAAO3PAAAmXEggAAKDBAAAAvfesA==
Date:   Wed, 15 Jan 2020 15:11:32 +0000
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From: Steven Rostedt
> Sent: 15 January 2020 13:19
> On Wed, 15 Jan 2020 12:44:19 +0000
> David Laight <David.Laight@ACULAB.COM> wrote:
> 
> > > Yes, even with CONFIG_PREEMPT, Linux has no guarantees of latency for
> > > any task regardless of priority. If you have latency requirements, then
> > > you need to apply the PREEMPT_RT patch (which may soon make it to
> > > mainline this year!), which spin locks and bh wont stop a task from
> > > scheduling (unless they need the same lock)
> 
> Every time you add something to allow higher priority processes to run
> with less latency you add overhead. By just adding that spinlock check
> or to migrate a process to a idle cpu will add a measurable overhead,
> and as you state, distros won't like that.
> 
> It's a constant game of give and take.

I know exactly how much effect innocuous changes can have...

Sorting out process migration on a 1024 cpu NUMA system must be a PITA.

For this case an idle cpu doing a unlocked check for a processes that has
been waiting 'ages' to preempt the running process may not be too
expensive.
I presume the locks are in place for the migrate itself.
The only downside is that the process's data is likely to be in the wrong cache,
but unless the original cpu becomes available just after the migrate it is
probably still a win.

	David

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