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From:   Aubrey Li <aubrey.intel@gmail.com>
Date:   Tue, 21 May 2019 16:19:00 +0800
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Subject: Re: [RFC PATCH v2 13/17] sched: Add core wide task selection and scheduling.
To:     Vineeth Pillai <vpillai@digitalocean.com>
Cc:     Phil Auld <pauld@redhat.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Aaron Lu <aaron.lwe@gmail.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>
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On Mon, May 20, 2019 at 10:04 PM Vineeth Pillai
<vpillai@digitalocean.com> wrote:
>
> > > The following patch improved my test cases.
> > > Welcome any comments.
> > >
> >
> > This is certainly better than violating the point of the core scheduler :)
> >
> > If I'm understanding this right what will happen in this case is instead
> > of using the idle process selected by the sibling we do the core scheduling
> > again. This may start with a newidle_balance which might bring over something
> > to run that matches what we want to put on the sibling. If that works then I
> > can see this helping.
> >
> > But I'd be a little concerned that we could end up thrashing. Once we do core
> > scheduling again here we'd force the sibling to resched and if we got a different
> > result which "helped" him pick idle we'd go around again.

Thrashing means more IPIs right? That's not what I observed, because idle task
has less chance onto CPU, rescheduling is reduced accordingly.

> > I think inherent in the concept of core scheduling (barring a perfectly aligned set
> > of jobs) is some extra idle time on siblings.

Yeah, I understand and agree with this, but 10-15% idle time on an overloaded
system makes me to try to figure out how this could happen and if we
can improve it.

> >
> >
> I was also thinking along the same lines. This change basically always
> tries to avoid idle and there by constantly interrupting the sibling.
> While this change might benefit a very small subset of workloads, it
> might introduce thrashing more often.

Thrashing is not observed under an overloaded case but may happen under a
light load or a mid load case, I need more investigation.

>
> One other reason you might be seeing performance improvement is
> because of the bugs that caused both siblings to go idle even though
> there are runnable and compatible threads in the queue. Most of the
> issues are fixed based on all the feedback received in v2. We have a
> github repo with the pre v3 changes here:
> https://github.com/digitalocean/linux-coresched/tree/coresched

Okay, thanks, it looks like the core functions pick_next_task() and pick_task()
have a lot of changes against v2. Need more brain power..

>
> Please try this and see how it compares with the vanilla v2. I think its
> time for a v3 now and we shall be posting it soon after some more
> testing and benchmarking.

Is there any potential change between pre v3 and v3? I prefer working
based on v3 so that everyone are on the same page.

Thanks,
-Aubrey