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From:   Vineeth Remanan Pillai <vpillai@digitalocean.com>
Date:   Wed, 15 Apr 2020 17:24:30 -0400
Message-ID: <CANaguZAXsjD=X-bB7dvQZ3FSqLioiZU=YREHH_7JqiFsZCTxHA@mail.gmail.com>
Subject: Re: [RFC PATCH 09/13] sched/fair: core wide vruntime comparison
To:     Aaron Lu <aaron.lwe@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        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>,
        Aaron Lu <aaron.lu@linux.alibaba.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        "Li, Aubrey" <aubrey.li@linux.intel.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>,
        Joel Fernandes <joelaf@google.com>,
        Joel Fernandes <joel@joelfernandes.org>
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> > > You forgot the time complexity analysis.
> >
> > This is a mistake and the adjust should be needed only once when core
> > scheduling is initially enabled. It is an initialization thing and there
> > is no reason to do it in every invocation of coresched_adjust_vruntime().
>
> Correction...
> I meant there is no need to call coresched_adjust_vruntime() in every
> invocation of update_core_cfs_rq_min_vruntime().

Due to the checks in place, update_core_cfs_rq_min_vruntime should
not be calling coresched_adjust_vruntime more than once between a
coresched enable/disable. Once the min_vruntime is adjusted, we depend
only on rq->core and the other sibling's min_vruntime will not grow
until coresched disable.

I did some micro benchmark tests today to verify this and observed
that coresched_adjust_vruntime called at most once between a coresched
enable/disable.

Thanks,
Vineeth