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From:   Vineeth Remanan Pillai <vpillai@digitalocean.com>
Date:   Mon, 12 Aug 2019 11:38:44 -0400
Message-ID: <CANaguZDEq4=5Q9pnFyWx0-Gfkoq-WxUXBYgiG6WKLTO5njAHUA@mail.gmail.com>
Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3
To:     Aaron Lu <aaron.lu@linux.alibaba.com>
Cc:     Tim Chen <tim.c.chen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        "Li, Aubrey" <aubrey.li@linux.intel.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.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>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>, Phil Auld <pauld@redhat.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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> I have two other small changes that I think are worth sending out.
>
> The first simplify logic in pick_task() and the 2nd avoid task pick all
> over again when max is preempted. I also refined the previous hack patch to
> make schedule always happen only for root cfs rq. Please see below for
> details, thanks.
>
I see a potential issue here. With the simplification in pick_task,
you might introduce a livelock where the match logic spins for ever.
But you avoid that with the patch 2, by removing the loop if a pick
preempts max. The potential problem is that, you miss a case where
the newly picked task might have a match in the sibling on which max
was selected before. By selecting idle, you ignore the potential match.
As of now, the potential match check does not really work because,
sched_core_find will always return the same task and we do not check
the whole core_tree for a next match. This is in my TODO list to have
sched_core_find to return the best next match, if match was preempted.
But its a bit complex and needs more thought.

Third patch looks good to me.

Thanks,
Vineeth