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From:   Vineeth Remanan Pillai <vpillai@digitalocean.com>
Date:   Thu, 10 Oct 2019 10:29:47 -0400
Message-ID: <CANaguZDCtmXpm_rpTkjsfPPBscHCwz4u1OHwUt3XztzgLJa_jA@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>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Dario Faggioli <dfaggioli@suse.com>,
        "Li, Aubrey" <aubrey.li@linux.intel.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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 didn't see why we need do this.
>
> We only need to have the root level sched entities' vruntime become core
> wide since we will compare vruntime for them across hyperthreads. For
> sched entities on sub cfs_rqs, we never(at least, not now) compare their
> vruntime outside their cfs_rqs.
>
The reason we need to do this is because, new tasks that gets created will
have a vruntime based on the new min_vruntime and old tasks will have it
based on the old min_vruntime and it can cause starvation based on how
you set the min_vruntime. With this new patch, we normalize the whole
tree so that new tasks and old tasks compare with the same min_vruntime.

Thanks,
Vineeth