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From:   Vineeth Remanan Pillai <vpillai@digitalocean.com>
To:     Phil Auld <pauld@redhat.com>
Cc:     Vineeth Remanan Pillai <vpillai@digitalocean.com>,
        Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>, mingo@kernel.org,
        tglx@linutronix.de, pjt@google.com, torvalds@linux-foundation.org,
        linux-kernel@vger.kernel.org, subhra.mazumdar@oracle.com,
        fweisbec@gmail.com, keescook@chromium.org, kerrnel@google.com,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@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>
Subject: Re: [RFC PATCH v2 00/17] Core scheduling v2
Date:   Tue, 23 Apr 2019 18:45:27 +0000
Message-Id: <20190423184527.6230-1-vpillai@digitalocean.com>
In-Reply-To: <20190423180238.GG22260@pauld.bos.csb>
Sender: linux-kernel-owner@vger.kernel.org
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>> - Processes with different tags can still share the core

> I may have missed something... Could you explain this statement?

> This, to me, is the whole point of the patch series. If it's not
> doing this then ... what?

What I meant was, the patch needs some more work to be accurate.
There are some race conditions where the core violation can still
happen. In our testing, we saw around 1 to 5% of the time being
shared with incompatible processes. One example of this happening
is as follows(let cpu 0 and 1 be siblings):
- cpu 0 selects a process with a cookie
- cpu 1 selects a higher priority process without cookie
- Selection process restarts for cpu 0 and it might select a
  process with cookie but with lesser priority.
- Since it is lesser priority, the logic in pick_next_task
  doesn't compare again for the cookie(trusts pick_task) and
  proceeds.

This is one of the scenarios that we saw from traces, but there
might be other race conditions as well. Fix seems a little
involved and We are working on that.

Thanks