linux-kernel - Re: [RFC PATCH v2 00/17] Core scheduling v2

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Message-Id: <20190423184527.6230-1-vpillai@digitalocean.com>
Date:   Tue, 23 Apr 2019 18:45:27 +0000
From:   Vineeth Remanan Pillai <vpillai@...italocean.com>
To:     Phil Auld <pauld@...hat.com>
Cc:     Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, subhra.mazumdar@...cle.com,
        fweisbec@...il.com, keescook@...omium.org, kerrnel@...gle.com,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v2 00/17] Core scheduling v2

>> - 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