linux-kernel - Re: [PATCH -tip 31/32] sched: Add a coresched command line option

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Message-ID: <20201126001128.GF163610@balbir-desktop>
Date:   Thu, 26 Nov 2020 11:11:28 +1100
From:   Balbir Singh <bsingharora@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        torvalds@...ux-foundation.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
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        benbjiang@...cent.com,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
        chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
        Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 31/32] sched: Add a coresched command line option

On Wed, Nov 25, 2020 at 02:45:37PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 17, 2020 at 06:20:01PM -0500, Joel Fernandes (Google) wrote:
> > Some hardware such as certain AMD variants don't have cross-HT MDS/L1TF
> > issues. Detect this and don't enable core scheduling as it can
> > needlessly slow those device down.
> > 
> > However, some users may want core scheduling even if the hardware is
> > secure. To support them, add a coresched= option which defaults to
> > 'secure' and can be overridden to 'on' if the user wants to enable
> > coresched even if the HW is not vulnerable. 'off' would disable
> > core scheduling in any case.
> 
> This is all sorts of wrong, and the reason is because you hard-coded
> that stupid policy.
> 
> Core scheduling should always be available on SMT (provided you did that
> CONFIG_ thing). Even on AMD systems RT tasks might want to claim the
> core exclusively.

Agreed, specifically if we need to have special cgroup tag/association to
enable it.

Balbir Singh.