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Date:   Thu, 26 Nov 2020 11:11:28 +1100
From:   Balbir Singh <bsingharora@gmail.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
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Subject: Re: [PATCH -tip 31/32] sched: Add a coresched command line option
Message-ID: <20201126001128.GF163610@balbir-desktop>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-32-joel@joelfernandes.org>
 <20201125134537.GA2414@hirez.programming.kicks-ass.net>
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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.