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Date:   Wed, 25 Nov 2020 14:45:37 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "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: <20201125134537.GA2414@hirez.programming.kicks-ass.net>
References: <20201117232003.3580179-1-joel@joelfernandes.org>
 <20201117232003.3580179-32-joel@joelfernandes.org>
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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.