linux-kernel - Re: [RFC PATCH 00/13] Core scheduling v5

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Date:   Tue, 14 Apr 2020 16:21:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     vpillai <vpillai@...italocean.com>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        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, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>, aubrey.li@...ux.intel.com,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Joel Fernandes <joelaf@...gle.com>, joel@...lfernandes.org
Subject: Re: [RFC PATCH 00/13] Core scheduling v5

On Wed, Mar 04, 2020 at 04:59:50PM +0000, vpillai wrote:
> TODO
> ----
> - Work on merging patches that are ready to be merged
> - Decide on the API for exposing the feature to userland
> - Experiment with adding synchronization points in VMEXIT to mitigate
>   the VM-to-host-kernel leaking

VMEXIT is too late, you need to hook irq_enter(), which is what makes
the whole thing so horrible.

> - Investigate the source of the overhead even when no tasks are tagged:
>   https://lkml.org/lkml/2019/10/29/242

 - explain why we're all still doing this ....

Seriously, what actual problems does it solve? The patch-set still isn't
L1TF complete and afaict it does exactly nothing for MDS.

Like I've written many times now, back when the world was simpler and
all we had to worry about was L1TF, core-scheduling made some sense, but
how does it make sense today?

It's cute that this series sucks less than it did before, but what are
we trading that performance for?