linux-kernel - Re: [PATCHSET RFC] sched: Implement BPF extensible scheduler class

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Date:   Mon, 12 Dec 2022 10:37:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     torvalds@...ux-foundation.org, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...nel.org, joshdon@...gle.com, brho@...gle.com,
        pjt@...gle.com, derkling@...gle.com, haoluo@...gle.com,
        dvernet@...a.com, dschatzberg@...a.com, dskarlat@...cmu.edu,
        riel@...riel.com, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCHSET RFC] sched: Implement BPF extensible scheduler class

On Tue, Nov 29, 2022 at 10:22:42PM -1000, Tejun Heo wrote:

> Core scheduling is an example of a feature that took a significant amount of
> time and effort to integrate into the kernel.

Mostly because I dropped it on the floor once I heard about MDS. That
made me lose interest entirely. The only reason it eventually happened
was ChromeOS (Joel) pushing for it again.

> Part of the difficulty with core
> scheduling was the inherent mismatch in abstraction between the desire to
> perform core-wide scheduling, and the per-cpu design of the kernel scheduler.

Not really; the main difficultly was due to me wanting to do it outside
of the scheduling classes so that it fundamentally covers all of them.

Doing it inside a class (say CFS) would've made it significantly simpler.

> This caused issues, for example ensuring proper fairness between the
> independent runqueues of SMT siblings.

Inter-runqueue fairness is a known issue of CFS and quite independent of
core scheduling.

Anyway, I hate all of this. Linus NAK'ed loadable schedulers a number of
times in the past and this is just that again -- with the extra downside
of the whole BPF thing on top  :/

You look to be exposing a ton of stuff I've so far even refused
tracepoints for :-(

Anyway, I'm just back from a heavy dose of Covid and still taking it
easy, but I'll go read through the whole thing, hopefully I'll finish
before vanishing again for the x-mas break.