linux-kernel - Re: [PATCH v2 7/9] sched: define TIF_ALLOW

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Message-ID: <87ttrothbb.ffs@tglx>
Date:   Thu, 21 Sep 2023 04:02:00 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, luto@...nel.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, rostedt@...dmis.org,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com,
        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Wed, Sep 20 2023 at 17:57, Ankur Arora wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
>> Find below a PoC which implements that scheme. It's not even close to
>> correct, but it builds, boots and survives lightweight testing.
>
> Whew, that was electric. I had barely managed to sort through some of
> the config maze.
> From a quick look this is pretty much how you described it.

Unsurpringly I spent at least 10x the time to describe it than to hack
it up.

IOW, I had done the analysis before I offered the idea and before I
changed a single line of code. The tools I used for that are git-grep,
tags, paper, pencil, accrued knowledge and patience, i.e. nothing even
close to rocket science.

Converting the analysis into code was mostly a matter of brain dumping
the analysis and adherence to accrued methodology.

What's electric about that?

I might be missing some meaning of 'electric' which is not covered by my
mostly Webster restricted old-school understanding of the english language :)

>> I did not even try to look into time-slice enforcement, but I really want
>> to share this for illustration and for others to experiment.
>>
>> This keeps all the existing mechanisms in place and introduces a new
>> config knob in the preemption model Kconfig switch: PREEMPT_AUTO
>>
>> If selected it builds a CONFIG_PREEMPT kernel, which disables the
>> cond_resched() machinery and switches the fair scheduler class to use
>> the NEED_PREEMPT_LAZY bit by default, i.e. it should be pretty close to
>> the preempt NONE model except that cond_resched() is a NOOP and I did
>> not validate the time-slice enforcement. The latter should be a
>> no-brainer to figure out and fix if required.
>
> Yeah, let me try this out.

That's what I hoped for :)

Thanks,

        tglx