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From:   Ankur Arora <ankur.a.arora@oracle.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ankur Arora <ankur.a.arora@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        akpm@linux-foundation.org, luto@kernel.org, bp@alien8.de,
        dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        willy@infradead.org, mgorman@suse.de, rostedt@goodmis.org,
        jon.grimm@amd.com, bharata@amd.com, raghavendra.kt@amd.com,
        boris.ostrovsky@oracle.com, konrad.wilk@oracle.com,
        jgross@suse.com, andrew.cooper3@citrix.com,
        Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED
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Date:   Wed, 20 Sep 2023 17:57:14 -0700
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Thomas Gleixner <tglx@linutronix.de> writes:

> On Tue, Sep 19 2023 at 14:30, Thomas Gleixner wrote:
>> On Mon, Sep 18 2023 at 18:57, Linus Torvalds wrote:
>>> Anyway, I'm definitely not opposed. We'd get rid of a config option
>>> that is presumably not very widely used, and we'd simplify a lot of
>>> issues, and get rid of all these badly defined "cond_preempt()"
>>> things.
>>
>> Hmm. Didn't I promise a year ago that I won't do further large scale
>> cleanups and simplifications beyond printk.
>>
>> Maybe I get away this time with just suggesting it. :)
>
> Maybe not. As I'm inveterate curious, I sat down and figured out how
> that might look like.
>
> To some extent I really curse my curiosity as the amount of macro maze,
> config options and convoluted mess behind all these preempt mechanisms
> is beyond disgusting.
>
> 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.

> 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.

Thanks
Ankur