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

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Date:   Thu, 21 Sep 2023 04:12:27 +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
Subject: Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

On Wed, Sep 20 2023 at 17:58, Ankur Arora wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
>> So no, we don't need yet another knob. We keep them chucking along and
>> if they really want they can adopt to the new world order. :)
>
> Will they chuckle along, or die trying ;)?

Either way is fine :)

> I grepped for "preempt_enable|preempt_disable" for all the archs and
> hexagon and m68k don't seem to do any explicit accounting at all.
> (Though, neither do nios2 and openrisc, and both csky and microblaze
> only do it in the tlbflush path.)
>
>         arch/hexagon      0
>         arch/m68k         0
...
>         arch/s390        91
>         arch/mips       115
>         arch/x86        146
>         arch/powerpc    201
>
> My concern is given that we preempt on timeslice expiration for all
> three preemption models, we could end up preempting at an unsafe
> location.

As I said in my reply to Linus, that count is not really conclusive.

arch/m68k has a count of 0 and supports PREEMPT for the COLDFIRE
sub-architecture and I know for sure that at some point in the past
PREEMPT_RT was supported on COLDFIRE with minimal changes to the
architecture code.

That said, I'm pretty sure that quite some of these
preempt_disable/enable pairs in arch/* are subject to voodoo
programming, but that's a different problem to analyze.

> Still, not the most pressing of problems.

Exactly :)

Thanks,

        tglx