linux-kernel - Re: [RFC PATCH 00/86] Make the kernel preemptible

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Message-ID: <87o7g3epcu.fsf@oracle.com>
Date:   Wed, 08 Nov 2023 16:34:41 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
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Subject: Re: [RFC PATCH 00/86] Make the kernel preemptible


Mark Rutland <mark.rutland@....com> writes:

> On Tue, Nov 07, 2023 at 01:56:46PM -0800, Ankur Arora wrote:
>> What's broken:
>>  - ARCH_NO_PREEMPT (See patch-45 "preempt: ARCH_NO_PREEMPT only preempts
>>    lazily")
>>  - Non-x86 architectures. It's trivial to support other archs (only need
>>    to add TIF_NEED_RESCHED_LAZY) but wanted to hold off until I got some
>>    comments on the series.
>>    (From some testing on arm64, didn't find any surprises.)
>
> When you say "testing on arm64, didn't find any surprises", I assume you mean
> with an additional patch adding TIF_NEED_RESCHED_LAZY?

Yeah. And, handling that in the user exit path.

> Note that since arm64 doesn't use the generic entry code, that also requires
> changes to arm64_preempt_schedule_irq() in arch/arm64/kernel/entry-common.c, to
> handle TIF_NEED_RESCHED_LAZY.

So, the intent (which got muddied due to this overly large series)
was to delay handling TIF_NEED_RESCHED_LAZY until we are about to
return to user.

I think arm64_preempt_schedule_irq() should only handle TIF_NEED_RESCHED
and the _TIF_NEED_RESCHED_LAZY should be handled via _TIF_WORK_MASK
and do_notify_resume().

(The design is much clearer in Thomas' PoC:
https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/)

>>  - ftrace support for need-resched-lazy is incomplete
>
> What exactly do we need for ftrace here?

Only support for TIF_NEED_RESCHED_LAZY which should be complete.
That comment was based on a misreading of the code.


Thanks

--
ankur