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From:   Ankur Arora <ankur.a.arora@oracle.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
        torvalds@linux-foundation.org, akpm@linux-foundation.org,
        luto@kernel.org, bp@alien8.de, dave.hansen@linux.intel.com,
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        rostedt@goodmis.org, vincent.guittot@linaro.org, jon.grimm@amd.com,
        bharata@amd.com, boris.ostrovsky@oracle.com,
        konrad.wilk@oracle.com, ankur.a.arora@oracle.com
Subject: Re: [PATCH 8/9] irqentry: define irqentry_exit_allow_resched()
In-reply-to: <87pm8kq96d.ffs@tglx>
Date:   Tue, 04 Apr 2023 22:29:15 -0700
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Thomas Gleixner <tglx@linutronix.de> writes:

> On Sun, Apr 02 2023 at 22:22, Ankur Arora wrote:
>> Allow threads marked TIF_ALLOW_RESCHED to be rescheduled in irqexit.
>>
>> This is only necessary under !preempt_model_preemptible() for which
>> we reuse the same logic as irqentry_exit_code_resched().
>
> This tells what this patch is doing but completely fails to explain why
> this is necessary and useful.

Thanks. Yeah, it does seem to miss that completely.

Needs some massaging, but does something like this clarify it's purpose?

On kernels with PREEMPTION_NONE/_VOLUNTARY, rescheduling of kernel
tasks happens when they allow it -- for instance by synchronously
calling cond_resched() in a long running task.

There are cases where it is not convenient to periodically call
cond_resched() -- for instance when executing a potentially
long running instruction (such as REP STOSB on x86).

To handle kernel code sections which can be safely preempted, but
cannot explicitly call cond_resched(), allow them to mark themselves
TIF_ALLOW_RESCHED.

Contexts marked such (via allow_resched()) can be rescheduled in the
irqexit path. This is, of course only needed with
!preempt_model_preemptible() and the rescheduling logic is functionally
same as irqentry_exit_code_resched().

--
ankur