linux-kernel - Re: [PATCH] task_work: allow TWA_SIGNAL without a rescheduling IPI

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Date:   Fri, 29 Apr 2022 00:28:18 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] task_work: allow TWA_SIGNAL without a rescheduling IPI

On Thu, Apr 28 2022 at 06:21, Jens Axboe wrote:
> On 4/28/22 3:08 AM, Peter Zijlstra wrote:
>> On Mon, Apr 25, 2022 at 07:52:31PM -0600, Jens Axboe wrote:
>>> On 4/22/22 8:34 AM, Jens Axboe wrote:
>>>> Some use cases don't always need an IPI when sending a TWA_SIGNAL
>>>> notification. Add TWA_SIGNAL_NO_IPI, which is just like TWA_SIGNAL,
>>>> except it doesn't send an IPI to the target task. It merely sets
>>>> TIF_NOTIFY_SIGNAL and wakes up the task.
>> 
>> Could you perphaps elaborate on those use-cases? How do they guarantee
>> the task_work is ran before userspace?
>
> The task is still marked as having task_work, so there should be no
> differences in how it's run before returning to userspace. That would
> not have delivered an IPI before, if it was in the kernel.
>
> The difference would be in the task currently running in userspace, and
> whether we force a reschedule to ensure the task_work gets run now.
> Without the forced reschedule, running of the task_work (from io_uring)
> becomes more cooperative - it'll happen when the task transitions to the
> kernel anyway (eg to wait for events).

I can see why you want that, but that needs to be part of the change log
and it also needs a comprehensive comment for TWA_SIGNAL_NO_IPI.

@TWA_SIGNAL_NO_IPI works like @TWA_SIGNAL.... does not really explain
much.

Thanks,

        tglx