Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Jens Axboe <axboe@kernel.dk>, Peter Zijlstra <peterz@infradead.org>
Cc:     LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] task_work: allow TWA_SIGNAL without a rescheduling IPI
In-Reply-To: <bf6cc541-4de7-3258-ebb8-caa3a8249bc7@kernel.dk>
References: <543452ca-f82e-4f00-cd51-88bb9723a975@kernel.dk>
 <33a6de15-f487-b12b-8ffe-af978ebb03c3@kernel.dk>
 <20220428090836.GQ2731@worktop.programming.kicks-ass.net>
 <bf6cc541-4de7-3258-ebb8-caa3a8249bc7@kernel.dk>
Date:   Fri, 29 Apr 2022 00:28:18 +0200
Message-ID: <87wnf8onb1.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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