linux-kernel - Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY

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Message-ID: <CAC2285F-0841-47F5-8567-24C1377DDFB6@vmware.com>
Date:   Mon, 9 Aug 2021 04:50:00 +0000
From:   Nadav Amit <namit@...are.com>
To:     Hao Xu <haoxu@...ux.alibaba.com>
CC:     Pavel Begunkov <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task
 work



> On Aug 8, 2021, at 9:07 PM, Hao Xu <haoxu@...ux.alibaba.com> wrote:
> 
> 在 2021/8/9 上午1:31, Nadav Amit 写道:
>>> On Aug 8, 2021, at 5:55 AM, Pavel Begunkov <asml.silence@...il.com> wrote:
>>> 
>>> On 8/8/21 1:13 AM, Nadav Amit wrote:
>>>> From: Nadav Amit <namit@...are.com>
>>>> 
>>>> When using SQPOLL, the submission queue polling thread calls
>>>> task_work_run() to run queued work. However, when work is added with
>>>> TWA_SIGNAL - as done by io_uring itself - the TIF_NOTIFY_SIGNAL remains
>>> 
>>> static int io_req_task_work_add(struct io_kiocb *req)
>>> {
>>> 	...
>>> 	notify = (req->ctx->flags & IORING_SETUP_SQPOLL) ? TWA_NONE : TWA_SIGNAL;
>>> 	if (!task_work_add(tsk, &tctx->task_work, notify))
>>> 	...
>>> }
>>> 
>>> io_uring doesn't set TIF_NOTIFY_SIGNAL for SQPOLL. But if you see it, I'm
>>> rather curious who does.
>> I was saying io-uring, but I meant io-uring in the wider sense:
>> io_queue_worker_create().
>> Here is a call trace for when TWA_SIGNAL is used. io_queue_worker_create()
>> uses TWA_SIGNAL. It is called by io_wqe_dec_running(), and not shown due
>> to inlining:
> Hi Nadav, Pavel,
> How about trying to make this kind of call to use TWA_NONE for sqthread,
> though I know for this case currently there is no info to get to know if
> task is sqthread. I think we shouldn't kick sqthread.

It is possible, but it would break the abstractions and propagating
it would be disgusting. Let me give it some thought.

Regardless, I think that this patch should go to 5.14 and stable,
and any solution to avoid kicking the SQ should come on top (to be
safe).