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Subject: Re: [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task
 work
To:     Nadav Amit <namit@vmware.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        "io-uring@vger.kernel.org" <io-uring@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20210808001342.964634-1-namit@vmware.com>
 <20210808001342.964634-2-namit@vmware.com>
 <488f005c-fd92-9881-2700-b2eb1adbb78e@gmail.com>
 <40885F33-F35D-49E9-A79F-DB3C35278F73@vmware.com>
From:   Pavel Begunkov <asml.silence@gmail.com>
Message-ID: <90d77c2e-23a2-4e77-3626-6bb08027b7d4@gmail.com>
Date:   Mon, 9 Aug 2021 11:18:56 +0100
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On 8/8/21 6:31 PM, Nadav Amit wrote:
>> On Aug 8, 2021, at 5:55 AM, Pavel Begunkov <asml.silence@gmail.com> wrote:
>> On 8/8/21 1:13 AM, Nadav Amit wrote:
>>> From: Nadav Amit <namit@vmware.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:
> 
> [   70.540761] Call Trace:
> [   70.541352]  dump_stack+0x7d/0x9c
> [   70.541930]  task_work_add.cold+0x9/0x12
> [   70.542591]  io_wqe_dec_running+0xd6/0xf0
> [   70.543259]  io_wq_worker_sleeping+0x3d/0x60
> [   70.544106]  schedule+0xa0/0xc0
> [   70.544673]  userfaultfd_read_iter+0x2c3/0x790
> [   70.545374]  ? wake_up_q+0xa0/0xa0
> [   70.545887]  io_iter_do_read+0x1e/0x40
> [   70.546531]  io_read+0xdc/0x340
> [   70.547148]  ? update_curr+0x72/0x1c0
> [   70.547887]  ? update_load_avg+0x7c/0x600
> [   70.548538]  ? __switch_to_xtra+0x10a/0x500
> [   70.549264]  io_issue_sqe+0xd99/0x1840
> [   70.549887]  ? lock_timer_base+0x72/0xa0
> [   70.550516]  ? try_to_del_timer_sync+0x54/0x80
> [   70.551224]  io_wq_submit_work+0x87/0xb0
> [   70.552001]  io_worker_handle_work+0x2b5/0x4b0
> [   70.552705]  io_wqe_worker+0xd6/0x2f0
> [   70.553364]  ? recalc_sigpending+0x1c/0x50
> [   70.554074]  ? io_worker_handle_work+0x4b0/0x4b0
> [   70.554813]  ret_from_fork+0x22/0x30
> 
> Does it answer your question?

Pretty much, thanks

-- 
Pavel Begunkov