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From: Xiaoguang Wang <[email protected]>
To: Jens Axboe <[email protected]>, io-uring <[email protected]>
Cc: Pavel Begunkov <[email protected]>,
	Stefano Garzarella <[email protected]>
Subject: Re: Question about patch "io_uring: add submission polling"
Date: Mon, 19 Oct 2020 10:52:18 +0800	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

hi,

> On 10/16/20 2:55 AM, Xiaoguang Wang wrote:
>> hi,
>>
>> I have questions about below code comments, which was included in patch
>> "io_uring: add submission polling",
>> ------------------------------------------------------------
>>       /*
>>        * Drop cur_mm before scheduling, we can't hold it for
>>        * long periods (or over schedule()). Do this before
>>        * adding ourselves to the waitqueue, as the unuse/drop
>>        * may sleep.
>>        */
>>       if (cur_mm) {
>>               unuse_mm(cur_mm);
>>               mmput(cur_mm);
>>               cur_mm = NULL;
>>       }
>>
>>       prepare_to_wait(&ctx->sqo_wait, &wait, TASK_INTERRUPTIBLE);
>> -------------------------------------------------------------
>> Stefano submited a patch "io_uring: prevent sq_thread from spinning when it should stop",
>> I understand what issue Stefano fixed, but don't understand below comments.
>>
>> Can anyone help to explain why we need to rop cur_mm before scheduling, or
>> why we can't hold it for long periods (or over schedule()), and if we
>> unuse/drop mm after adding ourselves to the waitqueue, what issue will
>> happen when unuse/drop sleeps, thanks.
> 
> The not holding it too long it just trying to be nice. But we can't drop
> it after we've done prepare_to_wait(), as that sets our task runstate to
> a non-running state. This conflicts with with mmput(), which might
> sleep.
I see, thanks.
Now I'm trying to improve io_sq_thread a bit and will send patches soon.

Regards,
Xiaoguang Wang
> 

      reply	other threads:[~2020-10-19  2:53 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-16  8:55 Question about patch "io_uring: add submission polling" Xiaoguang Wang
2020-10-16 13:27 ` Jens Axboe
2020-10-19  2:52   ` Xiaoguang Wang [this message]

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