From: Pavel Begunkov <[email protected]>
To: Jens Axboe <[email protected]>, [email protected]
Cc: [email protected], [email protected], Jann Horn <[email protected]>
Subject: Re: [PATCH 7/9] io_uring: add per-task callback handler
Date: Fri, 21 Feb 2020 22:22:45 +0300 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
On 21/02/2020 22:10, Jens Axboe wrote:
> On 2/21/20 11:30 AM, Pavel Begunkov wrote:
>> On 21/02/2020 17:50, Jens Axboe wrote:
>>> On 2/21/20 6:51 AM, Pavel Begunkov wrote:
>>>> On 20/02/2020 23:31, Jens Axboe wrote:
>>>>> For poll requests, it's not uncommon to link a read (or write) after
>>>>> the poll to execute immediately after the file is marked as ready.
>>>>> Since the poll completion is called inside the waitqueue wake up handler,
>>>>> we have to punt that linked request to async context. This slows down
>>>>> the processing, and actually means it's faster to not use a link for this
>>>>> use case.
>>>>>
>>>>> We also run into problems if the completion_lock is contended, as we're
>>>>> doing a different lock ordering than the issue side is. Hence we have
>>>>> to do trylock for completion, and if that fails, go async. Poll removal
>>>>> needs to go async as well, for the same reason.
>>>>>
>>>>> eventfd notification needs special case as well, to avoid stack blowing
>>>>> recursion or deadlocks.
>>>>>
>>>>> These are all deficiencies that were inherited from the aio poll
>>>>> implementation, but I think we can do better. When a poll completes,
>>>>> simply queue it up in the task poll list. When the task completes the
>>>>> list, we can run dependent links inline as well. This means we never
>>>>> have to go async, and we can remove a bunch of code associated with
>>>>> that, and optimizations to try and make that run faster. The diffstat
>>>>> speaks for itself.
>>>>
>>>> So, it piggybacks request execution onto a random task, that happens
>>>> to complete a poll. Did I get it right?
>>>>
>>>> I can't find where it setting right mm, creds, etc., or why it have
>>>> them already.
>>>
>>> Not a random task, the very task that initially tried to do the receive
>>> (or whatever the operation may be). Hence there's no need to set
>>> mm/creds/whatever, we're still running in the context of the original
>>> task once we retry the operation after the poll signals readiness.
>>
>> Got it. Then, it may happen in the future after returning from
>> __io_arm_poll_handler() and io_uring_enter(). And by that time io_submit_sqes()
>> should have already restored creds (i.e. personality stuff) on the way back.
>> This might be a problem.
>
> Not sure I follow, can you elaborate? Just to be sure, the requests that
> go through the poll handler will go through __io_queue_sqe() again. Oh I
> guess your point is that that is one level below where we normally
> assign the creds.
Yeah, exactly. Poll handler won't do the personality dancing, as it doesn't go
through io_submit_sqes().
>
>> BTW, Is it by design, that all requests of a link use personality creds
>> specified in the head's sqe?
>
> No, I think that's more by accident. We should make sure they use the
> specified creds, regardless of the issue time. Care to clean that up?
> Would probably help get it right for the poll case, too.
Ok, I'll prepare
--
Pavel Begunkov
next prev parent reply other threads:[~2020-02-21 19:23 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-20 20:31 [PATCHSET 0/9] io_uring: use polled async retry Jens Axboe
2020-02-20 20:31 ` [PATCH 1/9] io_uring: consider any io_read/write -EAGAIN as final Jens Axboe
2020-02-20 20:31 ` [PATCH 2/9] io_uring: io_accept() should hold on to submit reference on retry Jens Axboe
2020-02-20 20:31 ` [PATCH 3/9] sched: move io-wq/workqueue worker sched in/out into helpers Jens Axboe
2020-02-20 20:31 ` [PATCH 4/9] task_work_run: don't take ->pi_lock unconditionally Jens Axboe
2020-02-20 20:31 ` [PATCH 5/9] kernel: abstract out task work helpers Jens Axboe
2020-02-20 21:07 ` Peter Zijlstra
2020-02-20 21:08 ` Jens Axboe
2020-02-20 20:31 ` [PATCH 6/9] sched: add a sched_work list Jens Axboe
2020-02-20 21:17 ` Peter Zijlstra
2020-02-20 21:53 ` Jens Axboe
2020-02-20 22:02 ` Jens Axboe
2020-02-20 20:31 ` [PATCH 7/9] io_uring: add per-task callback handler Jens Axboe
2020-02-20 22:02 ` Jann Horn
2020-02-20 22:14 ` Jens Axboe
2020-02-20 22:18 ` Jens Axboe
2020-02-20 22:25 ` Jann Horn
2020-02-20 22:23 ` Jens Axboe
2020-02-20 22:38 ` Jann Horn
2020-02-20 22:56 ` Jens Axboe
2020-02-20 22:58 ` Jann Horn
2020-02-20 23:02 ` Jens Axboe
2020-02-20 22:23 ` Jann Horn
2020-02-20 23:00 ` Jens Axboe
2020-02-20 23:12 ` Jann Horn
2020-02-20 23:22 ` Jens Axboe
2020-02-21 1:29 ` Jann Horn
2020-02-21 17:32 ` Jens Axboe
2020-02-21 19:24 ` Jann Horn
2020-02-21 20:18 ` Jens Axboe
2020-02-20 22:56 ` Jann Horn
2020-02-21 10:47 ` Peter Zijlstra
2020-02-21 14:49 ` Jens Axboe
2020-02-21 15:02 ` Jann Horn
2020-02-21 16:12 ` Peter Zijlstra
2020-02-21 16:23 ` Peter Zijlstra
2020-02-21 20:13 ` Jens Axboe
2020-02-21 13:51 ` Pavel Begunkov
2020-02-21 14:50 ` Jens Axboe
2020-02-21 18:30 ` Pavel Begunkov
2020-02-21 19:10 ` Jens Axboe
2020-02-21 19:22 ` Pavel Begunkov [this message]
2020-02-23 6:00 ` Jens Axboe
2020-02-23 6:26 ` Jens Axboe
2020-02-23 11:02 ` Pavel Begunkov
2020-02-23 14:49 ` Jens Axboe
2020-02-23 14:58 ` Jens Axboe
2020-02-23 15:07 ` Jens Axboe
2020-02-23 18:04 ` Pavel Begunkov
2020-02-23 18:06 ` Jens Axboe
2020-02-23 17:55 ` Pavel Begunkov
2020-02-20 20:31 ` [PATCH 8/9] io_uring: mark requests that we can do poll async in io_op_defs Jens Axboe
2020-02-20 20:31 ` [PATCH 9/9] io_uring: use poll driven retry for files that support it Jens Axboe
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