From: Jann Horn <[email protected]>
To: Jens Axboe <[email protected]>
Cc: io-uring <[email protected]>,
Glauber Costa <[email protected]>,
Peter Zijlstra <[email protected]>,
Pavel Begunkov <[email protected]>
Subject: Re: [PATCH 7/9] io_uring: add per-task callback handler
Date: Fri, 21 Feb 2020 00:12:15 +0100 [thread overview]
Message-ID: <CAG48ez2vXYgW8WqBxeb=A=+_2WRL98b_Heoe8rPeXOMXuuf4oQ@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
On Fri, Feb 21, 2020 at 12:00 AM Jens Axboe <[email protected]> wrote:
> On 2/20/20 3:23 PM, Jann Horn wrote:
> > On Thu, Feb 20, 2020 at 11:14 PM Jens Axboe <[email protected]> wrote:
> >> On 2/20/20 3:02 PM, Jann Horn wrote:
> >>> On Thu, Feb 20, 2020 at 9:32 PM Jens Axboe <[email protected]> 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.
[...]
> >>>> -static void io_poll_trigger_evfd(struct io_wq_work **workptr)
> >>>> +static void io_poll_task_func(struct callback_head *cb)
> >>>> {
> >>>> - struct io_kiocb *req = container_of(*workptr, struct io_kiocb, work);
> >>>> + struct io_kiocb *req = container_of(cb, struct io_kiocb, sched_work);
> >>>> + struct io_kiocb *nxt = NULL;
> >>>>
> >>> [...]
> >>>> + io_poll_task_handler(req, &nxt);
> >>>> + if (nxt)
> >>>> + __io_queue_sqe(nxt, NULL);
> >>>
> >>> This can now get here from anywhere that calls schedule(), right?
> >>> Which means that this might almost double the required kernel stack
> >>> size, if one codepath exists that calls schedule() while near the
> >>> bottom of the stack and another codepath exists that goes from here
> >>> through the VFS and again uses a big amount of stack space? This is a
> >>> somewhat ugly suggestion, but I wonder whether it'd make sense to
> >>> check whether we've consumed over 25% of stack space, or something
> >>> like that, and if so, directly punt the request.
[...]
> >>> Also, can we recursively hit this point? Even if __io_queue_sqe()
> >>> doesn't *want* to block, the code it calls into might still block on a
> >>> mutex or something like that, at which point the mutex code would call
> >>> into schedule(), which would then again hit sched_out_update() and get
> >>> here, right? As far as I can tell, this could cause unbounded
> >>> recursion.
> >>
> >> The sched_work items are pruned before being run, so that can't happen.
> >
> > And is it impossible for new ones to be added in the meantime if a
> > second poll operation completes in the background just when we're
> > entering __io_queue_sqe()?
>
> True, that can happen.
>
> I wonder if we just prevent the recursion whether we can ignore most
> of it. Eg never process the sched_work list if we're not at the top
> level, so to speak.
>
> This should also prevent the deadlock that you mentioned with FUSE
> in the next email that just rolled in.
But there the first ->read_iter could be from outside io_uring. So you
don't just have to worry about nesting inside an already-running uring
work; you also have to worry about nesting inside more or less
anything else that might be holding mutexes. So I think you'd pretty
much have to whitelist known-safe schedule() callers, or something
like that.
Taking a step back: Do you know why this whole approach brings the
kind of performance benefit you mentioned in the cover letter? 4x is a
lot... Is it that expensive to take a trip through the scheduler?
I wonder whether the performance numbers for the echo test would
change if you commented out io_worker_spin_for_work()...
next prev parent reply other threads:[~2020-02-20 23:12 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 [this message]
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
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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