linux-kernel - Re: [PATCH v2] nbd: Don't use workqueue to handle recv work

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Date:   Wed, 5 Jan 2022 13:36:08 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Josef Bacik <josef@...icpanda.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, nbd@...er.debian.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] nbd: Don't use workqueue to handle recv work

On Wed, Jan 5, 2022 at 2:06 AM Josef Bacik <josef@...icpanda.com> wrote:
>
> On Tue, Jan 04, 2022 at 01:31:47PM +0800, Yongji Xie wrote:
> > On Tue, Jan 4, 2022 at 12:10 AM Josef Bacik <josef@...icpanda.com> wrote:
> > >
> > > On Thu, Dec 30, 2021 at 12:01:23PM +0800, Yongji Xie wrote:
> > > > On Thu, Dec 30, 2021 at 1:35 AM Christoph Hellwig <hch@...radead.org> wrote:
> > > > >
> > > > > On Mon, Dec 27, 2021 at 05:12:41PM +0800, Xie Yongji wrote:
> > > > > > The rescuer thread might take over the works queued on
> > > > > > the workqueue when the worker thread creation timed out.
> > > > > > If this happens, we have no chance to create multiple
> > > > > > recv threads which causes I/O hung on this nbd device.
> > > > >
> > > > > If a workqueue is used there aren't really 'receive threads'.
> > > > > What is the deadlock here?
> > > >
> > > > We might have multiple recv works, and those recv works won't quit
> > > > unless the socket is closed. If the rescuer thread takes over those
> > > > works, only the first recv work can run. The I/O needed to be handled
> > > > in other recv works would be hung since no thread can handle them.
> > > >
> > >
> > > I'm not following this explanation.  What is the rescuer thread you're talking
> >
> > https://www.kernel.org/doc/html/latest/core-api/workqueue.html#c.rescuer_thread
> >
>
> Ahhh ok now I see, thanks, I didn't know this is how this worked.
>
> So what happens is we do the queue_work(), this needs to do a GFP_KERNEL
> allocation internally, we are unable to satisfy this, and thus the work gets
> pushed onto the rescuer thread.
>
> Then the rescuer thread can't be used in the future because it's doing this long
> running thing.
>

Yes.

> I think the correct thing to do here is simply drop the WQ_MEM_RECLAIM bit.  It
> makes sense for workqueue's that are handling the work of short lived works that
> are in the memory reclaim path.  That's not what these workers are doing, yes
> they are in the reclaim path, but they run the entire time the device is up.
> The actual work happens as they process incoming requests.  AFAICT
> WQ_MEM_RECLAIM doesn't affect the actual allocations that the worker thread
> needs to do, which is what I think the intention was in using WQ_MEM_RECLAIM,
> which isn't really what it's used for.
>
> tl;dr, just remove thee WQ_MEM_RECLAIM flag completely and I think that's good
> enough?  Thanks,
>

In the reconnect case, we still need to call queue_work() while the
device is running. So it looks like we can't simply remove the
WQ_MEM_RECLAIM flag.

Thanks,
Yongji