Re: [PATCH V2] block: ignore RWF_HIPRI hint for sync dio

From: Ming Lei <ming.lei@redhat.com>
To: Keith Busch <kbusch@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	linux-mm@kvack.org, linux-xfs@vger.kernel.org,
	Changhui Zhong <czhong@redhat.com>
Subject: Re: [PATCH V2] block: ignore RWF_HIPRI hint for sync dio
Date: Tue, 24 May 2022 08:40:46 +0800	[thread overview]
Message-ID: <YowpjtLfZPld1H6T@T590> (raw)
In-Reply-To: <YowMVODoNIyaqVdC@kbusch-mbp.dhcp.thefacebook.com>

On Mon, May 23, 2022 at 04:36:04PM -0600, Keith Busch wrote:
> On Wed, Apr 20, 2022 at 10:31:10PM +0800, Ming Lei wrote:
> > So far bio is marked as REQ_POLLED if RWF_HIPRI/IOCB_HIPRI is passed
> > from userspace sync io interface, then block layer tries to poll until
> > the bio is completed. But the current implementation calls
> > blk_io_schedule() if bio_poll() returns 0, and this way causes io hang or
> > timeout easily.
> 
> Wait a second. The task's current state is TASK_RUNNING when bio_poll() returns
> zero, so calling blk_io_schedule() isn't supposed to hang.

void __sched io_schedule(void)
{
        int token;

        token = io_schedule_prepare();
        schedule();
        io_schedule_finish(token);
}

But who can wakeup this task after scheduling out? There can't be irq
handler for POLLED request.

The hang can be triggered on nvme/qemu reliably:

fio --bs=4k --size=1G --ioengine=pvsync2 --norandommap --hipri=1 --iodepth=64 \
	--slat_percentiles=1 --nowait=0 --filename=/dev/nvme0n1 --direct=1 \
	--runtime=10 --numjobs=1 --rw=rw --name=test --group_reporting

Thanks, 
Ming