Re: single aio thread is migrated crazily by scheduler

Ming Lei
2019-12-03T00:19:08Z

On Tue, Dec 03, 2019 at 10:53:21AM +1100, Dave Chinner wrote:
> On Mon, Dec 02, 2019 at 02:45:42PM +0100, Vincent Guittot wrote:
> > On Mon, 2 Dec 2019 at 05:02, Dave Chinner <david@fromorbit.com> wrote:
> > >
> > > On Mon, Dec 02, 2019 at 10:46:25AM +0800, Ming Lei wrote:
> > > > On Thu, Nov 28, 2019 at 10:53:33AM +0100, Vincent Guittot wrote:
> > > > > On Thu, 28 Nov 2019 at 10:40, Hillf Danton <hdanton@sina.com> wrote:
> > > > > > --- a/fs/iomap/direct-io.c
> > > > > > +++ b/fs/iomap/direct-io.c
> > > > > > @@ -157,10 +157,8 @@ static void iomap_dio_bio_end_io(struct
> > > > > > WRITE_ONCE(dio->submit.waiter, NULL);
> > > > > > blk_wake_io_task(waiter);
> > > > > > } else if (dio->flags & IOMAP_DIO_WRITE) {
> > > > > > - struct inode *inode = file_inode(dio->iocb->ki_filp);
> > > > > > -
> > > > > > INIT_WORK(&dio->aio.work, iomap_dio_complete_work);
> > > > > > - queue_work(inode->i_sb->s_dio_done_wq, &dio->aio.work);
> > > > > > + schedule_work(&dio->aio.work);
> > > > >
> > > > > I'm not sure that this will make a real difference because it ends up
> > > > > to call queue_work(system_wq, ...) and system_wq is bounded as well so
> > > > > the work will still be pinned to a CPU
> > > > > Using system_unbound_wq should make a difference because it doesn't
> > > > > pin the work on a CPU
> > > > > + queue_work(system_unbound_wq, &dio->aio.work);
> > > >
> > > > Indeed, just run a quick test on my KVM guest, looks the following patch
> > > > makes a difference:
> > > >
> > > > diff --git a/fs/direct-io.c b/fs/direct-io.c
> > > > index 9329ced91f1d..2f4488b0ecec 100644
> > > > --- a/fs/direct-io.c
> > > > +++ b/fs/direct-io.c
> > > > @@ -613,7 +613,8 @@ int sb_init_dio_done_wq(struct super_block *sb)
> > > > {
> > > > struct workqueue_struct *old;
> > > > struct workqueue_struct *wq = alloc_workqueue("dio/%s",
> > > > - WQ_MEM_RECLAIM, 0,
> > > > + WQ_MEM_RECLAIM |
> > > > + WQ_UNBOUND, 0,
> > > > sb->s_id);
> > >
> > > That's not an answer to the user task migration issue.
> > >
> > > That is, all this patch does is trade user task migration when the
> > > CPU is busy for migrating all the queued work off the CPU so the
> > > user task does not get migrated. IOWs, this forces all the queued
> > > work to be migrated rather than the user task. IOWs, it does not
> > > address the issue we've exposed in the scheduler between tasks with
> > > competing CPU affinity scheduling requirements - it just hides the
> > > symptom.
> > >
> > > Maintaining CPU affinity across dispatch and completion work has
> > > been proven to be a significant performance win. Right throughout
> > > the IO stack we try to keep this submitter/completion affinity,
> > > and that's the whole point of using a bound wq in the first place:
> > > efficient delayed batch processing of work on the local CPU.
> >
> > Do you really want to target the same CPU ? looks like what you really
> > want to target the same cache instead
>
> Well, yes, ideally we want to target the same cache, but we can't do
> that with workqueues.
>
> However, the block layer already does that same-cache steering for
> it's directed completions (see __blk_mq_complete_request()), so we
> are *already running in a "hot cache" CPU context* when we queue
> work. When we queue to the same CPU, we are simply maintaining the
> "cache-hot" context that we are already running in.

__blk_mq_complete_request() doesn't always complete the request on
the submission CPU, which is only done in case of 1:1 queue mapping
and N:1 mapping when nr_hw_queues < nr_nodes. Also, the default
completion flag is SAME_GROUP, which just requires the completion
CPU to share cache with submission CPU:

#define QUEUE_FLAG_SAME_COMP 4 /* complete on same CPU-group */

Thanks,
Ming