Re: [PATCH -next RFC v2 2/8] blk-mq: call 'bt_wait_ptr()' later in blk_mq_get_tag()

[yukuai (C)]

在 2022/04/08 22:20, Bart Van Assche 写道:
> On 4/8/22 00:39, Yu Kuai wrote:
> > bt_wait_ptr() will increase 'wait_index', however, if blk_mq_get_tag()
> > get a tag successfully after bt_wait_ptr() is called and before
> > sbitmap_prepare_to_wait() is called, then the 'ws' is skipped. This
> > behavior might cause 8 waitqueues to be unbalanced.
> >
> > Move bt_wait_ptr() later should reduce the problem when the disk is
> > under high io preesure.
> >
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   block/blk-mq-tag.c | 4 +---
> >   1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> > index 68ac23d0b640..228a0001694f 100644
> > --- a/block/blk-mq-tag.c
> > +++ b/block/blk-mq-tag.c
> > @@ -155,7 +155,6 @@ unsigned int blk_mq_get_tag(struct 
> > blk_mq_alloc_data *data)
> >       if (data->flags & BLK_MQ_REQ_NOWAIT)
> >           return BLK_MQ_NO_TAG;
> > -    ws = bt_wait_ptr(bt, data->hctx);
> >       do {
> >           struct sbitmap_queue *bt_prev;
> > @@ -174,6 +173,7 @@ unsigned int blk_mq_get_tag(struct 
> > blk_mq_alloc_data *data)
> >           if (tag != BLK_MQ_NO_TAG)
> >               break;
> > +        ws = bt_wait_ptr(bt, data->hctx);
> >           sbitmap_prepare_to_wait(bt, ws, &wait, TASK_UNINTERRUPTIBLE);
> >           tag = __blk_mq_get_tag(data, bt);
> > @@ -201,8 +201,6 @@ unsigned int blk_mq_get_tag(struct 
> > blk_mq_alloc_data *data)
> >            */
> >           if (bt != bt_prev)
> >               sbitmap_queue_wake_up(bt_prev);
> > -
> > -        ws = bt_wait_ptr(bt, data->hctx);
> >       } while (1);
>
> Is it necessary to call bt_wait_ptr() during every loop iteration or 
> only if bt != bt_prev? Would calling bt_wait_ptr() only if bt != bt_prev 
> help to reduce unfairness further?
Hi,

You are right, that sounds reasonable.

Thanks,
Kuai
> Thanks,
>
> Bart.
> .