Re: [PATCH 1/2] md/raid5: split wait_for_stripe and introduce wait_for_quiesce

[Yuanhan Liu]

On Mon, Apr 27, 2015 at 10:10:24AM +1000, NeilBrown wrote:
> On Fri, 24 Apr 2015 21:39:03 +0800 Yuanhan Liu <yuanhan.liu@xxxxxxxxxxxxxxx>
> wrote:
> 
> > If I read code correctly, current wait_for_stripe actually has 2 usage:
> > 
> > - wait for there is enough free stripe cache, triggered when
> >   get_free_stripe() failed. This is what wait_for_stripe intend
> >   for literally.
> > 
> > - wait for quiesce == 0 or
> >            active_aligned_reads == 0 && active_stripes == 0
> > 
> >   It has nothing to do with wait_for_stripe literally, and releasing
> >   an active stripe won't actually wake them up. On the contrary, wake_up
> >   from under this case won't actually wake up the process waiting for
> >   an free stripe being available.
> 
> I disagree.  Releasing an active stripe *will* (or *can*) wake up that third
> case, as it decrements "active_stripes" which will eventually reach zero.
> 
> I don't think your new code will properly wake up a process which is waiting
> for "active_stripes == 0".

Right, and thanks for pointing it out. So, is this enough?

---
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 2d8fcc1..3f23035 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -383,6 +383,9 @@ static void release_inactive_stripe_list(struct
r5conf *conf,
                        }
                }
        }
+
+       if (!atomic_read(&conf->active_stripes))
+               wake_up(&conf->wait_for_quiesce);
 }

 /* should hold conf->device_lock already */


Or, should I put it a bit ahead, trying to invoke wake_up(&conf->wait_for_quiesce)
after each atomic_dec(&conf->active_stripes)?

	if (atomic_dec_return(&conf->active_stripes) == 0)
		wake_up(&conf->wait_for_quiesce);

> 
> > 
> > Hence, we'd better split wait_for_stripe, and here I introduce
> > wait_for_quiesce for the second usage. The name may not well taken, or
> > even taken wrongly. Feel free to correct me then.
> > 
> > This is also a prepare patch for next patch: make wait_for_stripe
> > exclusive.
> 
> I think you have this commit description upside down :-)
> 
> The real motivation is that you are seeing contention on some spinlock and so
> you want to split 'wait_for_stripe' up in to multiple wait_queues so that you
> can use exclusive wakeup.  As this is the main motivation, it should be
> stated first.
> 
> Then explain that 'wait_for_stripe' is used to wait for the array to enter or
> leave the quiescent state, and also to wait for an available stripe in each
> of the hash lists.
> 
> So this patch splits the first usage off into a separate wait_queue, and the
> next patch will split the second usage into one waitqueue for each hash value.
> 
> Then explain just is what is needed for that first step.
> 
> When you put it that way around, the patch makes lots of sense.

It does, and thanks!

> 
> So: could you please resubmit with the description the right way around, and

To make sure I followed you correctly, my patch order is correct(I mean,
split lock first, and make wait_for_stripe per lock hash and exclusive
second), and what I need to do is re-writing the commit log as you suggested,
and fixing all issues you pointed out. Right?

	--yliu

> with an appropriate wakeup call to ensure raid5_quiesce is woken up when
> active_stripes reaches zero?
> 
> Thanks,
> NeilBrown
> 
> 
> > 
> > Signed-off-by: Yuanhan Liu <yuanhan.liu@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/md/raid5.c | 13 +++++++------
> >  drivers/md/raid5.h |  1 +
> >  2 files changed, 8 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> > index 9716319..b7e385f 100644
> > --- a/drivers/md/raid5.c
> > +++ b/drivers/md/raid5.c
> > @@ -667,7 +667,7 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
> >  	spin_lock_irq(conf->hash_locks + hash);
> >  
> >  	do {
> > -		wait_event_lock_irq(conf->wait_for_stripe,
> > +		wait_event_lock_irq(conf->wait_for_quiesce,
> >  				    conf->quiesce == 0 || noquiesce,
> >  				    *(conf->hash_locks + hash));
> >  		sh = __find_stripe(conf, sector, conf->generation - previous);
> > @@ -4725,7 +4725,7 @@ static void raid5_align_endio(struct bio *bi, int error)
> >  					 raid_bi, 0);
> >  		bio_endio(raid_bi, 0);
> >  		if (atomic_dec_and_test(&conf->active_aligned_reads))
> > -			wake_up(&conf->wait_for_stripe);
> > +			wake_up(&conf->wait_for_quiesce);
> >  		return;
> >  	}
> >  
> > @@ -4820,7 +4820,7 @@ static int chunk_aligned_read(struct mddev *mddev, struct bio * raid_bio)
> >  		align_bi->bi_iter.bi_sector += rdev->data_offset;
> >  
> >  		spin_lock_irq(&conf->device_lock);
> > -		wait_event_lock_irq(conf->wait_for_stripe,
> > +		wait_event_lock_irq(conf->wait_for_quiesce,
> >  				    conf->quiesce == 0,
> >  				    conf->device_lock);
> >  		atomic_inc(&conf->active_aligned_reads);
> > @@ -5659,7 +5659,7 @@ static int  retry_aligned_read(struct r5conf *conf, struct bio *raid_bio)
> >  		bio_endio(raid_bio, 0);
> >  	}
> >  	if (atomic_dec_and_test(&conf->active_aligned_reads))
> > -		wake_up(&conf->wait_for_stripe);
> > +		wake_up(&conf->wait_for_quiesce);
> >  	return handled;
> >  }
> >  
> > @@ -6390,6 +6390,7 @@ static struct r5conf *setup_conf(struct mddev *mddev)
> >  		goto abort;
> >  	spin_lock_init(&conf->device_lock);
> >  	seqcount_init(&conf->gen_lock);
> > +	init_waitqueue_head(&conf->wait_for_quiesce);
> >  	init_waitqueue_head(&conf->wait_for_stripe);
> >  	init_waitqueue_head(&conf->wait_for_overlap);
> >  	INIT_LIST_HEAD(&conf->handle_list);
> > @@ -7413,7 +7414,7 @@ static void raid5_quiesce(struct mddev *mddev, int state)
> >  		 * active stripes can drain
> >  		 */
> >  		conf->quiesce = 2;
> > -		wait_event_cmd(conf->wait_for_stripe,
> > +		wait_event_cmd(conf->wait_for_quiesce,
> >  				    atomic_read(&conf->active_stripes) == 0 &&
> >  				    atomic_read(&conf->active_aligned_reads) == 0,
> >  				    unlock_all_device_hash_locks_irq(conf),
> > @@ -7427,7 +7428,7 @@ static void raid5_quiesce(struct mddev *mddev, int state)
> >  	case 0: /* re-enable writes */
> >  		lock_all_device_hash_locks_irq(conf);
> >  		conf->quiesce = 0;
> > -		wake_up(&conf->wait_for_stripe);
> > +		wake_up(&conf->wait_for_quiesce);
> >  		wake_up(&conf->wait_for_overlap);
> >  		unlock_all_device_hash_locks_irq(conf);
> >  		break;
> > diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> > index 7dc0dd8..fab53a3 100644
> > --- a/drivers/md/raid5.h
> > +++ b/drivers/md/raid5.h
> > @@ -508,6 +508,7 @@ struct r5conf {
> >  	struct list_head	inactive_list[NR_STRIPE_HASH_LOCKS];
> >  	atomic_t		empty_inactive_list_nr;
> >  	struct llist_head	released_stripes;
> > +	wait_queue_head_t	wait_for_quiesce;
> >  	wait_queue_head_t	wait_for_stripe;
> >  	wait_queue_head_t	wait_for_overlap;
> >  	unsigned long		cache_state;
> 


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