[PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[Mikulas Patocka]

Hi Jens

Please would you consider taking this into the block tree? It seems to 
speed up device deletion enormously.

Mikulas

---

backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

synchronize_rcu sleeps several timer ticks. synchronize_rcu_expedited is 
much faster.

With 100Hz timer frequency, when we remove 10000 block devices with 
"dmsetup remove_all" command, it takes 27 minutes. With this patch, 
removing 10000 block devices takes only 15 seconds.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 mm/backing-dev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-3.0-rc7-fast/mm/backing-dev.c
===================================================================
--- linux-3.0-rc7-fast.orig/mm/backing-dev.c	2011-07-19 18:01:00.000000000 +0200
+++ linux-3.0-rc7-fast/mm/backing-dev.c	2011-07-19 18:01:07.000000000 +0200
@@ -505,7 +505,7 @@ static void bdi_remove_from_list(struct 
 	list_del_rcu(&bdi->bdi_list);
 	spin_unlock_bh(&bdi_lock);
 
-	synchronize_rcu();
+	synchronize_rcu_expedited();
 }
 
 int bdi_register(struct backing_dev_info *bdi, struct device *parent,

Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[Jens Axboe]

On 2011-07-21 02:29, Mikulas Patocka wrote:
> Hi Jens
> 
> Please would you consider taking this into the block tree? It seems to 
> speed up device deletion enormously.

Sure, looks like a good fix. Reminds me of a similar problem we had in
block core.


-- 
Jens Axboe

Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[Peter Zijlstra]

On Wed, 2011-07-20 at 20:29 -0400, Mikulas Patocka wrote:
> Hi Jens
> 
> Please would you consider taking this into the block tree? It seems to 
> speed up device deletion enormously.
> 
> Mikulas
> 
> ---
> 
> backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu
> 
> synchronize_rcu sleeps several timer ticks. synchronize_rcu_expedited is 
> much faster.
> 
> With 100Hz timer frequency, when we remove 10000 block devices with 
> "dmsetup remove_all" command, it takes 27 minutes. With this patch, 
> removing 10000 block devices takes only 15 seconds.
> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> 
> ---
>  mm/backing-dev.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Index: linux-3.0-rc7-fast/mm/backing-dev.c
> ===================================================================
> --- linux-3.0-rc7-fast.orig/mm/backing-dev.c	2011-07-19 18:01:00.000000000 +0200
> +++ linux-3.0-rc7-fast/mm/backing-dev.c	2011-07-19 18:01:07.000000000 +0200
> @@ -505,7 +505,7 @@ static void bdi_remove_from_list(struct 
>  	list_del_rcu(&bdi->bdi_list);
>  	spin_unlock_bh(&bdi_lock);
>  
> -	synchronize_rcu();
> +	synchronize_rcu_expedited();
>  }
>  

Urgh, I just noticed this crap in my tree.. You realize that what you're
effectively hammering a global sync primitive this way? Depending on
what RCU flavour you have any SMP variant will at least do a machine
wide IPI broadcast for every sync_rcu_exp(), some do significantly more.

The much better solution would've been to batch your block-dev removals
and use a single sync_rcu as barrier.

This is not cool.

Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[Paul E. McKenney]

On Tue, Jan 31, 2012 at 09:34:23PM +0100, Peter Zijlstra wrote:
> On Wed, 2011-07-20 at 20:29 -0400, Mikulas Patocka wrote:
> > Hi Jens
> > 
> > Please would you consider taking this into the block tree? It seems to 
> > speed up device deletion enormously.
> > 
> > Mikulas
> > 
> > ---
> > 
> > backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu
> > 
> > synchronize_rcu sleeps several timer ticks. synchronize_rcu_expedited is 
> > much faster.
> > 
> > With 100Hz timer frequency, when we remove 10000 block devices with 
> > "dmsetup remove_all" command, it takes 27 minutes. With this patch, 
> > removing 10000 block devices takes only 15 seconds.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > ---
> >  mm/backing-dev.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > Index: linux-3.0-rc7-fast/mm/backing-dev.c
> > ===================================================================
> > --- linux-3.0-rc7-fast.orig/mm/backing-dev.c	2011-07-19 18:01:00.000000000 +0200
> > +++ linux-3.0-rc7-fast/mm/backing-dev.c	2011-07-19 18:01:07.000000000 +0200
> > @@ -505,7 +505,7 @@ static void bdi_remove_from_list(struct 
> >  	list_del_rcu(&bdi->bdi_list);
> >  	spin_unlock_bh(&bdi_lock);
> >  
> > -	synchronize_rcu();
> > +	synchronize_rcu_expedited();
> >  }
> >  
> 
> Urgh, I just noticed this crap in my tree.. You realize that what you're
> effectively hammering a global sync primitive this way? Depending on
> what RCU flavour you have any SMP variant will at least do a machine
> wide IPI broadcast for every sync_rcu_exp(), some do significantly more.
> 
> The much better solution would've been to batch your block-dev removals
> and use a single sync_rcu as barrier.
> 
> This is not cool.

Indeed, synchronize_rcu_expedited() is quite heavyweight, so as Peter
suggests, if you can use batching you will get even better performance
with much less load on the rest of the system.

							Thanx, Paul

Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[Mikulas Patocka]

On Tue, 31 Jan 2012, Peter Zijlstra wrote:

> On Wed, 2011-07-20 at 20:29 -0400, Mikulas Patocka wrote:
> > Hi Jens
> > 
> > Please would you consider taking this into the block tree? It seems to 
> > speed up device deletion enormously.
> > 
> > Mikulas
> > 
> > ---
> > 
> > backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu
> > 
> > synchronize_rcu sleeps several timer ticks. synchronize_rcu_expedited is 
> > much faster.
> > 
> > With 100Hz timer frequency, when we remove 10000 block devices with 
> > "dmsetup remove_all" command, it takes 27 minutes. With this patch, 
> > removing 10000 block devices takes only 15 seconds.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > 
> > ---
> >  mm/backing-dev.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > Index: linux-3.0-rc7-fast/mm/backing-dev.c
> > ===================================================================
> > --- linux-3.0-rc7-fast.orig/mm/backing-dev.c	2011-07-19 18:01:00.000000000 +0200
> > +++ linux-3.0-rc7-fast/mm/backing-dev.c	2011-07-19 18:01:07.000000000 +0200
> > @@ -505,7 +505,7 @@ static void bdi_remove_from_list(struct 
> >  	list_del_rcu(&bdi->bdi_list);
> >  	spin_unlock_bh(&bdi_lock);
> >  
> > -	synchronize_rcu();
> > +	synchronize_rcu_expedited();
> >  }
> >  
> 
> Urgh, I just noticed this crap in my tree.. You realize that what you're
> effectively hammering a global sync primitive this way? Depending on
> what RCU flavour you have any SMP variant will at least do a machine
> wide IPI broadcast for every sync_rcu_exp(), some do significantly more.
> 
> The much better solution would've been to batch your block-dev removals
> and use a single sync_rcu as barrier.
> 
> This is not cool.

Do you have some measurable use case where the user is removing block 
devices so heavily that this causes a problem?

Mikulas

Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[Peter Zijlstra]

On Thu, 2012-02-02 at 15:43 -0500, Mikulas Patocka wrote:
> Do you have some measurable use case where the user is removing block 
> devices so heavily that this causes a problem? 

Even one can be a problem, we're having people spend lots of time and
effort to reduce machine wide jitter and interference. Adding it with
such disregard isn't cool.

There's no reason a management cpu adding or removing block devices
should perturb the high-freq trading or industrial laser control running
on the other side of the machine.

Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu

[Paul E. McKenney]

On Thu, Feb 02, 2012 at 10:59:04PM +0100, Peter Zijlstra wrote:
> On Thu, 2012-02-02 at 15:43 -0500, Mikulas Patocka wrote:
> > Do you have some measurable use case where the user is removing block 
> > devices so heavily that this causes a problem? 
> 
> Even one can be a problem, we're having people spend lots of time and
> effort to reduce machine wide jitter and interference. Adding it with
> such disregard isn't cool.
> 
> There's no reason a management cpu adding or removing block devices
> should perturb the high-freq trading or industrial laser control running
> on the other side of the machine.

Very true for real-time applications!

For the heavy trading apps, given Frederic's upcoming user-mode-idle work,
I can keep this stuff from perturbing the apps.  Still, batching would
be preferable.

							Thanx, Paul