linux-kernel - Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of synchronize

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Pine.LNX.4.64.1202021538570.9035@hs20-bc2-1.build.redhat.com>
Date:	Thu, 2 Feb 2012 15:43:01 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	axboe@...nel.dk, "Alasdair G. Kergon" <agk@...hat.com>,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH] backing-dev: use synchronize_rcu_expedited instead of
 synchronize_rcu



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@...hat.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/