linux-kernel - Re: Queue upcall locking (was: [dm-devel] [RFC][PATCH] fix dm_any_congested() to properly sync up with suspend code path)

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Message-Id: <1226328417.7685.198.camel@twins>
Date:	Mon, 10 Nov 2008 15:46:57 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Chandra Seetharaman <sekharan@...ibm.com>,
	Alasdair G Kergon <agk@...hat.com>,
	dm-devel <dm-devel@...hat.com>, linux-kernel@...r.kernel.org,
	axboe@...nel.dk
Subject: Re: Queue upcall locking (was: [dm-devel] [RFC][PATCH] fix
 dm_any_congested() to properly sync up with suspend code path)

On Mon, 2008-11-10 at 09:32 -0500, Mikulas Patocka wrote:
> 
> On Mon, 10 Nov 2008, Peter Zijlstra wrote:
> 
> > On Mon, 2008-11-10 at 09:19 -0500, Mikulas Patocka wrote:
> > > On Mon, 10 Nov 2008, Christoph Hellwig wrote:
> > > 
> > > > On Mon, Nov 10, 2008 at 08:11:51AM -0500, Mikulas Patocka wrote:
> > > > > For upstream Linux developers: you are holding a spinlock and calling 
> > > > > bdi*_congested functions that can take indefinite amount of time (there 
> > > > > are even users reporting having 50 disks in one logical volume or so). I 
> > > > > think it would be good to move these calls out of spinlocks.
> > > > 
> > > > Umm, they shouldn't block that long, as that completely defeats their
> > > > purpose.  These functions are mostly used to avoid throwing more I/O at
> > > > a congested device if pdflush could do more useful things instead.  But
> > > > if it blocks in those functions anyway we wouldn't have to bother using
> > > > them.  Do you have more details about the uses cases when this happens
> > > > and where the routines spend so much time?
> > > 
> > > For device mapper, congested_fn asks every device in the tree and make OR 
> > > of their bits --- so if the user has 50 devices, it asks them all.
> > > 
> > > For md-linear, md-raid0, md-raid1, md-raid10 and md-multipath it does the 
> > > same --- asking every device.
> > > 
> > > If you have a better idea how to implement congested_fn, say it.
> > 
> > Fix the infrastructure by adding a function call so that you can have
> > the individual devices report their congestion state to the aggregate.
> > 
> > Then congestion_fn can return a valid state in O(1) because the state is
> > keps up-to-date by the individual state changes.
> > 
> > IOW, add a set_congested_fn() and clear_congested_fn().
> 
> If you have a physical disk that has many LVM volumes on it, you end up in 
> a situation when disk congestion state change is reported to all the 
> volumes. So it will create O(n) problem at the other side.

*sigh* I can almost understand why people want to use lvm to combine
multiple disks, but why make the partition thing even worse...
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