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: <Pine.LNX.4.64.0811100914500.14825@hs20-bc2-1.build.redhat.com>
Date:	Mon, 10 Nov 2008 09:19:21 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
cc:	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, 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.

Mikulas
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