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

[Mikulas Patocka <mpatocka redhat com>]

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