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