Re: [dm-devel] [RFC PATCH 2/2] mm, mempool: do not throttle PF_LESS_THROTTLE tasks

[Michal Hocko <mhocko@kernel.org>]

On Wed 03-08-16 08:53:25, Mikulas Patocka wrote:
> 
> 
> On Thu, 28 Jul 2016, Michal Hocko wrote:
> 
> > > >> I think we'd end up with cleaner code if we removed the cute-hacks.  And
> > > >> we'd be able to use 6 more GFP flags!!  (though I do wonder if we really
> > > >> need all those 26).
> > > >
> > > > Well, maybe we are able to remove those hacks, I wouldn't definitely
> > > > be opposed.  But right now I am not even convinced that the mempool
> > > > specific gfp flags is the right way to go.
> > > 
> > > I'm not suggesting a mempool-specific gfp flag.  I'm suggesting a
> > > transient-allocation gfp flag, which would be quite useful for mempool.
> > > 
> > > Can you give more details on why using a gfp flag isn't your first choice
> > > for guiding what happens when the system is trying to get a free page
> > > :-?
> > 
> > If we get rid of throttle_vm_writeout then I guess it might turn out to
> > be unnecessary. There are other places which will still throttle but I
> > believe those should be kept regardless of who is doing the allocation
> > because they are helping the LRU scanning sane. I might be wrong here
> > and bailing out from the reclaim rather than waiting would turn out
> > better for some users but I would like to see whether the first approach
> > works reasonably well.
> 
> If we are swapping to a dm-crypt device, the dm-crypt device is congested 
> and the underlying block device is not congested, we should not throttle 
> mempool allocations made from the dm-crypt workqueue. Not even a little 
> bit.

But the device congestion is not the only condition required for the
throttling. The pgdat has also be marked congested which means that the
LRU page scanner bumped into dirty/writeback/pg_reclaim pages at the
tail of the LRU. That should only happen if we are rotating LRUs too
quickly. AFAIU the reclaim shouldn't allow free ticket scanning in that
situation.

> So, I think, mempool_alloc should set PF_NO_THROTTLE (or 
> __GFP_NO_THROTTLE).

As I've said earlier that would probably require to bail out from the
reclaim if we detect a potential pgdat congestion. What do you think
Mel?
-- 
Michal Hocko
SUSE Labs