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

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 3 Aug 2016, Michal Hocko wrote:

> 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.

The obvious problem here is that mempool allocations should sleep in 
mempool_alloc() on &pool->wait (until someone returns some entries into 
the mempool), they should not sleep inside the page allocator.

Mikulas

> > 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
> 

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