On Wed 03-08-16 08:53:25, Mikulas Patocka wrote:

On Thu, 28 Jul 2016, Michal Hocko wrote:

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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

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