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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>