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

[Mikulas Patocka <mpatocka@redhat.com>]

On Sun, 14 Aug 2016, Michal Hocko wrote:

> On Sat 13-08-16 13:34:29, Mikulas Patocka wrote:
> > 
> > 
> > On Fri, 12 Aug 2016, Michal Hocko wrote:
> > 
> > > On Thu 04-08-16 14:49:41, Mikulas Patocka wrote:
> > > 
> > > > On Wed, 3 Aug 2016, Michal Hocko wrote:
> > > > 
> > > > > 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.
> > > 
> > > I agree that mempool_alloc should _primarily_ sleep on their own
> > > throttling mechanism. I am not questioning that. I am just saying that
> > > the page allocator has its own throttling which it relies on and that
> > > cannot be just ignored because that might have other undesirable side
> > > effects. So if the right approach is really to never throttle certain
> > > requests then we have to bail out from a congested nodes/zones as soon
> > > as the congestion is detected.
> > > 
> > > Now, I would like to see that something like that is _really_ necessary.
> > 
> > Currently, it is not a problem - device mapper reports the device as 
> > congested only if the underlying physical disks are congested.
> > 
> > But once we change it so that device mapper reports congested state on its 
> > own (when it has too many bios in progress), this starts being a problem.
> 
> OK, can we wait until it starts becoming a real problem and solve it
> appropriately then?
> 
> I will repost the patch which removes thottle_vm_pageout in the meantime
> as it doesn't seem to be needed anymore.
> 
> -- 
> Michal Hocko
> SUSE Labs

Hi Michal

So, here Google developers hit a stacktrace where a block device driver is 
being throttled in the memory management:

https://www.redhat.com/archives/dm-devel/2016-November/msg00158.html

dm-bufio layer is something like a buffer cache, used by block device 
drivers. Unlike the real buffer cache, dm-bufio guarantees forward 
progress even if there is no memory free.

dm-bufio does something similar like a mempool allocation, it tries an 
allocation with GFP_NOIO | __GFP_NORETRY | __GFP_NOMEMALLOC | __GFP_NOWARN 
(just like a mempool) and if it fails, it will reuse some existing buffer.

Here, they caught it being throttled in the memory management:

   Workqueue: kverityd verity_prefetch_io
   __switch_to+0x9c/0xa8
   __schedule+0x440/0x6d8
   schedule+0x94/0xb4
   schedule_timeout+0x204/0x27c
   schedule_timeout_uninterruptible+0x44/0x50
   wait_iff_congested+0x9c/0x1f0
   shrink_inactive_list+0x3a0/0x4cc
   shrink_lruvec+0x418/0x5cc
   shrink_zone+0x88/0x198
   try_to_free_pages+0x51c/0x588
   __alloc_pages_nodemask+0x648/0xa88
   __get_free_pages+0x34/0x7c
   alloc_buffer+0xa4/0x144
   __bufio_new+0x84/0x278
   dm_bufio_prefetch+0x9c/0x154
   verity_prefetch_io+0xe8/0x10c
   process_one_work+0x240/0x424
   worker_thread+0x2fc/0x424
   kthread+0x10c/0x114

Will you consider removing vm throttling for __GFP_NORETRY allocations?

Mikulas

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