Re: çå: çå: [PATCH] mm/memcontrol.c: speed up to force empty a memory cgroup

[David Rientjes]

On Mon, 19 Mar 2018, Li,Rongqing wrote:

> > > Although SWAP_CLUSTER_MAX is used at the lower level, but the call
> > > stack of try_to_free_mem_cgroup_pages is too long, increase the
> > > nr_to_reclaim can reduce times of calling
> > > function[do_try_to_free_pages, shrink_zones, hrink_node ]
> > >
> > > mem_cgroup_resize_limit
> > > --->try_to_free_mem_cgroup_pages:  .nr_to_reclaim = max(1024,
> > > --->SWAP_CLUSTER_MAX),
> > >    ---> do_try_to_free_pages
> > >      ---> shrink_zones
> > >       --->shrink_node
> > >        ---> shrink_node_memcg
> > >          ---> shrink_list          <-------loop will happen in this place
> > [times=1024/32]
> > >            ---> shrink_page_list
> > 
> > Can you actually measure this to be the culprit. Because we should rethink
> > our call path if it is too complicated/deep to perform well.
> > Adding arbitrary batch sizes doesn't sound like a good way to go to me.
> 
> Ok, I will try
> 

Looping in mem_cgroup_resize_limit(), which takes memcg_limit_mutex on 
every iteration which contends with lowering limits in other cgroups (on 
our systems, thousands), calling try_to_free_mem_cgroup_pages() with less 
than SWAP_CLUSTER_MAX is lame.  It would probably be best to limit the 
nr_pages to the amount that needs to be reclaimed, though, rather than 
over reclaiming.

If you wanted to be invasive, you could change page_counter_limit() to 
return the count - limit, fix up the callers that look for -EBUSY, and 
then use max(val, SWAP_CLUSTER_MAX) as your nr_pages.