re: [PATCH] mm: avoid the unnecessary waiting when force empty a cgroup

[Li,Rongqing]

> -----邮件原件-----
> 发件人: Michal Hocko [mailto:mhocko@xxxxxxxxxx]
> 发送时间: 2018年4月3日 16:05
> 收件人: Li,Rongqing <lirongqing@xxxxxxxxx>
> 抄送: hannes@xxxxxxxxxxx; vdavydov.dev@xxxxxxxxx;
> cgroups@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx
> 主题: Re: [PATCH] mm: avoid the unnecessary waiting when force empty a
> cgroup
> 
> On Tue 03-04-18 15:12:09, Li RongQing wrote:
> > The number of writeback and dirty page can be read out from memcg, the
> > unnecessary waiting can be avoided by these counts
> 
> This changelog doesn't explain the problem and how the patch fixes it.

If a process in a memory cgroup takes some RSS, when force empty this memory cgroup, congestion_wait will be called unconditionally, there is 0.5 seconds delay

If use this patch, nearly no delay.


> Why do wee another throttling when we do already throttle in the reclaim
> path?

Do you mean we should remove congestion_wait(BLK_RW_ASYNC, HZ/10) from mem_cgroup_force_empty, since try_to_free_mem_cgroup_pages [shrink_inactive_list] has called congestion_wait


-RongQing

> 
> > Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> > ---
> >  mm/memcontrol.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c index
> > 9ec024b862ac..5258651bd4ec 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2613,9 +2613,13 @@ static int mem_cgroup_force_empty(struct
> mem_cgroup *memcg)
> >  		progress = try_to_free_mem_cgroup_pages(memcg, 1,
> >  							GFP_KERNEL, true);
> >  		if (!progress) {
> > +			unsigned long num;
> > +
> > +			num = memcg_page_state(memcg, NR_WRITEBACK) +
> > +					memcg_page_state(memcg, NR_FILE_DIRTY);
> >  			nr_retries--;
> > -			/* maybe some writeback is necessary */
> > -			congestion_wait(BLK_RW_ASYNC, HZ/10);
> > +			if (num)
> > +				congestion_wait(BLK_RW_ASYNC, HZ/10);
> >  		}
> >
> >  	}
> > --
> > 2.11.0
> 
> --
> Michal Hocko
> SUSE Labs