Re: [PATCH v1] mm:memcg: skip memcg of current in mem_cgroup_soft_limit_reclaim

From: Michal Hocko
Date: Fri Aug 03 2018 - 03:07:25 EST

On Fri 03-08-18 14:59:34, Zhaoyang Huang wrote:
> On Fri, Aug 3, 2018 at 2:18 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> >
> > On Fri 03-08-18 14:11:26, Zhaoyang Huang wrote:
> > > On Fri, Aug 3, 2018 at 1:48 PM Zhaoyang Huang <huangzhaoyang@xxxxxxxxx> wrote:
> > > >
> > > > for the soft_limit reclaim has more directivity than global reclaim, we40960
> > > > have current memcg be skipped to avoid potential page thrashing.
> > > >
> > > The patch is tested in our android system with 2GB ram. The case
> > > mainly focus on the smooth slide of pictures on a gallery, which used
> > > to stall on the direct reclaim for over several hundred
> > > millionseconds. By further debugging, we find that the direct reclaim
> > > spend most of time to reclaim pages on its own with softlimit set to
> > > 40960KB. I add a ftrace event to verify that the patch can help
> > > escaping such scenario. Furthermore, we also measured the major fault
> > > of this process(by dumpsys of android). The result is the patch can
> > > help to reduce 20% of the major fault during the test.
> >
> > I have asked already asked. Why do you use the soft limit in the first
> > place? It is known to cause excessive reclaim and long stalls.
> It is required by Google for applying new version of android system.
> There was such a mechanism called LMK in previous ANDROID version,
> which will kill process when in memory contention like OOM does. I
> think Google want to drop such rough way for reclaiming pages and turn
> to memcg. They setup different memcg groups for different process of
> the system and set their softlimit according to the oom_adj. Their
> original purpose is to reclaim pages gentlely in direct reclaim and
> kswapd. During the debugging process , it seems to me that memcg maybe
> tunable somehow. At least , the patch works on our system.

Then the suggestion is to use v2 and the high limit. This is much less
disruptive method for pro-active reclaim. Really softlimit semantic is
established for many years and you cannot change it even when it sucks
for your workload. Others might depend on the traditional behavior.

I have tried to change the semantic in the past and there was a general
consensus that changing the semantic is just too risky. So it is nice
that it helps for your particular workload but this is not an upstream
material, I am sorry.

Michal Hocko