Re: [PATCH] mm/memcg: add allocstall to memory.stat

[Michal Hocko <mhocko@kernel.org>]

On Thu 11-04-19 20:41:32, Yafang Shao wrote:
> On Thu, Apr 11, 2019 at 8:27 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Thu 11-04-19 19:59:51, Yafang Shao wrote:
> > > The current item 'pgscan' is for pages in the memcg,
> > > which indicates how many pages owned by this memcg are scanned.
> > > While these pages may not scanned by the taskes in this memcg, even for
> > > PGSCAN_DIRECT.
> > >
> > > Sometimes we need an item to indicate whehter the tasks in this memcg
> > > under memory pressure or not.
> > > So this new item allocstall is added into memory.stat.
> >
> > We do have memcg events for that purpose and those can even tell whether
> > the pressure is a result of high or hard limit. Why is this not
> > sufficient?
> >
> 
> The MEMCG_HIGH and MEMCG_LOW may not be tiggered by the tasks in this
> memcg neither.
> They all reflect the memory status of a memcg, rather than tasks
> activity in this memcg.

I do not follow. Can you give me an example when does this matter? I
thought it is more important to see that there is a reclaim activity
for a specific memcg as you account for that memcg.
If you want to see/measure a reclaim imposed latency on a task then
the counter doesn't make so much sense as you have no way to match that
to a task. We have tracepoints for that purpose.
-- 
Michal Hocko
SUSE Labs