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

[Michal Hocko <mhocko@kernel.org>]

On Fri 12-04-19 09:32:55, Yafang Shao wrote:
> On Thu, Apr 11, 2019 at 11:10 PM Michal Hocko <mhocko@kernel.org> wrote:
> >
> > On Thu 11-04-19 21:54:22, Yafang Shao wrote:
> > > On Thu, Apr 11, 2019 at 9:39 PM Michal Hocko <mhocko@kernel.org> wrote:
> > > >
> > > > 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
> > >
> > > For example, the tasks in this memcg may encounter direct page reclaim
> > > due to system memory pressure,
> > > meaning it is stalling in page alloc slow path.
> > > At the same time, maybe there's no memory pressure in this memcg, I
> > > mean, it could succussfully charge memcg.
> >
> > And that is exactly what those events aim for. They are measuring
> > _where_ the memory pressure comes from.
> >
> > Can you please try to explain what do you want to achieve again?
> 
> To know the impact of this memory pressure.
> The current events can tell us the source of this pressure, but can't
> tell us the impact of this pressure.

Can you give me a more specific example how you are going to use this
counter in a real life please?
-- 
Michal Hocko
SUSE Labs