Re: [PATCH resend] mm, memcg: reset memcg's memory.{min, low} for reclaiming itself

[Yafang Shao <laoar.shao@gmail.com>]

On Mon, Feb 17, 2020 at 10:04 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 17-02-20 21:51:23, Yafang Shao wrote:
> > On Mon, Feb 17, 2020 at 9:24 PM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > On Mon 17-02-20 21:08:12, Yafang Shao wrote:
> > > > On Mon, Feb 17, 2020 at 5:25 PM Michal Hocko <mhocko@kernel.org> wrote:
> > > > >
> > > > > On Sun 16-02-20 09:52:49, Yafang Shao wrote:
> > > > > > memory.{emin, elow} are set in mem_cgroup_protected(), and the values of
> > > > > > them won't be changed until next recalculation in this function. After
> > > > > > either or both of them are set, the next reclaimer to relcaim this memcg
> > > > > > may be a different reclaimer, e.g. this memcg is also the root memcg of
> > > > > > the new reclaimer, and then in mem_cgroup_protection() in get_scan_count()
> > > > > > the old values of them will be used to calculate scan count, that is not
> > > > > > proper. We should reset them to zero in this case.
> > > > > >
> > > > > > Here's an example of this issue.
> > > > > >
> > > > > >     root_mem_cgroup
> > > > > >          /
> > > > > >         A   memory.max=1024M memory.min=512M memory.current=800M
> > > > > >
> > > > > > Once kswapd is waked up, it will try to scan all MEMCGs, including
> > > > > > this A, and it will assign memory.emin of A with 512M.
> > > > > > After that, A may reach its hard limit(memory.max), and then it will
> > > > > > do memcg reclaim. Because A is the root of this reclaimer, so it will
> > > > > > not calculate its memory.emin. So the memory.emin is the old value
> > > > > > 512M, and then this old value will be used in
> > > > > > mem_cgroup_protection() in get_scan_count() to get the scan count.
> > > > > > That is not proper.
> > > > >
> > > > > Please document user visible effects of this patch. What does it mean
> > > > > that this is not proper behavior?
> > > >
> > > > In the memcg reclaim, if the target memcg is the root of the reclaimer,
> > > > the reclaimer should scan this memcg's all page cache pages in the LRU,
> > > > but now as the old memcg.{emin, elow} value are still there, it will get
> > > > a wrong protection value,
> > > > and the reclaimer can't reclaim the page cache pages protected by this
> > > > wrong protection.
> > >
> > > Could you be more specific please. Your example above says that emin is
> > > not going to be recalculated and stays at 512M even for a potential max
> > > limit reclaim. The min limit is still 512M so why is this value wrong?
> > >
> >
> > Because the relcaimers are changed or the root the relcaimer is changed.
> >
> > Kswapd begins to relcaim memcg-A.
> > kswapd
> >   |
> > calculate the {emin, elow} for memcg-A
> >  |
> > stores {emin, elow} in memory.{emin, elow} of memcg-A
> > |
> > This memory.{emin, elow} will protect the page cache pages in memcg-A
> > (See get_scan_count->mem_cgroup_protection)
> > |
> > exit
> > (And it won't relcaim memcg-A for a long time)
> >
> >
> > Then memcg relcaimer is woke up (reached the hard limit of memcg-A),
> > and the root of this new reclaimer is memcg-A.
> >
> > This memcg relcaimer begins to reclaim memcg-A.
> > memcg relcaimer
> >       |
> > As the root of the relcaimer is memcg-A, it won't calculate emin, elow
> > for memcg-A.
> > (See if (memcg == root) in mem_cgroup_protected())
> >      |
> > The old memory.{emin, elow} will protect the page cache pages in memcg-A
> > (SO WE SHOULD CLEAR THE OLD VALUE)
>
> I am sorry but I still do not follow. Could you focus on _why_ the old
> value is no longer valid?

Because for the new reclaimer the memory.{emin, elow} should be 0.
The old value may be not 0, but it was thought as 0 in the if
statement (if (memcg == root)).

>
> Btw. have you seen the latest patch from Johannes touching this area
> [1]? Is it possible that the issue you are referring to is related with
> the one he has fixed?
>
> [1] http://lkml.kernel.org/r/20191219200718.15696-2-hannes@cmpxchg.org
>

I haven't taken a look at it yet.


-- 
Yafang Shao
DiDi