Re: [PATCH 3/3] mm: improvements on memcg protection functions

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

On Mon, Apr 27, 2020 at 7:24 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 27-04-20 19:06:52, Yafang Shao wrote:
> > On Mon, Apr 27, 2020 at 6:50 PM Michal Hocko <mhocko@kernel.org> wrote:
> > >
> > > On Mon 27-04-20 18:09:27, Yafang Shao wrote:
> > > > On Mon, Apr 27, 2020 at 5:40 PM Michal Hocko <mhocko@kernel.org> wrote:
> > > > >
> > > > > On Sat 25-04-20 11:24:18, Yafang Shao wrote:
> > > > > > Since proportional memory.{min, low} reclaim is introduced in
> > > > > > commit 9783aa9917f8 ("mm, memcg: proportional memory.{low,min} reclaim"),
> > > > > > it have been proved that the proportional reclaim is hard to understand and
> > > > > > the issues caused by it is harder to understand.[1]. That dilemma faced by
> > > > > > us is caused by that the proportional reclaim mixed up memcg and the
> > > > > > reclaim context.
> > > > > >
> > > > > > In proportional reclaim, the whole reclaim context - includes the memcg
> > > > > > to be reclaimed and the reclaimer, should be considered, rather than
> > > > > > memcg only.
> > > > > >
> > > > > > To make it clear, a new member 'protection' is introduced in the reclaim
> > > > > > context (struct shrink_control) to replace mem_cgroup_protection(). This
> > > > >
> > > > > s@shrink_control@scan_control@
> > > > >
> > > >
> > > > Thanks for pointing this out.
> > > >
> > > > > > one is set when we check whether the memcg is protected or not.
> > > > > >
> > > > > > After this change, the issue pointed by me[1] - a really old left-over
> > > > > > value can slow donw target reclaim - can be fixed, and I think it could
> > > > > > also avoid some potential race.
> > > > >
> > > > > The patch would have been much esier to review if you only focused on
> > > > > the effective protection value caching. I really fail to see why you had
> > > > > to make mem_cgroup_protected even more convoluted with more side effects
> > > > > (e.g. sc->memcg_low_skipped). This goes directly opposite to what
> > > > > Johannes was proposing in other email AFAICS.
> > > > >
> > > >
> > > > Sorry, I failed to see what the advantage of Johannes's proposal
> > > > except the better naming.
> > >
> > > The immediate advantage is that predicate should better not have any
> > > side effect.
> >
> > No, it still has side effect. That is not an immediate advantage neither.
> > See bellow,
>
> I believe you have misunderstood the proposal.
> mem_cgroup_below_{min,low,protection} won't have any side effect on the
> memcg. It would be only mem_cgroup_calculate_protection which updates
> the cached state. So there won't be any predicate to have side effect.
>

Maybe  I misunderstood this porposal, or both of us misunderstood this
proposal.

> > > So splitting into the calculation part which has clearly
> > > defined side effects and having a simple predicate that consults
> > > pre-calculated values makes a lot of sense to me.
> > >
> >
> > When you calculate the effective values,  the source values to
> > calculate it  may be changed with these values when you do the
> > predicate.
> > IOW, this proposal greatly increase the race window.
>
> Why do you think so?
>

See above, I don't think it is proper to disccuss it any more until we
see the code.

> > > > > Your changelog doesn't explain why double caching the effective value is
> > > > > an improvement.
> > > >
> > > > The improvement is, to avoid getting an wrong value calculated by
> > > > other reclaimers and avoid issues in mem_cgroup_protection() that we
> > > > haven't noticed.
> > >
> > > This is not true in general. There is still parallel calculation done
> > > and so parallel reclaimers might affect each other. Your patch only
> > > makes a real difference for leaf memcgs which are the reclaim target as
> > > well.
> >
> > The race between  parallel reclaimers is fundamentally exist, and we
> > can do now is reducing the race window as far as possible.
>
> Reducing the race window is futile. The situation changes after each
> reclaim attempt. All we need is to keep a protection consistent
> regardless of where the reclaim root. That means that hierarchies deeper
> in the tree cannot override the protection from those which are higher.
>

That is what I have implemented in this patchset.

> > > All intermediate nodes really do not care about the cached values
> > > because they do not have any pages on the LRU lists.
> > >
> >
> > But read these cached values can save lot of time against the existing
> > code, as the existing code still calculates them even if they are
> > useless.
>
> They are not useless. Intermediate values are necessary for the
> protection distribution for lower level memcgs.
>



-- 
Thanks
Yafang