Re: [RFC PATCH] memcg: use root_mem_cgroup when css is inherited

[Zhaoyang Huang]

On Tue, Aug 23, 2022 at 7:51 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Tue 23-08-22 17:20:59, Zhaoyang Huang wrote:
> > On Tue, Aug 23, 2022 at 4:33 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Tue 23-08-22 14:03:04, Zhaoyang Huang wrote:
> > > > On Tue, Aug 23, 2022 at 1:21 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > >
> > > > > On Tue 23-08-22 10:31:57, Zhaoyang Huang wrote:
> > > [...]
> > > > > > I would like to quote the comments from google side for more details
> > > > > > which can also be observed from different vendors.
> > > > > > "Also be advised that when you enable memcg v2 you will be using
> > > > > > per-app memcg configuration which implies noticeable overhead because
> > > > > > every app will have its own group. For example pagefault path will
> > > > > > regress by about 15%. And obviously there will be some memory overhead
> > > > > > as well. That's the reason we don't enable them in Android by
> > > > > > default."
> > > > >
> > > > > This should be reported and investigated. Because per-application memcg
> > > > > vs. memcg in general shouldn't make much of a difference from the
> > > > > performance side. I can see a potential performance impact for no-memcg
> > > > > vs. memcg case but even then 15% is quite a lot.
> > > > Less efficiency on memory reclaim caused by multi-LRU should be one of
> > > > the reason, which has been proved by comparing per-app memcg on/off.
> > > > Besides, theoretically workingset could also broken as LRU is too
> > > > short to compose workingset.
> > >
> > > Do you have any data to back these claims? Is this something that could
> > > be handled on the configuration level? E.g. by applying low limit
> > > protection to keep the workingset in the memory?
> > I don't think so. IMO, workingset works when there are pages evicted
> > from LRU and then refault which provide refault distance for pages.
> > Applying memcg's protection will have all LRU out of evicted which
> > make the mechanism fail.
>
> It is really hard to help you out without any actual data. The idea was
> though to use the low limit protection to adaptively configure
> respective memcgs to reduce refaults. You already have data about
> refaults ready so increasing the limit for often refaulting memcgs would
> reduce the trashing.
>
> [...]
> > > A.cgroup.controllers = memory
> > > A.cgroup.subtree_control = memory
> > >
> > > A/B.cgroup.controllers = memory
> > > A/B.cgroup.subtree_control = memory
> > > A/B/B1.cgroup.controllers = memory
> > >
> > > A/C.cgroup.controllers = memory
> > > A/C.cgroup.subtree_control = ""
> > > A/C/C1.cgroup.controllers = ""
> > Yes for above hierarchy and configuration.
> > >
> > > Is your concern that C1 is charged to A/C or that you cannot actually make
> > > A/C.cgroup.controllers = "" because you want to maintain memory in A?
> > > Because that would be breaking the internal node constrain rule AFAICS.
> > No. I just want to keep memory on B.
>
> That would require A to be without controllers which is not possible due
> to hierarchical constrain.
>
> > > Or maybe you just really want a different hierarchy where
> > > A == root_cgroup and want the memory acocunted in B
> > > (root/B.cgroup.controllers = memory) but not in C (root/C.cgroup.controllers = "")?
> > Yes.
> > >
> > > That would mean that C memory would be maintained on the global (root
> > > memcg) LRUs which is the only internal node which is allowed to have
> > > resources because it is special.
> > Exactly. I would like to have all groups like C which have no parent's
> > subtree_control = memory charge memory to root. Under this
> > implementation, memory under enabled group will be protected by
> > min/low while other groups' memory share the same LRU to have
> > workingset things take effect.
>
> One way to achieve that would be shaping the hierarchy the following way
>             root
>         /         \
> no_memcg[1]      memcg[2]
> ||||||||         |||||
> app_cgroups     app_cgroups
>
> with
> no_memcg.subtree_control = ""
> memcg.subtree_control = memory
>
> no?
According to my understanding, No as there will be no no_memcg. All
children groups under root would have its cgroup.controllers = memory
as long as root has memory enabled. Under this circumstance, all
descendants group under 'no_memcg' will charge memory to its parent
group. This is caused by e_css  policy when apply subsys control which
have child group use its first level ancestors css.
>
> You haven't really described why you need per application freezer cgroup
> but I suspect you want to selectively freeze applications. Is there
> any obstacle to have a dedicated frozen cgroup and migrate tasks to be
> frozen there?
> --
> Michal Hocko
> SUSE Labs