Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes

Michal Hocko
2018-01-11T16:29:47Z

On Thu 11-01-18 18:23:57, Andrey Ryabinin wrote:
> On 01/11/2018 03:46 PM, Michal Hocko wrote:
> > On Thu 11-01-18 15:21:33, Andrey Ryabinin wrote:
> >>
> >>
> >> On 01/11/2018 01:42 PM, Michal Hocko wrote:
> >>> On Wed 10-01-18 15:43:17, Andrey Ryabinin wrote:
> >>> [...]
> >>>> @@ -2506,15 +2480,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> >>>> if (!ret)
> >>>> break;
> >>>>
> >>>> - try_to_free_mem_cgroup_pages(memcg, 1, GFP_KERNEL, !memsw);
> >>>> -
> >>>> - curusage = page_counter_read(counter);
> >>>> - /* Usage is reduced ? */
> >>>> - if (curusage >= oldusage)
> >>>> - retry_count--;
> >>>> - else
> >>>> - oldusage = curusage;
> >>>> - } while (retry_count);
> >>>> + usage = page_counter_read(counter);
> >>>> + if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
> >>>> + GFP_KERNEL, !memsw)) {
> >>>
> >>> If the usage drops below limit in the meantime then you get underflow
> >>> and reclaim the whole memcg. I do not think this is a good idea. This
> >>> can also lead to over reclaim. Why don't you simply stick with the
> >>> original SWAP_CLUSTER_MAX (aka 1 for try_to_free_mem_cgroup_pages)?
> >>>
> >>
> >> Because, if new limit is gigabytes bellow the current usage, retrying to set
> >> new limit after reclaiming only 32 pages seems unreasonable.
> >
> > Who would do insanity like that?
> >
>
> What's insane about that?

I haven't seen this being done in practice. Why would you want to
reclaim GBs of memory from a cgroup? Anyway, if you believe this is
really needed then simply do it in a separate patch.

> >> @@ -2487,8 +2487,8 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
> >> if (!ret)
> >> break;
> >>
> >> - usage = page_counter_read(counter);
> >> - if (!try_to_free_mem_cgroup_pages(memcg, usage - limit,
> >> + nr_pages = max_t(long, 1, page_counter_read(counter) - limit);
> >> + if (!try_to_free_mem_cgroup_pages(memcg, nr_pages,
> >> GFP_KERNEL, !memsw)) {
> >> ret = -EBUSY;
> >> break;
> >
> > How does this address the over reclaim concern?
>
> It protects from over reclaim due to underflow.

I do not think so. Consider that this reclaim races with other
reclaimers. Now you are reclaiming a large chunk so you might end up
reclaiming more than necessary. SWAP_CLUSTER_MAX would reduce the over
reclaim to be negligible.
--
Michal Hocko
SUSE Labs