On 2/22/19 10:15 PM, Johannes Weiner wrote:
> On Fri, Feb 22, 2019 at 08:58:25PM +0300, Andrey Ryabinin wrote:
>> In a presence of more than 1 memory cgroup in the system our reclaim
>> logic is just suck. When we hit memory limit (global or a limit on
>> cgroup with subgroups) we reclaim some memory from all cgroups.
>> This is sucks because, the cgroup that allocates more often always wins.
>> E.g. job that allocates a lot of clean rarely used page cache will push
>> out of memory other jobs with active relatively small all in memory
>> working set.
>>
>> To prevent such situations we have memcg controls like low/max, etc which
>> are supposed to protect jobs or limit them so they to not hurt others.
>> But memory cgroups are very hard to configure right because it requires
>> precise knowledge of the workload which may vary during the execution.
>> E.g. setting memory limit means that job won't be able to use all memory
>> in the system for page cache even if the rest the system is idle.
>> Basically our current scheme requires to configure every single cgroup
>> in the system.
>>
>> I think we can do better. The idea proposed by this patch is to reclaim
>> only inactive pages and only from cgroups that have big
>> (!inactive_is_low()) inactive list. And go back to shrinking active lists
>> only if all inactive lists are low.
>
> Yes, you are absolutely right.
>
> We shouldn't go after active pages as long as there are plenty of
> inactive pages around. That's the global reclaim policy, and we
> currently fail to translate that well to cgrouped systems.
>
> Setting group protections or limits would work around this problem,
> but they're kind of a red herring. We shouldn't ever allow use-once
> streams to push out hot workingsets, that's a bug.
>
>> @@ -2489,6 +2491,10 @@ static void get_scan_count(struct lruvec *lruvec,
>> struct mem_cgroup *memcg,
>>
>> scan >>= sc->priority;
>>
>> + if (!sc->may_shrink_active && inactive_list_is_low(lruvec,
>> + file, memcg, sc, false))
>> + scan = 0;
>> +
>> /*
>> * If the cgroup's already been deleted, make sure to
>> * scrape out the remaining cache.
>> @@ -2733,6 +2739,7 @@ static bool shrink_node(pg_data_t *pgdat, struct
>> scan_control *sc)
>> struct reclaim_state *reclaim_state = current->reclaim_state;
>> unsigned long nr_reclaimed, nr_scanned;
>> bool reclaimable = false;
>> + bool retry;
>>
>> do {
>> struct mem_cgroup *root = sc->target_mem_cgroup;
>> @@ -2742,6 +2749,8 @@ static bool shrink_node(pg_data_t *pgdat, struct
>> scan_control *sc)
>> };
>> struct mem_cgroup *memcg;
>>
>> + retry = false;
>> +
>> memset(&sc->nr, 0, sizeof(sc->nr));
>>
>> nr_reclaimed = sc->nr_reclaimed;
>> @@ -2813,6 +2822,13 @@ static bool shrink_node(pg_data_t *pgdat, struct
>> scan_control *sc)
>> }
>> } while ((memcg = mem_cgroup_iter(root, memcg, &reclaim)));
>>
>> + if ((sc->nr_scanned - nr_scanned) == 0 &&
>> + !sc->may_shrink_active) {
>> + sc->may_shrink_active = 1;
>> + retry = true;
>> + continue;
>> + }
>
> Using !scanned as the gate could be a problem. There might be a cgroup
> that has inactive pages on the local level, but when viewed from the
> system level the total inactive pages in the system might still be low
> compared to active ones. In that case we should go after active pages.
>
> Basically, during global reclaim, the answer for whether active pages
> should be scanned or not should be the same regardless of whether the
> memory is all global or whether it's spread out between cgroups.
>
> The reason this isn't the case is because we're checking the ratio at
> the lruvec level - which is the highest level (and identical to the
> node counters) when memory is global, but it's at the lowest level
> when memory is cgrouped.
>
> So IMO what we should do is:
>
> - At the beginning of global reclaim, use node_page_state() to compare
> the INACTIVE_FILE:ACTIVE_FILE ratio and then decide whether reclaim
> can go after active pages or not. Regardless of what the ratio is in
> individual lruvecs.
>
> - And likewise at the beginning of cgroup limit reclaim, walk the
> subtree starting at sc->target_mem_cgroup, sum up the INACTIVE_FILE
> and ACTIVE_FILE counters, and make inactive_is_low() decision on
> those sums.
>
Sounds reasonable.
