Re: [PATCH RFC] mm/vmscan: try to protect active working set of cgroup from reclaim.

[Andrey Ryabinin]

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.