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

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

On 2/26/19 3:50 PM, Andrey Ryabinin wrote:
> 
> 
> 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.
> 

On the second thought it seems to be better to keep the decision on lru level.
There are couple reasons for this:

1) Using bare node_page_state() (or sc->targe_mem_cgroup's total_[in]active counters) would be wrong.
 Because some cgroups might have protection set (memory.low) and we must take it into account. Also different
cgroups have different available swap space/memory.swappiness and it must be taken into account as well to.

So it has to be yet another full memcg-tree iteration.

2) Let's consider simple case. Two cgroups, one with big 'active' set of pages the other allocates one-time used pages.
So the total inactive is low, thus checking inactive ratio on higher level will result in reclaiming pages.
While with check on lru-level only inactive will be reclaimed.

I've tried to come up with a scenario in which checking ratio on higher level would better but failed.