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

[Andrey Ryabinin]

On 3/1/19 8:49 PM, Johannes Weiner wrote:
> Hello Andrey,
> 
> On Fri, Mar 01, 2019 at 01:38:26PM +0300, Andrey Ryabinin wrote:
>> 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.
> 
> It should be possible to take that into account on the first iteration
> and adjust the inactive/active counters in proportion to how much of
> the cgroup's total memory is exempt by memory.low or min, right?
> 

Should be possible, more complexity though to this subtle code.


>> 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.
> 
> It's the other way around. Let's say you have two cgroups, A and B:
> 
>         A:  500M inactive   10G active -> inactive is low
>         B:   10G inactive  500M active -> inactive is NOT low
>    ----------------------------------------------------------
>    global: 10.5G inactive 10.5G active -> inactive is NOT low
> 
> Checking locally will scan active pages from A.

No, checking locally will not scan active from A. Initial state of sc->may_shrink_active = 0, so A group
will be skipped completely, and will reclaim from B. Since overall reclaim was successful, sc->may_shrink_active remain 0
and A will be protected as long as B supply enough inactive pages. 

> Checking globally will
> not, because there is plenty of use-once pages from B.
> 

That is correct. So in this example global vs local check will not make a difference.

> So if you check globally, without any protection, A and B compete
> evenly during global reclaim. Under the same reclaim pressure, A has
> managed to activate most of its pages whereas B has not. That means A
> is hotter and B provides the better reclaim candidates.
> 
> If you apply this decision locally, on the other hand, you are no
> longer aging the groups at the same rate. And then the LRU orders
> between groups will no longer be comparable, and you won't be
> reclaiming the coldest memory in the system anymore.
> 

I really don't see any how global check will make any difference in this example.
In both cases, we reclaim only from B and don't touch A. And this what we actually want.