Re: [PATCH] mm, slab: Extend slab/shrink to shrink all the memcg caches

[Vlastimil Babka]

On 7/23/19 4:30 PM, Waiman Long wrote:
> On 7/22/19 8:46 AM, peter enderborg wrote:
>> On 7/2/19 8:37 PM, Waiman Long wrote:
>>> Currently, a value of '1" is written to /sys/kernel/slab/<slab>/shrink
>>> file to shrink the slab by flushing all the per-cpu slabs and free
>>> slabs in partial lists. This applies only to the root caches, though.
>>>
>>> Extends this capability by shrinking all the child memcg caches and
>>> the root cache when a value of '2' is written to the shrink sysfs file.
>>>
>>> On a 4-socket 112-core 224-thread x86-64 system after a parallel kernel
>>> build, the the amount of memory occupied by slabs before shrinking
>>> slabs were:
>>>
>>>  # grep task_struct /proc/slabinfo
>>>  task_struct         7114   7296   7744    4    8 : tunables    0    0
>>>  0 : slabdata   1824   1824      0
>>>  # grep "^S[lRU]" /proc/meminfo
>>>  Slab:            1310444 kB
>>>  SReclaimable:     377604 kB
>>>  SUnreclaim:       932840 kB
>>>
>>> After shrinking slabs:
>>>
>>>  # grep "^S[lRU]" /proc/meminfo
>>>  Slab:             695652 kB
>>>  SReclaimable:     322796 kB
>>>  SUnreclaim:       372856 kB
>>>  # grep task_struct /proc/slabinfo
>>>  task_struct         2262   2572   7744    4    8 : tunables    0    0
>>>  0 : slabdata    643    643      0
>>
>> What is the time between this measurement points? Should not the shrinked memory show up as reclaimable?
> 
> In this case, I echoed '2' to all the shrink sysfs files under
> /sys/kernel/slab. The purpose of shrinking caches is to reclaim as much
> unused memory slabs from all the caches, irrespective if they are
> reclaimable or not.

Well, SReclaimable counts pages allocated by kmem caches with
SLAB_RECLAIM_ACCOUNT flags, which should match those that have a shrinker
associated and can thus actually reclaim objects. That shrinking slabs affected
SReclaimable just a bit while reducing SUnreclaim by more than 50% looks
certainly odd.
For example the task_struct cache is not a reclaimable one, yet shows massive
reduction. Could be that the reclaimable objects were pinning non-reclaimable
ones, so the shrinking had secondary effects in non-reclaimable caches.

> We do not reclaim any used objects. That is why we
> see the numbers were reduced in both cases.
> 
> Cheers,
> Longman
>