Re: [PATCH] mm: Make count list_lru_one::nr_items lockless

From: Shakeel Butt
Date: Wed Nov 29 2017 - 19:27:52 EST


On Fri, Sep 29, 2017 at 1:15 AM, Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> On 29.09.2017 00:02, Andrew Morton wrote:
>> On Thu, 28 Sep 2017 10:48:55 +0300 Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>>
>>>>> This patch aims to make super_cache_count() (and other functions,
>>>>> which count LRU nr_items) more effective.
>>>>> It allows list_lru_node::memcg_lrus to be RCU-accessed, and makes
>>>>> __list_lru_count_one() count nr_items lockless to minimize
>>>>> overhead introduced by locking operation, and to make parallel
>>>>> reclaims more scalable.
>>>>
>>>> And... what were the effects of the patch? Did you not run the same
>>>> performance tests after applying it?
>>>
>>> I've just detected the such high usage of shrink slab on production node. It's rather
>>> difficult to make it use another kernel, than it uses, only kpatches are possible.
>>> So, I haven't estimated how it acts on node's performance.
>>> On test node I see, that the patch obviously removes raw_spin_lock from perf profile.
>>> So, it's a little bit untested in this way.
>>
>> Well that's a problem. The patch increases list_lru.o text size by a
>> lot (4800->5696) which will have a cost. And we don't have proof that
>> any benefit is worth that cost. It shouldn't be too hard to cook up a
>> synthetic test to trigger memcg slab reclaim and then run a
>> before-n-after benchmark?
>
> Ok, then, please, ignore this for a while, I'll try to do it a little bit later.
>

I rebased this patch on linus tree (replacing kfree_rcu with call_rcu
as there is no kvfree_rcu) and did some experiments. I think the patch
is worth to be included.

Setup: running a fork-bomb in a memcg of 200MiB on a 8GiB and 4 vcpu
VM and recording the trace with 'perf record -g -a'.

The trace without the patch:

+ 34.19% fb.sh [kernel.kallsyms] [k] queued_spin_lock_slowpath
+ 30.77% fb.sh [kernel.kallsyms] [k] _raw_spin_lock
+ 3.53% fb.sh [kernel.kallsyms] [k] list_lru_count_one
+ 2.26% fb.sh [kernel.kallsyms] [k] super_cache_count
+ 1.68% fb.sh [kernel.kallsyms] [k] shrink_slab
+ 0.59% fb.sh [kernel.kallsyms] [k] down_read_trylock
+ 0.48% fb.sh [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
+ 0.38% fb.sh [kernel.kallsyms] [k] shrink_node_memcg
+ 0.32% fb.sh [kernel.kallsyms] [k] queue_work_on
+ 0.26% fb.sh [kernel.kallsyms] [k] count_shadow_nodes

With the patch:

+ 0.16% swapper [kernel.kallsyms] [k] default_idle
+ 0.13% oom_reaper [kernel.kallsyms] [k] mutex_spin_on_owner
+ 0.05% perf [kernel.kallsyms] [k] copy_user_generic_string
+ 0.05% init.real [kernel.kallsyms] [k] wait_consider_task
+ 0.05% kworker/0:0 [kernel.kallsyms] [k] finish_task_switch
+ 0.04% kworker/2:1 [kernel.kallsyms] [k] finish_task_switch
+ 0.04% kworker/3:1 [kernel.kallsyms] [k] finish_task_switch
+ 0.04% kworker/1:0 [kernel.kallsyms] [k] finish_task_switch
+ 0.03% binary [kernel.kallsyms] [k] copy_page


Kirill, can you resend your patch with this info or do you want me
send the rebased patch?