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

[Kirill Tkhai]

On 30.11.2017 03:27, Shakeel Butt wrote:
> 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?

Shakeel, thanks you for the testing! I'll resend the patch as "v2".