Message-ID: <c949c89a-0b0d-723f-4c60-f6374b1993f3@bytedance.com>
Date:   Tue, 28 Feb 2023 18:11:34 +0800
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Subject: Re: [PATCH v3 0/8] make slab shrink lockless
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To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Andrew Morton <akpm@linux-foundation.org>, tkhai@ya.ru,
        hannes@cmpxchg.org, shakeelb@google.com, mhocko@kernel.org,
        muchun.song@linux.dev, david@redhat.com, shy828301@gmail.com,
        sultan@kerneltoast.com, dave@stgolabs.net,
        penguin-kernel@I-love.SAKURA.ne.jp, paulmck@kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
References: <20230226144655.79778-1-zhengqi.arch@bytedance.com>
 <20230226115100.7e12bda7931dd65dbabcebe3@linux-foundation.org>
 <b7e8929c-8fd5-a248-f8a8-d9177fc01b4b@bytedance.com>
 <Y/z+P5cgeunD6u21@P9FQF9L96D>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
In-Reply-To: <Y/z+P5cgeunD6u21@P9FQF9L96D>
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On 2023/2/28 03:02, Roman Gushchin wrote:
> On Mon, Feb 27, 2023 at 09:31:51PM +0800, Qi Zheng wrote:
>>
>>
>> On 2023/2/27 03:51, Andrew Morton wrote:
>>> On Sun, 26 Feb 2023 22:46:47 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote:
>>>
>> Save the above script, then run test and touch commands.
>>
>> Then we can use the following perf command to view hotspots:
>>
>> perf top -U -F 999
>>
>> 1) Before applying this patchset:
>>
>>    32.31%  [kernel]           [k] down_read_trylock
>>    19.40%  [kernel]           [k] pv_native_safe_halt
>>    16.24%  [kernel]           [k] up_read
>>    15.70%  [kernel]           [k] shrink_slab
>>     4.69%  [kernel]           [k] _find_next_bit
>>     2.62%  [kernel]           [k] shrink_node
>>     1.78%  [kernel]           [k] shrink_lruvec
>>     0.76%  [kernel]           [k] do_shrink_slab
>>
>> 2) After applying this patchset:
>>
>>    27.83%  [kernel]           [k] _find_next_bit
>>    16.97%  [kernel]           [k] shrink_slab
>>    15.82%  [kernel]           [k] pv_native_safe_halt
>>     9.58%  [kernel]           [k] shrink_node
>>     8.31%  [kernel]           [k] shrink_lruvec
>>     5.64%  [kernel]           [k] do_shrink_slab
>>     3.88%  [kernel]           [k] mem_cgroup_iter
> 
> Not opposing the intention of the patchset in any way (I actually think
> it's a good idea to make the shrinkers list lockless), but looking at
> both outputs above I think that the main problem is not the contention on
> the semaphore, but the reason of this contention.

Yes, in the above scenario, there is indeed no lock contention problem.

> 
> It seems like often there is a long list of shrinkers which barely
> can reclaim any memory, but we're calling them again and again.
> In order to achieve real wins with real-life workloads, I guess
> it's what we should optimize.
> 
> Thanks!

-- 
Thanks,
Qi