Re: [PATCH v3 0/8] make slab shrink lockless

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2023/2/28 03:20, Kirill Tkhai wrote:
> On 27.02.2023 18:08, Mike Rapoport wrote:
>> Hi,
>>
>> 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:
>>>>
>>>>> Hi all,
>>>>>
>>>>> This patch series aims to make slab shrink lockless.
>>>>
>>>> What an awesome changelog.
>>>>
>>>>> 2. Survey
>>>>> =========
>>>>
>>>> Especially this part.
>>>>
>>>> Looking through all the prior efforts and at this patchset I am not
>>>> immediately seeing any statements about the overall effect upon
>>>> real-world workloads.  For a good example, does this patchset
>>>> measurably improve throughput or energy consumption on your servers?
>>>
>>> Hi Andrew,
>>>
>>> I re-tested with the following physical machines:
>>>
>>> Architecture:        x86_64
>>> CPU(s):              96
>>> On-line CPU(s) list: 0-95
>>> Model name:          Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz
>>>
>>> I found that the reason for the hotspot I described in cover letter is
>>> wrong. The reason for the down_read_trylock() hotspot is not because of
>>> the failure to trylock, but simply because of the atomic operation
>>> (cmpxchg). And this will lead to a significant reduction in IPC (insn
>>> per cycle).
>>
>> ...
>>   
>>> 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
>>>
>>> 2. At the same time, we use the following perf command to capture IPC
>>> information:
>>>
>>> perf stat -e cycles,instructions -G test -a --repeat 5 -- sleep 10
>>>
>>> 1) Before applying this patchset:
>>>
>>>   Performance counter stats for 'system wide' (5 runs):
>>>
>>>        454187219766      cycles                    test                    (
>>> +-  1.84% )
>>>         78896433101      instructions              test #    0.17  insn per
>>> cycle           ( +-  0.44% )
>>>
>>>          10.0020430 +- 0.0000366 seconds time elapsed  ( +-  0.00% )
>>>
>>> 2) After applying this patchset:
>>>
>>>   Performance counter stats for 'system wide' (5 runs):
>>>
>>>        841954709443      cycles                    test                    (
>>> +- 15.80% )  (98.69%)
>>>        527258677936      instructions              test #    0.63  insn per
>>> cycle           ( +- 15.11% )  (98.68%)
>>>
>>>            10.01064 +- 0.00831 seconds time elapsed  ( +-  0.08% )
>>>
>>> We can see that IPC drops very seriously when calling
>>> down_read_trylock() at high frequency. After using SRCU,
>>> the IPC is at a normal level.
>>
>> The results you present do show improvement in IPC for an artificial test
>> script. But more interesting would be to see how a real world workloads
>> benefit from your changes.
> 
> One of the real workloads from my experience is start of an overcommitted node
> containing many starting containers after node crash (or many resuming containers
> after reboot for kernel update). In these cases memory pressure is huge, and
> the node goes round in long reclaim.

Thanks a lot for providing this real workload! :)

> 
> This patch patchset makes prealloc_memcg_shrinker() independent of do_shrink_slab(),
> so prealloc_memcg_shrinker() won't have to wait till shrink_slab_memcg() completes its
> current bit iteration, sees rwsem_is_contended() and the iteration breaks.
> 
> Also, it's important to mention that currently we have the strange behavior:
> 
> prealloc_memcg_shrinker()
>    down_write(&shrinker_rwsem)
>    idr_alloc()
>      reclaim
>        for each child memcg
>          shrink_slab_memcg()
>            down_read_trylock(&shrinker_rwsem) -> fail
> 
> All the slab reclaim in this behavior is just a parasite work, and it just wastes
> our cpu time, which does not look a good design.
> 
> Kirill

-- 
Thanks,
Qi