Re: [PATCH V2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes

[Jesper Dangaard Brouer <hawk@kernel.org>]

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On 27/06/2024 00.07, Yosry Ahmed wrote:
> On Wed, Jun 26, 2024 at 2:35 PM Jesper Dangaard Brouer <hawk@kernel.org> wrote:
>>
>> On 26/06/2024 00.59, Yosry Ahmed wrote:
>>> On Tue, Jun 25, 2024 at 3:35 PM Christoph Lameter (Ampere) <cl@linux.com> wrote:
>>>>
>>>> On Tue, 25 Jun 2024, Yosry Ahmed wrote:
>>>>
[...]
>>
>> I implemented a variant using completions as Yosry asked for:
>>
>> [V3] https://lore.kernel.org/all/171943668946.1638606.1320095353103578332.stgit@firesoul/
> 
> Thanks! I will take a look at this a little bit later. I am wondering
> if you could verify if that solution fixes the problem with kswapd
> flushing?

I will deploy V3 on some production metals and report back in that thread.

For this patch V2, I already have some results that show it solves the
kswapd lock contention. Attaching grafana screenshot comparing two
machines without/with this V2 patch. Green (16m1253) without patch, and
Yellow line (16m1254) with patched kernel.  These machines have 12 NUMA
nodes and thus 12 kswapd threads, and CPU time is summed for these threads.

Zooming in with perf record we can also see the lock contention is gone.
  - sudo perf record -g -p $(pgrep -d, kswapd) -F 499 sleep 60
  - sudo perf report --no-children  --call-graph graph,0.01,callee 
--sort symbol


On a machine (16m1254) with this V2 patch:

  Samples: 7K of event 'cycles:P', Event count (approx.): 61228473670
    Overhead  Symbol
  +    8.28%  [k] mem_cgroup_css_rstat_flush
  +    6.69%  [k] xfs_perag_get_tag
  +    6.51%  [k] radix_tree_next_chunk
  +    5.09%  [k] queued_spin_lock_slowpath
  +    3.94%  [k] srso_alias_safe_ret
  +    3.62%  [k] srso_alias_return_thunk
  +    3.11%  [k] super_cache_count
  +    2.96%  [k] mem_cgroup_iter
  +    2.95%  [k] down_read_trylock
  +    2.48%  [k] shrink_lruvec
  +    2.12%  [k] isolate_lru_folios
  +    1.76%  [k] dentry_lru_isolate
  +    1.74%  [k] radix_tree_gang_lookup_tag


On a machine (16m1253) without patch:

  Samples: 65K of event 'cycles:P', Event count (approx.): 492125554022
    Overhead  SymbolCoverage]
  +   55.84%  [k] queued_spin_lock_slowpath
    - 55.80% queued_spin_lock_slowpath
       + 53.10% __cgroup_rstat_lock
       + 2.63% evict
  +    7.06%  [k] mem_cgroup_css_rstat_flush
  +    2.07%  [k] page_vma_mapped_walk
  +    1.76%  [k] invalid_folio_referenced_vma
  +    1.72%  [k] srso_alias_safe_ret
  +    1.37%  [k] shrink_lruvec
  +    1.23%  [k] srso_alias_return_thunk
  +    1.17%  [k] down_read_trylock
  +    0.98%  [k] perf_adjust_freq_unthr_context
  +    0.97%  [k] super_cache_count

I think this (clearly) shows that the patch works and eliminates kswapd
lock contention.

--Jesper

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