Re: [PATCH v4 1/4] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags

Vlastimil Babka
2020-09-03T09:00:34Z

On 9/3/20 10:40 AM, Alex Shi wrote:
>
>
> 在 2020/9/3 下午4:32, Alex Shi 写道:
>>>
>> I have run thpscale with 'always' defrag setting of THP. The Amean stddev is much
>> larger than a very little average run time reducing.
>>
>> But the left patch 4 could show the cmpxchg retry reduce from thousands to hundreds
>> or less.
>>
>> Subject: [PATCH v4 4/4] add cmpxchg tracing
>
>
> It's a typical result with the patchset:
>
> Performance counter stats for './run-mmtests.sh -c configs/config-workload-thpscale pageblock-c':
>
> 9,564 compaction:mm_compaction_isolate_migratepages
> 6,430 compaction:mm_compaction_isolate_freepages
> 5,287 compaction:mm_compaction_migratepages
> 45,299 compaction:mm_compaction_begin
> 45,299 compaction:mm_compaction_end
> 30,557 compaction:mm_compaction_try_to_compact_pages
> 95,540 compaction:mm_compaction_finished
> 149,379 compaction:mm_compaction_suitable
> 0 compaction:mm_compaction_deferred
> 0 compaction:mm_compaction_defer_compaction
> 3,949 compaction:mm_compaction_defer_reset
> 0 compaction:mm_compaction_kcompactd_sleep
> 0 compaction:mm_compaction_wakeup_kcompactd
> 0 compaction:mm_compaction_kcompactd_wake
> 68 pageblock:hit_cmpxchg
>
> 113.570974583 seconds time elapsed
>
> 14.664451000 seconds user
> 96.847116000 seconds sys
>
> It's 5.9-rc2 base kernel result:
>
> Performance counter stats for './run-mmtests.sh -c configs/config-workload-thpscale rc2-e':
>
> 15,920 compaction:mm_compaction_isolate_migratepages
> 20,523 compaction:mm_compaction_isolate_freepages
> 9,752 compaction:mm_compaction_migratepages
> 27,773 compaction:mm_compaction_begin
> 27,773 compaction:mm_compaction_end
> 16,391 compaction:mm_compaction_try_to_compact_pages
> 62,809 compaction:mm_compaction_finished
> 69,821 compaction:mm_compaction_suitable
> 0 compaction:mm_compaction_deferred
> 0 compaction:mm_compaction_defer_compaction
> 7,875 compaction:mm_compaction_defer_reset
> 0 compaction:mm_compaction_kcompactd_sleep
> 0 compaction:mm_compaction_wakeup_kcompactd
> 0 compaction:mm_compaction_kcompactd_wake
> 1,208 pageblock:hit_cmpxchg
>
> 116.440414591 seconds time elapsed
>
> 15.326913000 seconds user
> 103.752758000 seconds sys

The runs wildly differ in many of other stats, so I'm not sure they are really
comparable. I guess you could show the fraction of hit_cmpxchg to all cmpxchg.
But there's also danger of tracepoints widening the race window.

In the end what matters is how these 1208 retries contribute to runtime. I doubt
they could be really visible in a 100+ seconds run though.