Re: [PATCH v2] vmpressure: wake up work only when there is registration event
From: Michal Hocko <hidden>
Date: 2021-09-15 12:42:07
Also in:
linux-mm, lkml
On Tue 14-09-21 09:05:51, yongw.pur-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org wrote:
From: wangyong <redacted> Use the global variable num_events to record the number of vmpressure events registered by the system, and wake up work only when there is registration event. Usually, the vmpressure event is not registered in the system, this patch can avoid waking up work and doing nothing.
I have asked in the previous version and this changelog doesn't that explain again. Why don't you simply bail out early in vmpressure() entry?
Test with 5.14.0-rc5-next-20210813 on x86_64 4G ram. Consume cgroup memory until it is about to be reclaimed, then execute "perf stat -I 2000 malloc.out" command to trigger memory reclamation and get performance results. The context-switches is reduced by about 20 times.
Is this test somewhere available so that it can be reproduced by others. Also while the number of context switches can be an interesting it is not really clear from this evaluation whether that actually matters or not. E.g. what does an increase of task-clock and twice as many instructions recorded tell us?
unpatched: Average of 10 test results 582.4674048 task-clock(msec) 19910.8 context-switches 0 cpu-migrations 1292.9 page-faults 414784733.1 cycles
<not supported> stalled-cycles-frontend <not supported> stalled-cycles-backend
Why is this a part of the data?
580070698.4 instructions 125572244.7 branches 2073541.2 branch-misses patched Average of 10 test results 973.6174796 task-clock(msec) 988.6 context-switches 0 cpu-migrations 1785.2 page-faults 772883602.4 cycles <not supported> stalled-cycles-frontend <not supported> stalled-cycles-backend 1360280911 instructions 290519434.9 branches 3378378.2 branch-misses Tested-by: Zeal Robot <redacted> Signed-off-by: wangyong <redacted> ---
[...]
quoted hunk ↗ jump to hunk
@@ -272,6 +277,9 @@ void vmpressure(gfp_t gfp, struct mem_cgroup *memcg, bool tree, return; if (tree) { + if (!static_branch_unlikely(&num_events)) + return;
We usually hide the change behind a static inline helper (e.g. vmpressure_disabled()). I would also put it to the beginning of vmpressure or put an explanation why it makes sense only in this branch. -- Michal Hocko SUSE Labs