Messages in this thread | | | Date | Wed, 18 May 2022 14:57:35 +0800 | Subject | Re: [External] Re: [PATCH] bpf: avoid grabbing spin_locks of all cpus when no free elems | From | Feng Zhou <> |
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在 2022/5/18 下午2:32, Alexei Starovoitov 写道: > On Tue, May 17, 2022 at 11:27 PM Feng zhou <zhoufeng.zf@bytedance.com> wrote: >> From: Feng Zhou <zhoufeng.zf@bytedance.com> >> >> We encountered bad case on big system with 96 CPUs that >> alloc_htab_elem() would last for 1ms. The reason is that after the >> prealloc hashtab has no free elems, when trying to update, it will still >> grab spin_locks of all cpus. If there are multiple update users, the >> competition is very serious. >> >> So this patch add is_empty in pcpu_freelist_head to check freelist >> having free or not. If having, grab spin_lock, or check next cpu's >> freelist. >> >> Before patch: hash_map performance >> ./map_perf_test 1 >> 0:hash_map_perf pre-alloc 975345 events per sec >> 4:hash_map_perf pre-alloc 855367 events per sec >> 12:hash_map_perf pre-alloc 860862 events per sec >> 8:hash_map_perf pre-alloc 849561 events per sec >> 3:hash_map_perf pre-alloc 849074 events per sec >> 6:hash_map_perf pre-alloc 847120 events per sec >> 10:hash_map_perf pre-alloc 845047 events per sec >> 5:hash_map_perf pre-alloc 841266 events per sec >> 14:hash_map_perf pre-alloc 849740 events per sec >> 2:hash_map_perf pre-alloc 839598 events per sec >> 9:hash_map_perf pre-alloc 838695 events per sec >> 11:hash_map_perf pre-alloc 845390 events per sec >> 7:hash_map_perf pre-alloc 834865 events per sec >> 13:hash_map_perf pre-alloc 842619 events per sec >> 1:hash_map_perf pre-alloc 804231 events per sec >> 15:hash_map_perf pre-alloc 795314 events per sec >> >> hash_map the worst: no free >> ./map_perf_test 2048 >> 6:worse hash_map_perf pre-alloc 28628 events per sec >> 5:worse hash_map_perf pre-alloc 28553 events per sec >> 11:worse hash_map_perf pre-alloc 28543 events per sec >> 3:worse hash_map_perf pre-alloc 28444 events per sec >> 1:worse hash_map_perf pre-alloc 28418 events per sec >> 7:worse hash_map_perf pre-alloc 28427 events per sec >> 13:worse hash_map_perf pre-alloc 28330 events per sec >> 14:worse hash_map_perf pre-alloc 28263 events per sec >> 9:worse hash_map_perf pre-alloc 28211 events per sec >> 15:worse hash_map_perf pre-alloc 28193 events per sec >> 12:worse hash_map_perf pre-alloc 28190 events per sec >> 10:worse hash_map_perf pre-alloc 28129 events per sec >> 8:worse hash_map_perf pre-alloc 28116 events per sec >> 4:worse hash_map_perf pre-alloc 27906 events per sec >> 2:worse hash_map_perf pre-alloc 27801 events per sec >> 0:worse hash_map_perf pre-alloc 27416 events per sec >> 3:worse hash_map_perf pre-alloc 28188 events per sec >> >> ftrace trace >> >> 0) | htab_map_update_elem() { >> 0) 0.198 us | migrate_disable(); >> 0) | _raw_spin_lock_irqsave() { >> 0) 0.157 us | preempt_count_add(); >> 0) 0.538 us | } >> 0) 0.260 us | lookup_elem_raw(); >> 0) | alloc_htab_elem() { >> 0) | __pcpu_freelist_pop() { >> 0) | _raw_spin_lock() { >> 0) 0.152 us | preempt_count_add(); >> 0) 0.352 us | native_queued_spin_lock_slowpath(); >> 0) 1.065 us | } >> | ... >> 0) | _raw_spin_unlock() { >> 0) 0.254 us | preempt_count_sub(); >> 0) 0.555 us | } >> 0) + 25.188 us | } >> 0) + 25.486 us | } >> 0) | _raw_spin_unlock_irqrestore() { >> 0) 0.155 us | preempt_count_sub(); >> 0) 0.454 us | } >> 0) 0.148 us | migrate_enable(); >> 0) + 28.439 us | } >> >> The test machine is 16C, trying to get spin_lock 17 times, in addition >> to 16c, there is an extralist. > Is this with small max_entries and a large number of cpus? > > If so, probably better to fix would be to artificially > bump max_entries to be 4x of num_cpus. > Racy is_empty check still wastes the loop.
This hash_map worst testcase with 16 CPUs, map's max_entries is 1000.
This is the test case I constructed, it is to fill the map on purpose, and then
continue to update, just to reproduce the problem phenomenon.
The bad case we encountered with 96 CPUs, map's max_entries is 10240.
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