Messages in this thread | | | Date | Wed, 18 May 2022 13:39:50 -0700 | Subject | Re: [External] Re: [PATCH] bpf: avoid grabbing spin_locks of all cpus when no free elems | From | Yonghong Song <> |
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On 5/17/22 11:57 PM, Feng Zhou wrote: > 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
could you explain what parameter '1' means here?
>>> 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.
For such cases, most likely the map is *almost* full. What is the performance if we increase map size, e.g., from 10240 to 16K(16192)?
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