Messages in this thread | | | Date | Thu, 19 May 2022 11:12:48 +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 2022/5/19 上午4:39, Yonghong Song 写道: > > > 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?
This code is here: samples/bpf/map_perf_test_user.c samples/bpf/map_perf_test_kern.c parameter '1' means testcase flag, test hash_map's performance parameter '2048' means test hash_map's performance when free=0. testcase flag '2048' is added by myself to reproduce the problem phenomenon.
> >>>> 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)?
Yes, increasing max_entries can temporarily solve this problem, but when 16k is used up, it will still encounter this problem. This patch is to try to fix this corner case.
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