Re: [External] Re: [PATCH] bpf: avoid grabbing spin_locks of all cpus when no free elems

[Feng Zhou <zhoufeng.zf@bytedance.com>]

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.