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

[Alexei Starovoitov]

On Wed, May 18, 2022 at 8:12 PM Feng Zhou <zhoufeng.zf@xxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxx> wrote:
> >>>> From: Feng Zhou <zhoufeng.zf@xxxxxxxxxxxxx>
> >>>>
> >>>> 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.

Please convert it to selftests/bpf/bench,
so that everyone can reproduce the issue you're seeing
and can assess whether it's a real issue or a corner case.

Also please avoid adding indent in the patch.
Instead of
 if (!s->extralist.is_empty) {
  .. churn

do

 if (s->extralist.is_empty)