Re: [External] Re: [PATCH] bpf: use count for prealloc hashtab too

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Oct 18, 2021 at 9:31 PM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> 在 2021/10/19 上午11:45, Alexei Starovoitov 写道:
> > On Mon, Oct 18, 2021 at 8:14 PM Chengming Zhou
> > <zhouchengming@bytedance.com> wrote:
> >>
> >> 在 2021/10/19 上午9:57, Alexei Starovoitov 写道:
> >>> On Sun, Oct 17, 2021 at 10:49 PM Chengming Zhou
> >>> <zhouchengming@bytedance.com> wrote:
> >>>>
> >>>> 在 2021/10/16 上午3:58, Alexei Starovoitov 写道:
> >>>>> On Fri, Oct 15, 2021 at 11:04 AM Chengming Zhou
> >>>>> <zhouchengming@bytedance.com> wrote:
> >>>>>>
> >>>>>> We only use count for kmalloc hashtab not for prealloc hashtab, because
> >>>>>> __pcpu_freelist_pop() return NULL when no more elem in pcpu freelist.
> >>>>>>
> >>>>>> But the problem is that __pcpu_freelist_pop() will traverse all CPUs and
> >>>>>> spin_lock for all CPUs to find there is no more elem at last.
> >>>>>>
> >>>>>> We encountered bad case on big system with 96 CPUs that alloc_htab_elem()
> >>>>>> would last for 1ms. This patch use count for prealloc hashtab too,
> >>>>>> avoid traverse and spin_lock for all CPUs in this case.
> >>>>>>
> >>>>>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> >>>>>
> >>>>> It's not clear from the commit log what you're solving.
> >>>>> The atomic inc/dec in critical path of prealloc maps hurts performance.
> >>>>> That's why it's not used.
> >>>>>
> >>>> Thanks for the explanation, what I'm solving is when hash table hasn't free
> >>>> elements, we don't need to call __pcpu_freelist_pop() to traverse and
> >>>> spin_lock all CPUs. The ftrace output of this bad case is below:
> >>>>
> >>>>  50)               |  htab_map_update_elem() {
> >>>>  50)   0.329 us    |    _raw_spin_lock_irqsave();
> >>>>  50)   0.063 us    |    lookup_elem_raw();
> >>>>  50)               |    alloc_htab_elem() {
> >>>>  50)               |      pcpu_freelist_pop() {
> >>>>  50)   0.209 us    |        _raw_spin_lock();
> >>>>  50)   0.264 us    |        _raw_spin_lock();
> >>>
> >>> This is LRU map. Not hash map.
> >>> It will grab spin_locks of other cpus
> >>> only if all previous cpus don't have free elements.
> >>> Most likely your map is actually full and doesn't have any free elems.
> >>> Since it's an lru it will force free an elem eventually.
> >>>
> >>
> >> Maybe I missed something, the map_update_elem function of LRU map is
> >> htab_lru_map_update_elem() and the htab_map_update_elem() above is the
> >> map_update_elem function of hash map.
> >> Because of the implementation of percpu freelist used in hash map, it
> >> will spin_lock all other CPUs when there is no free elements.
> >
> > Ahh. Right. Then what's the point of optimizing the error case
> > at the expense of the fast path?
> >
>
> Yes, this patch only optimized the very bad case that no free elements left,
> and add atomic operation in the fast path. Maybe the better workaround is not
> allowing full hash map in our case or using LRU map?

No idea, since you haven't explained your use case.

> But the problem of spinlock contention also exist even when the map is not full,
> like some CPUs run out of its freelist but other CPUs seldom used, then have to
> grab those CPUs' spinlock to get free element.

In theory that would be correct. Do you see it in practice?
Please describe the use case.

> Should we change the current implementation of percpu freelist to percpu lockless freelist?

Like llist.h ? That was tried already and for typical hash map usage
it's slower than percpu free list.
Many progs still do a lot of hash map update/delete on all cpus at once.
That is the use case hashmap optimized for.
Please see commit 6c9059817432 ("bpf: pre-allocate hash map elements")
that also lists different alternative algorithms that were benchmarked.