Re: [PATCH] mm:zswap: fix zswap entry reclamation failure in two scenarios

[Huang, Ying]

Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:

> On Sun, Nov 19, 2023 at 7:20 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Chris Li <chriscli@xxxxxxxxxx> writes:
>>
>> > On Thu, Nov 16, 2023 at 12:19 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>> >>
>> >> Not bypassing the swap slot cache, just make the callbacks to
>> >> invalidate the zswap entry, do memg uncharging, etc when the slot is
>> >> no longer used and is entering the swap slot cache (i.e. when
>> >> free_swap_slot() is called), instead of when draining the swap slot
>> >> cache (i.e. when swap_range_free() is called). For all parts of MM
>> >> outside of swap, the swap entry is freed when free_swap_slot() is
>> >> called. We don't free it immediately because of caching, but this
>> >> should be transparent to other parts of MM (e.g. zswap, memcg, etc).
>> >
>> > That will cancel the batching effect on the swap slot free, making the
>> > common case for  swapping  faults take longer to complete, righ?
>> > If I recall correctly, the uncharge is the expensive part of the swap
>> > slot free operation.
>> > I just want to figure out what we are trading off against. This is not
>> > one side wins all situations.
>>
>> Per my understanding, we don't batch memcg uncharging in
>> swap_entry_free() now.  Although it's possible and may improve
>> performance.
>
> Yes. It actually causes a long tail in swapin fault latency as Chris
> discovered in our prod. I am wondering if doing the memcg uncharging
> outside the slots cache will actually amortize the cost instead.
>
> Regardless of memcg charging, which is more complicated, I think we
> should at least move the call to zswap_invalidate() before the slots
> cache. I would prefer that we move everything non-swapfile specific
> outside the slots cache layer (zswap_invalidate(),
> arch_swap_invalidate_page(),  clear_shadow_from_swap_cache(),
> mem_cgroup_uncharge_swap(), ..).  However, if some of those are
> controversial, we can move some of them for now.

That makes sense for me.

> When draining free swap slots from the cache, swap_range_free() is
> called with nr_entries == 1 anyway, so I can't see how any batching is
> going on. If anything it should help amortize the cost.

In swapcache_free_entries(), the sis->lock will be held to free multiple
swap slots via swap_info_get_cont() if possible.  This can reduce
sis->lock contention.

--
Best Regards,
Huang, Ying