Re: [RFC PATCH 10/10] mm/swap: optimize synchronous swapin

Huang, Ying
2024-03-27T06:49:41Z

Kairui Song <ryncsn@gmail.com> writes:

> On Wed, Mar 27, 2024 at 2:24 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Kairui Song <ryncsn@gmail.com> writes:
>>
>> > From: Kairui Song <kasong@tencent.com>
>> >
>> > Interestingly the major performance overhead of synchronous is actually
>> > from the workingset nodes update, that's because synchronous swap in
>>
>> If it's the major overhead, why not make it the first optimization?
>
> This performance issue became much more obvious after doing other
> optimizations, and other optimizations are for general swapin not only
> for synchronous swapin, that's also how I optimized things step by
> step, so I kept my patch order...
>
> And it is easier to do this after Patch 8/10 which introduces the new
> interface for swap cache.
>
>>
>> > keeps adding single folios into a xa_node, making the node no longer
>> > a shadow node and have to be removed from shadow_nodes, then remove
>> > the folio very shortly and making the node a shadow node again,
>> > so it has to add back to the shadow_nodes.
>>
>> The folio is removed only if should_try_to_free_swap() returns true?
>>
>> > Mark synchronous swapin folio with a special bit in swap entry embedded
>> > in folio->swap, as we still have some usable bits there. Skip workingset
>> > node update on insertion of such folio because it will be removed very
>> > quickly, and will trigger the update ensuring the workingset info is
>> > eventual consensus.
>>
>> Is this safe? Is it possible for the shadow node to be reclaimed after
>> the folio are added into node and before being removed?
>
> If a xa node contains any non-shadow entry, it can't be reclaimed,
> shadow_lru_isolate will check and skip such nodes in case of race.

In shadow_lru_isolate(),

/*
* The nodes should only contain one or more shadow entries,
* no pages, so we expect to be able to remove them all and
* delete and free the empty node afterwards.
*/
if (WARN_ON_ONCE(!node->nr_values))
goto out_invalid;
if (WARN_ON_ONCE(node->count != node->nr_values))
goto out_invalid;

So, this isn't considered normal and will cause warning now.

>>
>> If so, we may consider some other methods. Make shadow_nodes per-cpu?
>
> That's also an alternative solution if there are other risks.

This appears a general optimization and more clean.

--
Best Regards,
Huang, Ying