Re: [PATCH v2] mm: swap: prejudgement swap_has_cache to avoid page allocation

[Huang, Ying]

Zhaoyu Liu <liuzhaoyu.zackary@xxxxxxxxxxxxx> writes:

> On Tue, Apr 09, 2024 at 09:07:29AM +0800, Huang, Ying wrote:
>> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
>> 
>> > On Mon, 8 Apr 2024 20:14:39 +0800 Zhaoyu Liu <liuzhaoyu.zackary@xxxxxxxxxxxxx> wrote:
>> >
>> >> Based on qemu arm64 - latest kernel + 100M memory + 1024M swapfile.
>> >> Create 1G anon mmap and set it to shared, and has two processes
>> >> randomly access the shared memory. When they are racing on swap cache,
>> >> on average, each "alloc_pages_mpol + swapcache_prepare + folio_put"
>> >> took about 1475 us.
>> >
>> > And what effect does this patch have upon the measured time?  ANd upon
>> > overall runtime?
>> 
>> And the patch will cause increased lock contention, please test with
>> more processes and perhaps HDD swap device too.
>
> Hi Ying,
>
> Thank you for your suggestion.
> It may indeed cause some lock contention, as mentioned by Kairui before.
>
> If so, is it recommended?
> ---
>   unsigned char swap_map, mapcount, hascache;
>   ...
>   /* Return raw data of the si->swap_map[offset] */
>   swap_map = __swap_map(si, entry);
>   mapcount = swap_map & ~SWAP_HAS_CACHE;
>   if (!mapcount && swap_slot_cache_enabled)
>   ...
>   hascache = swap_map & SWAP_HAS_CACHE;
>   /* Could judge that it's being added to swap cache with high probability */
>   if (mapcount && hascache)
>     goto skip_alloc;
>   ...
> ---
> In doing so, there is no additional use of locks.

Yes.  This can remove the lock-contention.  But, you need to prove that
it's necessary in the first place.

--
Best Regards,
Huang, Ying

>> >> So skip page allocation if SWAP_HAS_CACHE was set, just
>> >> schedule_timeout_uninterruptible and continue to acquire page
>> >> via filemap_get_folio() from swap cache, to speedup
>> >> __read_swap_cache_async.