Re: [PATCH] block: Skip the folio lock if the folio is already dirty

[Hannes Reinecke <hare@suse.de>]

On 1/24/25 23:48, Matthew Wilcox (Oracle) wrote:
> Postgres sees significant contention on the hashed folio waitqueue lock
> when performing direct I/O to 1GB hugetlb pages.  This is because we
> mark the destination pages as dirty, and the locks end up 512x more
> contended with 1GB pages than with 2MB pages.
> 
> We can skip the locking if the folio is already marked as dirty.
> The writeback path clears the dirty flag before commencing writeback,
> if we see the dirty flag set, the data written to the folio will be
> written back.
> 
> In one test, throughput increased from 18GB/s to 20GB/s and moved the
> bottleneck elsewhere.
> 
> Reported-by: Andres Freund <andres@anarazel.de>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>   block/bio.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index f0c416e5931d..e8d18a0fecb5 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1404,6 +1404,8 @@ void bio_set_pages_dirty(struct bio *bio)
>   	struct folio_iter fi;
>   
>   	bio_for_each_folio_all(fi, bio) {
> +		if (folio_test_dirty(folio))
> +			continue;
>   		folio_lock(fi.folio);
>   		folio_mark_dirty(fi.folio);
>   		folio_unlock(fi.folio);

The same reasoning can probably applied to __bio_release_pages().

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
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