Re: [PATCH v2 11/13] percpu: fix contig hints that are too low

[Chris Down <chris@chrisdown.name>]

Hi Lorenzo,

Lorenzo Stoakes (ARM) writes:
>Secondly, when a free stays within one block,
>pcpu_block_update_hint_free() passes the end of the free area to the
>chunk hints as a block offset rather than a chunk offset, so in any block
>but the first the chunk hints are not updated at all.

Joonwon Kang sent this part earlier this year as "percpu: Fix wrong chunk
hints update" [1]. It has Dennis' Reviewed-by, but as far as I can tell it
was never picked up. The rest of that series goes after the scan_hint
problems by fixing how pcpu_block_update() maintains scan_hint, rather
than by dropping the promotion, and Dennis was still reviewing that in
May [2]. It touches the same code as the first case here, so the two won't
apply on top of each other. It's probably worth adding Joonwon to Cc and
working out between you which one goes in first.

>Thirdly, that free area stops at the block boundary, even if the free
>bits continue into a neighbouring block.

Wasn't this one deliberate? The comment this patch removes says that
chunk_md->contig_hint may be off by up to a page, as a tradeoff to avoid
iterating over the block metadata on every free. I don't mind changing
that, but it'd be good for the changelog to say so, because any free
whose area reaches a block boundary now ends up in
pcpu_chunk_refresh_hint().

>Fix these by always rescanning when a contig_hint is broken, passing the
>correct offset, and refreshing the chunk hints whenever a free area
>reaches a block boundary.

Similarly, da3afdd5bb54 ("percpu: use block scan_hint to only scan
forward") and d33d9f3dd96b ("percpu: use chunk scan_hint to skip some
scanning") added the scan_hint promotion to avoid exactly these rescans,
so do you have any numbers for what always rescanning from first_free
costs on the allocation path?

The result does look right to me, though. From a quick check with the
hint code lifted into a userspace harness and random allocations and
frees thrown at it, contig_hint regularly ends up below the largest free
area before this patch (for both blocks and the chunk) but is fine after
this patch is applied.

[1] https://lore.kernel.org/r/20260513085117.1024175-1-joonwonkang@google.com
[2] https://lore.kernel.org/r/ahVWu38ymnSeLnv3@palisades.local

Thanks!

Chris