linux-kernel - Re: [PATCH v2 1/2] mm: swap: fix performance regression on sparsetruncate-tiny

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Date:   Wed, 5 Apr 2023 17:45:11 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Qi Zheng <zhengqi.arch@...edance.com>
Cc:     akpm@...ux-foundation.org, lstoakes@...il.com, mgorman@...e.de,
        vbabka@...e.cz, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: swap: fix performance regression on
 sparsetruncate-tiny

On Thu, Apr 06, 2023 at 12:18:53AM +0800, Qi Zheng wrote:
> The ->percpu_pvec_drained was originally introduced by
> commit d9ed0d08b6c6 ("mm: only drain per-cpu pagevecs once per
> pagevec usage") to drain per-cpu pagevecs only once per pagevec
> usage. But after converting the swap code to be more folio-based,
> the commit c2bc16817aa0 ("mm/swap: add folio_batch_move_lru()")
> breaks this logic, which would cause ->percpu_pvec_drained to be
> reset to false, that means per-cpu pagevecs will be drained
> multiple times per pagevec usage.

My mistake.  I didn't reaise that we'd need a folio_batch_reinit(),
and indeed we didn't have one until 811561288397 (January 2023).
I thought this usage of percpu_pvec_drained was going to be fine
with being set to false each time.  Thanks for showing I was wrong.

> Fixes: c2bc16817aa0 ("mm/swap: add folio_batch_move_lru()")
> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>

Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>