linux-kernel - Re: [PATCH] fs/buffer.c: remove per-CPU buffer

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Message-ID: <ZJndTjktg17nulcs@casper.infradead.org>
Date:   Mon, 26 Jun 2023 19:47:42 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Frederic Weisbecker <frederic@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Leonardo Bras <leobras@...hat.com>,
        Yair Podemsky <ypodemsk@...hat.com>, P J P <ppandit@...hat.com>
Subject: Re: [PATCH] fs/buffer.c: remove per-CPU buffer_head lookup cache

On Mon, Jun 26, 2023 at 03:04:53PM -0300, Marcelo Tosatti wrote:
> Upon closer investigation, it was found that in current codebase, lookup_bh_lru
> is slower than __find_get_block_slow:
> 
>  114 ns per __find_get_block
>  68 ns per __find_get_block_slow
> 
> So remove the per-CPU buffer_head caching.

LOL.  That's amazing.  I can't even see why it's so expensive.  The
local_irq_disable(), perhaps?  Your test case is the best possible
one for lookup_bh_lru() where you're not even doing the copy.

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