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

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Message-ID: <ZNS9tqX9s7NbQq3c@lothringen>
Date:   Thu, 10 Aug 2023 12:36:38 +0200
From:   Frederic Weisbecker <frederic@...nel.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>,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        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: disable per-CPU buffer_head cache for
 isolated CPUs

On Fri, Aug 04, 2023 at 08:54:37PM -0300, Marcelo Tosatti wrote:
> > So what happens if they ever do I/O then? Like if they need to do
> > some prep work before entering an isolated critical section?
> 
> Then instead of going through the per-CPU LRU buffer_head cache
> (__find_get_block), isolated CPUs will work as if their per-CPU
> cache is always empty, going through the slowpath 
> (__find_get_block_slow). The algorithm is:
> 
> /*
>  * Perform a pagecache lookup for the matching buffer.  If it's there, refresh
>  * it in the LRU and mark it as accessed.  If it is not present then return
>  * NULL
>  */
> struct buffer_head *
> __find_get_block(struct block_device *bdev, sector_t block, unsigned size)
> {
>         struct buffer_head *bh = lookup_bh_lru(bdev, block, size);
> 
>         if (bh == NULL) {
>                 /* __find_get_block_slow will mark the page accessed */
>                 bh = __find_get_block_slow(bdev, block);
>                 if (bh)
>                         bh_lru_install(bh);
>         } else
>                 touch_buffer(bh);
> 
>         return bh;
> }
> EXPORT_SYMBOL(__find_get_block);
> 
> I think the performance difference between the per-CPU LRU cache
> VS __find_get_block_slow was much more significant when the cache 
> was introduced. Nowadays its only 26ns (moreover modern filesystems 
> do not use buffer_head's).

Sounds good then!

Acked-by: Frederic Weisbecker <frederic@...nel.org>

Thanks!