linux-kernel - Re: [RESEND][PATCH -mm 2/2] vfs: add cond_resched

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Date:	Mon, 23 Jun 2008 19:11:40 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kentaro Makita <k-makita@...css.fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	viro@...IV.linux.org.uk, harvey.harrison@...il.com
Subject: Re: [RESEND][PATCH -mm 2/2] vfs: add cond_resched_lock while
 scanning dentry LRU lists

On Tue, 24 Jun 2008 10:45:23 +0900 Kentaro Makita <k-makita@...css.fujitsu.com> wrote:

> o add cond_resched_lock(&dcache_lock) while scanning LRU lists on superblocks
>   in __shrink_dcache_sb()
> 
> Signed-off-by: Kentaro Makita <k-makita@...css.fujitsu.com>
> ---
>  fs/dcache.c |    1 +
>  1 files changed, 1 insertion(+)
> 
> Index: b/fs/dcache.c
> ===================================================================
> --- a/fs/dcache.c	2008-06-05 11:48:57.000000000 +0900
> +++ b/fs/dcache.c	2008-06-05 11:49:19.000000000 +0900
> @@ -484,6 +484,7 @@ restart:
>  				if (--cnt == 0)
>  					break;
>  			}
> +			cond_resched_lock(&dcache_lock);
>  		}
>  	}
>  	while (!list_empty(&tmp)) {

The changelog is insufficient.  It tells us what the patch did (which
was completely obvious anyway) but it fails to tell us _why_ the patch
did it.

That is is easily guessable but a good changelog would have described
the problem which you observed and would have described how the the
patch changed the runtime behaviour.

And this is not a trivial formality either.  Because if, for example,
the problem whcih you are fixing here is "machine goes comatose for ten
minutes" then we'll need algorithmic changes and not a bandaid of this
nature.

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