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Date: Wed, 30 May 2007 10:56:29 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Venki Pallipadi <venkatesh.pallipadi@intel.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/4] Make page-writeback timers 1 sec jiffy aligned
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On Tue, 29 May 2007 10:59:51 -0700
Venki Pallipadi <venkatesh.pallipadi@intel.com> wrote:

> 
> 
> timer round_jiffies in page-writeback.
> 
> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
> 
> Index: linux-2.6.22-rc-mm/mm/page-writeback.c
> ===================================================================
> --- linux-2.6.22-rc-mm.orig/mm/page-writeback.c	2007-05-25 10:49:11.000000000 -0700
> +++ linux-2.6.22-rc-mm/mm/page-writeback.c	2007-05-25 10:49:29.000000000 -0700
> @@ -469,7 +469,7 @@
>  	if (time_before(next_jif, jiffies + HZ))
>  		next_jif = jiffies + HZ;
>  	if (dirty_writeback_interval)
> -		mod_timer(&wb_timer, next_jif);
> +		mod_timer(&wb_timer, round_jiffies(next_jif));
>  }
>  
>  /*
> @@ -481,7 +481,7 @@
>  	proc_dointvec_userhz_jiffies(table, write, file, buffer, length, ppos);
>  	if (dirty_writeback_interval) {
>  		mod_timer(&wb_timer,
> -			jiffies + dirty_writeback_interval);
> +			round_jiffies(jiffies + dirty_writeback_interval));
>  		} else {
>  		del_timer(&wb_timer);
>  	}

mutter.

These tunables are in units of centiseconds, so the user can indeed set the
writeback interval to, say, 0.2 seconds.  People have played with that sort
of thing.

I doubt if this patch will hurt anyone much, but it is an incompatible
user-visible change.

Probably a suitable heuristic for fixing this would be to only do the
rounding if dirty_writeback_interval is a multiple of HZ.  In the vast
majority of cases, that will be true.