linux-kernel - Re: [PATCH 002 of 2] md: Improve the is_mddev

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Date:	Thu, 10 May 2007 20:04:03 +1000
From:	Neil Brown <neilb@...e.de>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 002 of 2] md: Improve the is_mddev_idle test

On Thursday May 10, jengelh@...ux01.gwdg.de wrote:
> 
> On May 10 2007 16:22, NeilBrown wrote:
> >
> >diff .prev/drivers/md/md.c ./drivers/md/md.c
> >--- .prev/drivers/md/md.c	2007-05-10 15:51:54.000000000 +1000
> >+++ ./drivers/md/md.c	2007-05-10 16:05:10.000000000 +1000
> >@@ -5095,7 +5095,7 @@ static int is_mddev_idle(mddev_t *mddev)
> > 		 *
> > 		 * Note: the following is an unsigned comparison.
> > 		 */
> >-		if ((curr_events - rdev->last_events + 4096) > 8192) {
> >+		if ((long)curr_events - (long)rdev->last_events > 4096) {
> > 			rdev->last_events = curr_events;
> > 			idle = 0;
> > 		}
> 
> What did really change? Unless I am seriously mistaken,
> 
>     curr_events - last_evens + 4096 > 8192
> 
> is mathematically equivalent to
> 
>     curr_events - last_evens        > 4096
> 
> The casting to (long) may however force a signed comparison which turns
> things quite upside down, and the comment does not apply anymore.

Yes, the use of a signed comparison is the significant difference.
And yes, the comment becomes wrong.  I'm in the process of redrafting
that.  It currently stands at:

		/* sync IO will cause sync_io to increase before the disk_stats
		 * as sync_io is counted when a request starts, and 
		 * disk_stats is counted when it completes.
		 * So resync activity will cause curr_events to be smaller than
		 * when there was no such activity.
		 * non-sync IO will cause disk_stat to increase without
		 * increasing sync_io so curr_events will (eventually)
		 * be larger than it was before.  Once it becomes
		 * substantially larger, the test below will cause
		 * the array to appear non-idle, and resync will slow
		 * down.
		 * If there is a lot of outstanding resync activity when
		 * we set last_event to curr_events, then all that activity
		 * completing might cause the array to appear non-idle
		 * and resync will be slowed down even though there might
		 * not have been non-resync activity.  This will only
		 * happen once though.  'last_events' will soon reflect
		 * the state where there is little or no outstanding
		 * resync requests, and further resync activity will
		 * always make curr_events less than last_events.
		 *
		 */


Does that read at all well?

NeilBrown
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