Re: [RFC][PATCH] iowait statistics

[Jens Axboe <axboe@suse.de>]

On Thu, May 16 2002, Leigh Brown wrote:
> Today, Rik van Riel wrote:
> > On Thu, 16 May 2002, Leigh Brown wrote:
> >
> >> I've tried this patch against Red Hat's 2.4.18 kernel on my laptop,
> >> and patched top to display the results.  It certainly seems to be
> >> working correctly running a few little contrived tests.
> >
> > Cool, could you please post the patch to top so other people
> > can enjoy it too ? ;)
> 
> I'd call it a hack rather than a patch.  I might be able to look at
> it later.
> 
> [...]
> >> CPU states: 0.3% user,  8.9% system,  0.0% nice, 77.2% idle, 13.3%
> >> wait
> >>
> >> I'm not sure if that can be explained by the way the raw I/O stuff
> >> works, or because I'm running it against 2.4.  Anyway, overall it's
> >> looking good.
> >
> > Most likely the patch forgets to increment nr_iowait_tasks in
> > some raw IO code path...
> 
> Ah yes, could this be it?  It makes the output look right:
> 
> --- linux-2.4.18-3/fs/iobuf.c	Fri Apr 27 22:23:25 2001
> +++ linux-2.4.18-5/fs/iobuf.c	Thu May 16 16:07:32 2002
> @@ -136,7 +136,9 @@
>  	set_task_state(tsk, TASK_UNINTERRUPTIBLE);
>  	if (atomic_read(&kiobuf->io_count) != 0) {
> 
> 	run_task_queue(&tq_disk);
> +
> 	atomic_inc(&nr_iowait_tasks);
> 
> 	schedule();
> +
> 	atomic_dec(&nr_iowait_tasks);
> 
> 	if (atomic_read(&kiobuf->io_count) != 0)
> 
> 		goto repeat;
>  	}

Would it not be easier to simply have the paths which are waiting for
I/O completion to signal that?

	set_task_state(tsk, TASK_UNINTERRUPTIBLE | TASK_IOWAIT);

and have schedule() inc and dec nr_iowait_tasks for us.

-- 
Jens Axboe

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