linux-kernel - Re: [2.6.24 BUG] 100% iowait on host while UML is running

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Date:	Mon, 03 Dec 2007 10:53:21 -0800
From:	Zach Brown <zach.brown@...cle.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Miklos Szeredi <miklos@...redi.hu>, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, jdike@...toit.com,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [2.6.24 BUG] 100% iowait on host while UML is running


> We could check ctx->reqs_active before scheduling to determine whether
> or not we are waiting for I/O, but this would require taking the
> context lock in order to be accurate.  Given that the test would be
> only for the sake of book keeping, it might be okay to do it outside
> of the lock.
> 
> Zach, what are your thoughts on this?

I agree that it'd be OK to test it outside the lock, though we'll want
some commentary:

	/* Try to only show up in io wait if there are ops in flight */
	if (ctx->reqs_active)
		io_schedule();
	else
		schedule();

It's cheap, safe, and accurate the overwhelming majority of the time :).

We only need it in read_events().  The other two io_schedule() calls are
only reached to wait on pending reqs specifically.

It still won't make sense for iocbs which aren't performing IO, but I
guess that's one more bridge to cross when we come to it.

Do you want to throw this tiny patch together and submit it?

- z
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