From: Jeff Moyer <jmoyer@redhat.com>
To: Zach Brown <zach.brown@oracle.com>, akpm@linux-foundation.org
Cc: Miklos Szeredi <miklos@szeredi.hu>,
torvalds@linux-foundation.org, jdike@addtoit.com,
user-mode-linux-devel@lists.sourceforge.net,
linux-kernel@vger.kernel.org
Subject: Re: [2.6.24 BUG] 100% iowait on host while UML is running
Date: Mon, 03 Dec 2007 14:33:06 -0500 [thread overview]
Message-ID: <x49zlwrfua5.fsf@segfault.boston.devel.redhat.com> (raw)
In-Reply-To: <475450A1.9000103@oracle.com> (Zach Brown's message of "Mon\, 03 Dec 2007 10\:53\:21 -0800")
Zach Brown <zach.brown@oracle.com> writes:
>> 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?
Sure. I tested this on a system that I used to reproduce the problem,
and it shows I/O Wait back at normal levels on an idle system with 1
uml guest running.
Andrew, do you need a separate email with a [patch] heading or will
this do?
Cheers,
Jeff
Only account I/O wait time in read_events if there are active
requests.
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
diff --git a/fs/aio.c b/fs/aio.c
index f12db41..9dec7d2 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -1161,7 +1161,12 @@ retry:
ret = 0;
if (to.timed_out) /* Only check after read evt */
break;
- io_schedule();
+ /* Try to only show up in io wait if there are ops
+ * in flight */
+ if (ctx->reqs_active)
+ io_schedule();
+ else
+ schedule();
if (signal_pending(tsk)) {
ret = -EINTR;
break;
prev parent reply other threads:[~2007-12-03 19:34 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-12-03 15:32 [2.6.24 BUG] 100% iowait on host while UML is running Miklos Szeredi
2007-12-03 16:55 ` Jeff Moyer
2007-12-03 18:53 ` Zach Brown
2007-12-03 19:33 ` Jeff Moyer [this message]
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