On 2.6.24, top started showing 100% iowait on one CPU when a UML
instance was running (but completely idle).  I've traced it to this
commit.  Reverting it cures the problem.

Miklos



commit 41d10da3717409de33d5441f2f6d8f072ab3fbb6
Author: Jeff Moyer <[EMAIL PROTECTED]>
Date:   Tue Oct 16 23:27:20 2007 -0700

    aio: account I/O wait time properly
    
    Some months back I proposed changing the schedule() call in
    read_events to an io_schedule():
        http://osdir.com/ml/linux.kernel.aio.general/2006-10/msg00024.html
    This was rejected as there are AIO operations that do not initiate
    disk I/O.  I've had another look at the problem, and the only AIO
    operation that will not initiate disk I/O is IOCB_CMD_NOOP.  However,
    this command isn't even wired up!
    
    Given that it doesn't work, and hasn't for *years*, I'm going to
    suggest again that we do proper I/O accounting when using AIO.
    
    Signed-off-by: Jeff Moyer <[EMAIL PROTECTED]>
    Acked-by: Zach Brown <[EMAIL PROTECTED]>
    Cc: Benjamin LaHaise <[EMAIL PROTECTED]>
    Cc: Suparna Bhattacharya <[EMAIL PROTECTED]>
    Cc: Badari Pulavarty <[EMAIL PROTECTED]>
    Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
    Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>

diff --git a/fs/aio.c b/fs/aio.c
index ea2e198..d02f43b 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -303,7 +303,7 @@ static void wait_for_all_aios(struct kioctx *ctx)
        set_task_state(tsk, TASK_UNINTERRUPTIBLE);
        while (ctx->reqs_active) {
                spin_unlock_irq(&ctx->ctx_lock);
-               schedule();
+               io_schedule();
                set_task_state(tsk, TASK_UNINTERRUPTIBLE);
                spin_lock_irq(&ctx->ctx_lock);
        }
@@ -323,7 +323,7 @@ ssize_t fastcall wait_on_sync_kiocb(struct kiocb *iocb)
                set_current_state(TASK_UNINTERRUPTIBLE);
                if (!iocb->ki_users)
                        break;
-               schedule();
+               io_schedule();
        }
        __set_current_state(TASK_RUNNING);
        return iocb->ki_user_data;
@@ -1170,7 +1170,7 @@ retry:
                        ret = 0;
                        if (to.timed_out)       /* Only check after read evt */
                                break;
-                       schedule();
+                       io_schedule();
                        if (signal_pending(tsk)) {
                                ret = -EINTR;
                                break;
--
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