On Tue, 2008-09-23 at 00:06 +0900, kamezawa.hiroyu@jp.fujitsu.com wrote:

----- Original Message -----

quoted

On Mon, 2008-09-22 at 23:50 +0900, kamezawa.hiroyu@jp.fujitsu.com wrote:

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----- Original Message -----

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+			spin_lock_irqsave(&mz->lru_lock, flags);
+		} else {
+			unlock_page(page);
+			put_page(page);
+		}
+		if (atomic_read(&mem->css.cgroup->count) > 0)
+			break;
 	}
 	spin_unlock_irqrestore(&mz->lru_lock, flags);

do _NOT_ use yield() ever! unless you know what you're doing, and
probably not even then.

NAK!

Hmm, sorry. cond_resched() is ok ?

depends on what you want to do, please explain what you're trying to do.

Sorry again.

This force_empty is called only in following situation
 - there is no user threas in this cgroup.
 - a user tries to rmdir() this cgroup or explicitly type
   echo 1 > ../memory.force_empty.

force_empty() scans lru list of this cgroup and check page_cgroup on the
list one by one. Because there are no tasks in this group, force_empty can
see following racy condtions while scanning.

 - global lru tries to remove the page which pointed by page_cgroup 
   and it is not-on-LRU.

So you either skip the page because it already got un-accounted, or you
retry because its state is already updated to some new state.

 - the page is locked by someone.
   ....find some lock contetion with invalidation/truncate.

Then you just contend the lock and get woken when you obtain?

 - in later patch, page_cgroup can be on pagevec(i added) and we have to drain
   it to remove from LRU.

Then unlock, drain, lock, no need to sleep some arbitrary amount of time
[0-inf).

In above situation, force_empty() have to wait for some event proceeds.

Hmm...detecting busy situation in loop and sleep in out-side-of-loop
is better ? Anyway, ok, I'll rewrite this.

The better solution is to wait for events in a non-polling fashion, for
example by using wait_event().

yield() might not actually wait at all, suppose you're the highest
priority FIFO task on the system - if you used yield and rely on someone
else to run you'll deadlock.

Also, depending on sysctl_sched_compat_yield, SCHED_OTHER tasks using
yield() can behave radically different.

BTW, sched.c::yield() is for what purpose now ?

There are some (lagacy) users of yield, sadly they are all incorrect,
but removing them is non-trivial for various reasons.

The -rt kernel has 2 sites where yield() is the correct thing to do. In
both cases its where 2 SCHED_FIFO-99 tasks (migration and stop_machine)
depend on each-other.

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