Re: [PATCH v2] Use atomic_t and wait_event to track dquot pincount

[Dave Chinner <david@fromorbit.com>]

On Fri, Sep 26, 2008 at 11:32:43AM +1000, Lachlan McIlroy wrote:
> Dave Chinner wrote:
>> On Thu, Sep 25, 2008 at 06:43:43PM +1000, Peter Leckie wrote:
>>> However xfssyncd has had a long history of the task being woken up 
>>> from  other code,
>>> so it looks like it's simply not safe for either the aild or xfssyncd 
>>> to  sleep on a queue assuming that
>>> no one else will wake the processes up.
>>
>> Given that both xfsaild and xfssyncd are supposed to be doing
>> non-blocking flushes, neither of them should ever be waiting on a
>> pinned item, therefore fixing that problem in xfs_qm_dqflush()
>> should make this problem go away. It will also substantially
>> reduce tehnumber of log forces being triggered by dquot writeback
>> which will have positive impact on performance, too.
>>
>>> So I would say the fix I proposed is a good solution for this issue.
>>
>> but it doesn't fix the underlying problem that was causing the
>> spurious wakeups, which is the fact that xfs_qm_dqflush() is not
>> obeying non-blocking flush directions.
>
> The underlying problem has nothing to do with xfs_qm_dqflush() - the
> spurious wakeups are caused by calls to wake_up_process() that arbitrarily
> wake up a process that is in a state where it shouldn't be woken up.

Spurious wakeups are causing problems in a place where we should not 
even be sleeping. If you don't sleep there, you can't get spurious
wakeups....

> If we don't fix the spurious wakeups then we could easily re-introduce this
> problem again.

Right, but keep in mind that the patch doesn't prevent spurious
wakeups - it merely causes the thread to wakeup and go back to sleep
when a spurious wakeup occurs. The patch I posted avoids the
spurious wakeup problem completely, which is what we should be
aiming to do given it avoids the overhead of 2 context switches
and speeds up the rate at which we can flush unpinned dquots.

That being said, I agree that the original patch is still desirable,
though not from a bug-fix perspective. It's a cleanup and
optimisation patch, with the nice side effect of preventing future
occurrences of the spurious wakeup problem....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com