[2.6.36-rc3] Workqueues, XFS, dependencies and deadlocks
tj at kernel.org
Wed Sep 8 03:20:27 CDT 2010
On 09/08/2010 09:34 AM, Dave Chinner wrote:
>> I see. The use case itself shouldn't be problematic at all for cmwq
>> (sans bugs of course). In the other reply, you said "the system is
>> 100% unresponsive when the livelock occurs", which is kind of
>> puzzling. It isn't really a livelock.
> Actually, it is. You don't need to burn CPU to livelock, you just
> need a loop in the state machine that cannot be broken by internal
> or external events to be considered livelocked.
Yeah, but for the system to be completely unresponsive even to sysrq,
the system needs to be live/dead locked in a pretty specific way.
> However, this is not what I was calling the livelock problem - this
> is what I was calling the deadlock problem because to all external
> appearences the state machine is deadlocked on the inode lock....
> The livelock case I described where the system is completely
> unresponsive is the one I'm testing the WQ_HIGHPRI mod against.
> FWIW, having considered the above case again, and seeing what the
> WQ_HIGHPRI mod does in terms of queuing, I think that it may also
> solve this deadlock as the log IO completionwill always be queued
> ahead of the data IO completion now.
Cool, but please keep in mind that the nr_active underflow bug may end
up stalling or loosening ordering rules for a workqueue. Linus has
pulled in the pending fixes today.
>> Hmm... The point where I'm confused is that *delay()'s are busy waits.
>> They burn CPU cycles. I suppose you're referring to *sleep()'s,
> static inline void delay(long ticks)
Heh yeah, there's my confusion.
>> Probably I have overloaded the term 'concurrency' too much. In this
>> case, I meant the number of workers assigned to work items of the wq.
>> If you fire off N work items which sleep at the same time, cmwq will
>> eventually try to create N workers as each previous worker goes to
>> sleep so that the CPU doesn't sit idle while there are work items to
>> process as long as N < @wq->nr->active.
> Ok, so if I queue N items on a single CPU when max_active == N, they
> get spread across N worker threads on different CPUs?
They may if necessary to keep the workqueue progressing.
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