Messages in this thread | | | Subject | Re: [PATCHv6 00/15] kill unnecessary bdi wakeups + cleanups | From | Artem Bityutskiy <> | Date | Tue, 03 Aug 2010 17:11:21 +0300 |
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On Tue, 2010-08-03 at 14:27 +0200, Jens Axboe wrote: > On 2010-07-25 13:29, Artem Bityutskiy wrote: > > Hi, > > > > here is v6 of the patch series which clean-ups bdi threads and substantially > > lessens amount of unnecessary kernel wake-ups, which is very important on > > battery-powered devices. > > > > This patch-set is also available at: > > git://git.infradead.org/users/dedekind/misc-2.6.git flushers_v6 > > Thanks Artem, for sticking around long enough to get this into > shape. I have finally merged it. > > > 1. Use 'spin_lock_bh' for the 'bdi->wb_lock' (changed patch N12) > > I'd rather not, question is how to avoid it. Either just wakeup the > default thread, or punt the lock-and-check bdi->wb.task to a thread.
Jens, here are my quick thoughts, will come back to this tomorrow.
The spin_lock_bh(&bdi->wb_lock) in 'wakeup_timer_fn()' is needed:
a) to make sure the forker thread does not decide to kill the bdi thread at the same time, which could cause an oops on 'wake_up_process(bdi->wb.task)'. b) to make sure the forker thread does not decide to spawn a bdi thread at the same time, in which case we could lose a wake-up.
I without the "_bh" suffix lockdep complains with a warning. Cannot cite the complained, but it is a fair warning about a possible deadlock if the timer function interrupts the CPU while it is already holding the spinlock, or something like that. The easiest way to address it was to use "_bh".
The only way to avoid "_bh" I see right now is to not 'bdi->wb_lock' at all in 'wakeup_timer_fn()'. In this case we cannot touch 'bdi->wb.task' because it can become NULL at any point of time.
Your first suggestion ("just wakeup the default thread") will work only if we add a new BDI_wakeup_thread or something like that. Not sure it is worth it.
The second suggestion ("punt the lock-and-check bdi->wb.task to a thread") is vague. "A thread" - this must be the forker thread, what else could that be? So basically this is the same as the first suggestion - we set a flag in 'bdi->wb.state' and wake up the forker, which should wake up the bdi thread?
-- Best Regards, Artem Bityutskiy (Артём Битюцкий)
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