Peter, > On Wed, 2008-01-16 at 16:29 -0800, stephane eranian wrote: > > Hello, > > > > As suggested by people on this list, I have changed perfmon2 to use > > the high resolution timers as the interface to allow timeout-based > > event set multiplexing. This works around the problems I had with > > tickless-enabled kernels. > > > > Multiplexing is supported in per-thread as well. In that case, the > > timeout measures virtual time. When the thread is context switched > > out, we need to save the remainder of the timeout and cancel the > > timer. When the thread is context switched in, we need to reinstall > > the timer. These timer save/restore operations have to be done in the > > switch_to() code near the end of schedule(). > > > > There are situations where hrtimer_start() may end up trying to > > acquire the runqueue lock. This happens on a context switch where the > > current thread is blocking (not preempted) and the new timeout happens > > to be either in the past or just expiring. We've run into such > > situations with simple tests. > > > > On all architectures, but IA-64, it seems thet the runqueue lock is > > held until the end of schedule(). On IA-64, the lock is released > > BEFORE switch_to() for some reason I don't quite remember. That may > > not even be needed anymore. > > > > The early unlocking is controlled by a macro named > > __ARCH_WANT_UNLOCKED_CTXSW. Defining this macros on X86 (or PPC) fixed > > our problem. > > > > It is not clear to me why the runqueue lock needs to be held up until > > the end of schedule() on some platforms and not on others. Not that > > releasing the lock earlier does not necessarily introduce more > > overhead because the lock is never re-acquired later in the schedule() > > function. > > > > Question: > > - is it safe to release the lock before switch_to() on all > architectures? > > I had similar problem when using hrtimers from the scheduler, I extended > the HRTIMER_CB_IRQSAFE_NO_SOFTIRQ time type to run with cpu_base->lock > unlocked. > I am running into an issue when enabling this flag. Basically, the timer never fires when it gets into this situation where in hrtimer_start() the timer ends up being the next one to fire. In this mode, hrtimer_enqueue_reprogram() become a NOP. But then nobody never inserts the time into any queue. There is a comment that says "caller site takes care of this". Could you elaborate on this?
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