[ At the very least CC'ing the scheduler maintainer would be helpful :-) ]
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. http://git.kernel.org/?p=linux/kernel/git/mingo/linux-2.6-sched-devel.git;a=commitdiff;h=7e7cbd617833dde5b442e03f69aac39d17d02ec7 http://git.kernel.org/?p=linux/kernel/git/mingo/linux-2.6-sched-devel.git;a=commitdiff;h=45d10aad580a5cdd376e80848aeeaaaf1f97cc18 http://git.kernel.org/?p=linux/kernel/git/mingo/linux-2.6-sched-devel.git;a=commitdiff;h=5ae5d6c5850d4735798bc0e4526d8c61199e9f93 As for your __ARCH_WANT_UNLOCKED_CTXSW question I have to defer to Ingo, as I'm unaware of the arch ramifications there. - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
