Nick, On Jan 18, 2008 3:07 AM, Nick Piggin <[EMAIL PROTECTED]> wrote: > > On Friday 18 January 2008 00:24, Peter Zijlstra wrote: > > [ 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. > > It is arch specific. If an architecture wants interrupts on during context > switch, or runqueue unlocked, then they set it (btw INTERRUPTS_ON_CTXSW > also implies UNLOCKED_CTXSW). > Yes , I noticed that. I am only interested in UNLOCKED_CTXSW. But it appears that the approach suggested my Peter does work. We are running some tests.
> Although, eg on x86, you would hold off interrupts and runqueue lock for > slightly less time if you defined those, it results in _slightly_ more > complicated context switching... although I did once find a workload > where the reduced runqueue contention improved throughput a bit, it is > not much problem in general to hold the lock. > By complicated you mean that now you'd have to make sure you don't need to access runqueue data? Thanks. - 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
