Re: runqueue locks in schedule()

[Peter Zijlstra <peterz@infradead.org>]

[ 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.