Re: runqueue locks in schedule()

[Nick Piggin]

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).

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.
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