Re: runqueue locks in schedule()

[stephane eranian]

Nick,

On Jan 18, 2008 3:07 AM, Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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.
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