svn commit: r213985 - head/sys/sparc64/sparc64
Marius Strobl
marius at alchemy.franken.de
Mon Oct 18 20:52:26 UTC 2010
On Mon, Oct 18, 2010 at 11:28:13PM +0300, Alexander Motin wrote:
> Marius Strobl wrote:
> > On Mon, Oct 18, 2010 at 10:03:12AM -0400, John Baldwin wrote:
> >> On Sunday, October 17, 2010 12:46:54 pm Marius Strobl wrote:
> >>> Author: marius
> >>> Date: Sun Oct 17 16:46:54 2010
> >>> New Revision: 213985
> >>> URL: http://svn.freebsd.org/changeset/base/213985
> >>>
> >>> Log:
> >>> - In oneshot-mode it doesn't make sense to try to compensate the clock
> >>> drift in order to achieve a more stable clock as the tick intervals may
> >>> vary in the first place. In fact I haven't seen this code kick in when
> >>> in oneshot-mode so just skip it in that case.
> >>> - There's no need to explicitly stop the (S)TICK counter in oneshot-mode
> >>> with every tick as it just won't trigger again with the (S)TICK compare
> >>> register set to a value in the past (with a wrap-around once every ~195
> >>> years of uptime at 1.5 GHz this isn't something we have to worry about
> >>> in practice).
> >>> - Given that we'll disable interrupts completely anyway there's no
> >>> need to enter critical sections.
> >> This last is not entirely true. The purpose of the critical section is to
> >> prevent the kernel from preempting to the softclock swi thread until all of
> >> the hardclock handler has finished execution. Thus, places that actually
> >> actually call hardclock() should probably still be wrapped in a critical
> >> section.
> >
> > It's currently unclear to me how on architectures converted to the
> > event timer world order hardclock() is called eventually but in any case
> > shouldn't it be the responsibility of the code actually calling it (or
> > the equivalent code) to wrap it in a critical section instead then? After
> > all the MD part just enrolls in calling _something_ in one-shot and/or
> > periodic mode without knowing what it actually calls (and IMO it also
> > should no longer need to). In handleevents() of kern_clocksource.c
> > hardclock_anycpu() is called so i think that is what actually needs to
> > be wrapped in a critical section.
>
> At this time on most (all?) platforms critical section is grabbed by MD
> interrupt code. It is important to be there, as soon as there touched
> td_intr_nesting_level and td_intr_frame fields of curthread. We can't
> allow thread migration until all counted interrupt handlers complete.
>
AFAICT this is not true; intr_event_handle() in sys/kern/kern_intr.c
is what enters a critical section and f.e. on amd64 I don't see where
anywhere in the path from ISR_VEC() to intr_execute_handlers()
calling intr_event_handle() a critical section would be entered,
which also means that in intr_execute_handlers() td_intr_nesting_level
is incremented outside of a critical section.
Marius
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