svn commit: r213985 - head/sys/sparc64/sparc64
Alexander Motin
mav at FreeBSD.org
Mon Oct 18 20:28:20 UTC 2010
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.
--
Alexander Motin
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