Date: Mon, 18 Oct 2010 23:30:55 +0200 From: Marius Strobl <marius@alchemy.franken.de> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, Alexander Motin <mav@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org Subject: Re: svn commit: r213985 - head/sys/sparc64/sparc64 Message-ID: <20101018213055.GP1416@alchemy.franken.de> In-Reply-To: <201010181705.24879.jhb@freebsd.org> References: <201010171646.o9HGks2U038501@svn.freebsd.org> <4CBCADDD.5070109@FreeBSD.org> <20101018205224.GO1416@alchemy.franken.de> <201010181705.24879.jhb@freebsd.org>
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On Mon, Oct 18, 2010 at 05:05:24PM -0400, John Baldwin wrote: > On Monday, October 18, 2010 4:52:24 pm Marius Strobl wrote: > > 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. > > Not all of the clock interrupts use intr_event_handle(). The local APIC > timer uses its own interrupt entry point on x86 for example and uses an > explicit critical section as a result. I suspect the sparc64 tick interrupt > is closer to the local APIC timer case and doesn't use intr_event_handle(). Correct; but still you can't say that the MD interrupt code enters a critical section in general, neither is incrementing td_intr_nesting_level in intr_execute_handlers() protected by a critical section. > > The fact that some clock interrupts do use intr_event_handle() (e.g. the > atrtc driver on x86 now) does indicate that the low-level interrupt code > probably does not belong in the time events code but in the caller. > Well, I agree that entering a critical section in the time events code would mean entering a nested critical section unnecessarily in case the clock driver uses a regular "fast" interrupt handler and that should be avoided. Still I don't think the event time front-end actually should need to worry about wrapping the callback in a critical section. Marius
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