Date: Mon, 18 Oct 2010 23:28:13 +0300 From: Alexander Motin <mav@FreeBSD.org> To: Marius Strobl <marius@alchemy.franken.de> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, John Baldwin <jhb@freebsd.org> Subject: Re: svn commit: r213985 - head/sys/sparc64/sparc64 Message-ID: <4CBCADDD.5070109@FreeBSD.org> In-Reply-To: <20101018190615.GL1416@alchemy.franken.de> References: <201010171646.o9HGks2U038501@svn.freebsd.org> <201010181003.13045.jhb@freebsd.org> <20101018190615.GL1416@alchemy.franken.de>
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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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