Date: Tue, 19 Oct 2010 16:37:15 +0300 From: Alexander Motin <mav@FreeBSD.org> To: John Baldwin <jhb@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Marius Strobl <marius@alchemy.franken.de> Subject: Re: svn commit: r213985 - head/sys/sparc64/sparc64 Message-ID: <4CBD9F0B.80701@FreeBSD.org> In-Reply-To: <201010190836.37272.jhb@freebsd.org> References: <201010171646.o9HGks2U038501@svn.freebsd.org> <20101018213055.GP1416@alchemy.franken.de> <4CBCBF25.8010902@FreeBSD.org> <201010190836.37272.jhb@freebsd.org>
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John Baldwin wrote: > On Monday, October 18, 2010 5:41:57 pm Alexander Motin wrote: >> Marius Strobl wrote: >>> 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: >>>>> 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. >> Interrupt frame, required for hard-/stat-/profclock() operation is >> stored in curthread. So critical section is effectively mandatory there >> now. Correct td_intr_nesting_level value is also important for proper >> interrupt threads scheduling - one more reason to have critical section >> there. It is indeed strange that td_intr_nesting_level in >> intr_event_handle() is not covered by critical section, but probably it >> should. > > I don't see why the interrupt frame logic requires bumping > td_intr_nesting_level. It doesn't. It was two different sentences. I've just noticed that td_intr_nesting_level used by SCHED_ULE to bind interrupt threads on their CPUs. I agree that it may be not a very good idea, but it is what we have now. -- Alexander Motin
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