Date: Thu, 11 Nov 2004 22:22:45 +0000 (GMT) From: Robert Watson <rwatson@freebsd.org> To: Anurekh Saxena <anurekh@gmail.com> Cc: freebsd-current@freebsd.org Subject: Re: kernel: return from interrupt Message-ID: <Pine.NEB.3.96L.1041111221149.6545E-100000@fledge.watson.org> In-Reply-To: <aa26c8a904111114087d4415a7@mail.gmail.com>
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On Thu, 11 Nov 2004, Anurekh Saxena wrote: > > Even normal "options PREEMPTION" should do this. I know from tracing the > > kernel in 6.x that that's the way the system behaves out of the box; with > > PREEMPTION turned on in 5.x you should see the same behavior. One thing I > > often do see, FWIW, is that if you're on an SMP box, the ithread will get > > scheduled to run immediately on another CPU that's idle, so you won't > > actually preempt the thread on the current CPU other than for the > > interrupt handler. What behavior are you seeing that suggests this isn't > > happening with PREEMPTION compiled in? > > I may be missing something fundamental here, but, doreti (exceptions.s) > does not call 'ast' for an interrupted task, that does not have RPL of 3 > (user). So, even if an interrupt is pending, and the 'NEEDRESCHED' is > set, the scheduling decision is delayed till the kernel thread or > whatever was running in the kernel sleeps, or give up the cpu(call > mi_switch), or returns to user mode. > > AFAIK this is the only return path from an interrupt. Unless there is > another return path for the interrupts, the scheduler is not invoked on > a return. Assuming we're talking about i386, lapic_handle_intr() will call intr_execute_handlers(), which will walk the list of handlers for the interrupt, and either directly invoke the fast handlers of the interrupts, or call ithread_schedule() to schedule the ithread. ithread_schedule() will invoke setrunqueue(), which enters the scheduler and is a preemption point. If you dig down a bit, you'll find a call to maybe_preempt(), which may preempt if appropriate, resulting in a call to mi_switch() to the ithread. The maybe_preempt() code will only kick in to actually switch if PREEMPTION is defined. Robert N M Watson FreeBSD Core Team, TrustedBSD Projects robert@fledge.watson.org Principal Research Scientist, McAfee Research
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