Re: kernel: return from interrupt

From: John Baldwin (jhb_at_FreeBSD.org)
Date: 11/15/04

  • Next message: Jimmy Selgen: "Re: Reproducible panic in -CURRENT" 
    To: freebsd-current@FreeBSD.org, Anurekh Saxena <anurekh@gmail.com>
Date: Mon, 15 Nov 2004 14:33:57 -0500

    On Thursday 11 November 2004 10:59 pm, 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.
    >
    > Yeah, I got it wrong. Without the FULL_PREEMPTION enabled, it does not
    > preempt unless the current thread is in the idle priority band.
    > I was expecting the NEEDRESCHED flag to be used for preemption on
    > return paths, especially for interrupt context. I think this method
    > works better since preemption points become well defined in the
    > kernel.
    > Thanks for helping me figure this out.

    NEEDRESCHED (albeit rather broken at the moment) is used to implement
    preemptino of user threads. As Robert mentioned above, in-kernel preemption
    is managed via either direct switches in setrunqueue() or deferred
    preemptions via TDF_OWEPREEMPT in critical_exit(). 

    -- 
John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org
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  • Next message: Jimmy Selgen: "Re: Reproducible panic in -CURRENT"

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