Date: Tue, 6 Jun 2000 07:55:40 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: "G.B.Naidu" <gbnaidu@sasi.com> Cc: Duncan Barclay <dmlb@ragnet.demon.co.uk>, freebsd-hackers@FreeBSD.ORG Subject: Re: wakeup() question Message-ID: <200006061455.HAA93643@apollo.backplane.com> References: <Pine.LNX.4.21.0006061219080.763-100000@pcd75.sasi.com>
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:Hi,
:
:For me it appears that the process should be marked as runnable. But I am
:not sure.
:
:thanks
:--gb
:
:On Mon, 5 Jun 2000, Duncan Barclay wrote:
:
:> Hi all
:>
:> Does wakeup() ever cause a sleeping processes to run before the wakeup()
:> function returns, or does it just mark the process as running?
:>
:> Duncan
:>
:> Duncan Barclay | God smiles upon the little children,
wakeup() is callable from an interrupt so it doesn't synchronously
switch to the newly woken up process. However, it does call
need_resched() (see i386/include/cpu.h) which queues the astpending
software interrupt to handle switching to the new process.
The astpending software interrupt is processed just before the system
goes from a supervisor context to a user context (whether it is
returning from an interrupt or returning from a system call).
Thus if you have an interrupt which calls wakeup() which results in
the new processes getting rescheduled, the system will switch to a
new (may or may not be the one you woke up) process when the interrupt
handler returns.
So, in general, the newly ready process will not start running until
after whoever called wakeup() tries to return to user mode.
*BUT*, on an SMP box we have a different story. On an SMP box another
cpu may pick-up the new process and start running it the instant
wakeup() puts it on the run queue. This pickup is currently predicated on
whether the context doing the waking up is holding the MP lock or not.
You can't depend on that, though. The scheduler will almost certainly
get its own separate locking thingy for the new SMP design.
-Matt
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