Date: Wed, 15 May 2002 20:06:35 +0900 From: Seigo Tanimura <tanimura@r.dl.itc.u-tokyo.ac.jp> To: jhb@FreeBSD.org Cc: current@FreeBSD.org, tanimura@r.dl.itc.u-tokyo.ac.jp Subject: preemption across processors Message-ID: <200205151106.g4FB6Z3i059559@rina.r.dl.itc.u-tokyo.ac.jp>
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Currently, a new runnable thread cannot preempt the thread on any
processor other than the thread that called mi_switch(). For
instance, we do something like the following in _mtx_unlock_sleep():
--- v --- _mtx_unlock_sleep() --- v ---
setrunqueue(th_waken_up);
if (curthread->preemptable && th_waken_up->priority < curthread->priority) {
setrunqueue(curthread);
mi_switch();
}
--- ^ --- _mtx_unlock_sleep() --- ^ ---
If the priority of curthread is higher than th_waken_up, we cannot run
it immediately even if there is another processor running a thread
with a priority lower than th_waken_up. th_waken_up should preempt
that processor, or we would end up with a priority inversion.
Maybe we have to dispatch a runnable thread to the processor running
a thread with the lowest priority. Solaris seems to take the
following steps to do that:
1. If a new thread has slept for longer than 3/100 seconds (this
should be tunable), linearly search the processor running a thread
with the lowest priority. Otherwise, choose the processor that ran
the new thread most recently.
2. Make an inter-processor interrupt to the processor chosen in 1.
3. The chosen processor puts its current thread back to the dispatch
queue and performs a context switch to run the new thread.
Above is only a rough sketch. We have to watch out for a race of
inter-processor interrupts and a processor entering a critical section.
If no one is working on preemption across processors, I would like to
see if I can do that.
Thanks.
--
Seigo Tanimura <tanimura@r.dl.itc.u-tokyo.ac.jp> <tanimura@FreeBSD.org>
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