From: "Russ Cox" <rsc@swtch.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] Re: [sources] 20070413: /rc/bin/cpurc.local
Date: Sun, 15 Apr 2007 13:59:19 -0400 [thread overview]
Message-ID: <20070415175854.B53311E8C26@holo.morphisms.net> (raw)
In-Reply-To: <9ab217670704151043j1da7acc2j61869c6d54f736f7@mail.gmail.com>
>> p->mach is a pointer back to a machine structure. I'm assuming this
>> only gets set when a process is being run. I suppose this test is here
>> in case another CPU took over running the process.
>
> Though, this can't be the case, because m->readied isn't locked when
> we come in here, and if we're running MP, wouldn't it be possible
> (though unlikely) that another CPU takes the process between the
> assignment and the test? Especially if running heavily threaded
> applications on MP systems?
/* cooperative scheduling until the clock ticks */
if((p=m->readied) && p->mach==0 && p->state==Ready
&& runq[Nrq-1].head == nil && runq[Nrq-2].head == nil){
The tests are:
p=m->readied
some process p was last readied on this cpu (Mach)
p->mach == 0
p is not running on any cpu right now
p->state == Ready
p is still Ready (waiting to run)
runq[Nrq-1].head == nil && runq[Nrq-2].head == nil
there are no real-time processes waiting to run
If all those succeed, then the code tries to choose
p to run next. But it might not -- the next thing that
happens is
p = dequeueproc(rq, p);
which can return nil if p has already been grabbed
or even if there is contention for the runq lock.
All the accesses in the if condition are just fine --
they happen without a lock but dequeueproc double-checks
that p is okay to schedule.
If dequeueproc returns nil, then runproc won't pick
the readied p after all -- it will fall into the regular
scheduling loop to find a process.
Russ
next prev parent reply other threads:[~2007-04-15 17:59 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <f93786a38200d44b7406b3181b4e2c20@cat-v.org>
2007-04-15 8:06 ` Uriel
2007-04-15 17:00 ` Uriel
2007-04-15 17:26 ` Devon H. O'Dell
2007-04-15 17:35 ` erik quanstrom
2007-04-15 17:37 ` Devon H. O'Dell
2007-04-15 17:38 ` Russ Cox
2007-04-15 17:43 ` Uriel
2007-04-15 20:31 ` Devon H. O'Dell
2007-04-15 17:43 ` Devon H. O'Dell
2007-04-15 17:59 ` Russ Cox [this message]
2007-04-15 20:34 ` Devon H. O'Dell
2007-04-17 5:21 ` Russ Cox
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