linux-kernel - Re: Q: select_fallback_rq() && cpuset

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Message-ID: <20100313192823.GA21462@redhat.com>
Date:	Sat, 13 Mar 2010 20:28:23 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Lai Jiangshan <laijs@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: Q: select_fallback_rq() && cpuset_lock()

On 03/11, Oleg Nesterov wrote:
>
> > > @@ -2289,10 +2289,9 @@ static int select_fallback_rq(int cpu, s
> > >
> > >  	/* No more Mr. Nice Guy. */
> > >  	if (dest_cpu >= nr_cpu_ids) {
> > > -		rcu_read_lock();
> > > -		cpuset_cpus_allowed_locked(p, &p->cpus_allowed);
> > > -		rcu_read_unlock();
> > > -		dest_cpu = cpumask_any_and(cpu_active_mask, &p->cpus_allowed);
> > > +		// XXX: take cpu_rq(cpu)->lock ???
> > > +		cpumask_copy(&p->cpus_allowed, cpu_possible_mask);
> > > +		dest_cpu = cpumask_any(cpu_active_mask);
> >
> >
> > Right, this seems safe.
>
> OK, I'll try to read this code a bit more and then send this patch.

No, it is not safe :/

Peter, I seem to see the simple fix for the discussed cpuset problems,
but it turns out sched.c has more problems evem without cpusets.

I'll try to send the whole series on Monday, but perhaps you can look
at the attached compile-tested patches (especially 2 and 3) to quickly
correct me if the 3rd one is wrong.

The subsequent fix in cpuset.c depends on the locking rules enforced
by 2-3.

Oleg.


View attachment "1_KILL_CPUSET_LOCK.patch" of type "text/plain" (5625 bytes)

View attachment "2_MTODC_TAKE_RQ_LOCK.patch" of type "text/plain" (2121 bytes)

View attachment "3_SCHED_EXEC_DONT_FALLBACK.patch" of type "text/plain" (1918 bytes)

View attachment "4_CPU_DOWN_AFFINITY.patch" of type "text/plain" (4179 bytes)