Date:   Tue, 16 May 2017 07:27:42 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 0/5] perf/tracing/cpuhotplug: Fix locking order

On Tue, May 16, 2017 at 05:46:06AM -0700, Paul E. McKenney wrote:
> On Tue, May 16, 2017 at 10:19:23AM +0200, Peter Zijlstra wrote:
> > On Mon, May 15, 2017 at 11:40:43AM -0700, Paul E. McKenney wrote:
> > 
> > > Given that you acquire the global pmus_lock when doing the
> > > get_online_cpus(), and given that CPU hotplug is rare, is it possible
> > > to momentarily acquire the global pmus_lock in perf_event_init_cpu()
> > > and perf_event_exit_cpu() and interact directly with that?  Then perf
> > > would presumably leave alone any outgoing CPU that had already executed
> > > perf_event_exit_cpu(), and also any incoming CPU that had not already
> > > executed perf_event_init_cpu().
> > > 
> > > What prevents this approach from working?
> > 
> > Lack of sleep probably ;-)
> 
> I know that feeling...
> 
> > I'd blame the kids, but those have actually been very good lately.
> 
> I don't get that excuse anymore, all are on their own.  So I need
> to come up with some fresh excuses.  ;-)
> 
> > You're suggesting the below on top, right? I'll run it with lockdep
> > enabled after I chase some regression..
> 
> Something like this, yes.  Maybe even exactly like this.  ;-)

Ah, one thing I forgot...  If you are avoiding use of get_online_cpus(),
you usually also have to be very careful about how you use things like
cpu_online() and cpu_is_offline.

							Thanx, Paul

> > ---
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -8997,7 +8997,6 @@ int perf_pmu_register(struct pmu *pmu, c
> >  {
> >  	int cpu, ret;
> > 
> > -	get_online_cpus();
> >  	mutex_lock(&pmus_lock);
> >  	ret = -ENOMEM;
> >  	pmu->pmu_disable_count = alloc_percpu(int);
> 
> There is usually also some state check in here somewhere for the CPU
> being offline from a perf perspective.  Such a check might already exist,
> but I must plead ignorance of perf.
> 
> > @@ -9093,7 +9092,6 @@ int perf_pmu_register(struct pmu *pmu, c
> >  	ret = 0;
> >  unlock:
> >  	mutex_unlock(&pmus_lock);
> > -	put_online_cpus();
> > 
> >  	return ret;
> > 
> > @@ -11002,10 +11000,9 @@ static void perf_event_exit_cpu_context(
> >  	struct perf_cpu_context *cpuctx;
> >  	struct perf_event_context *ctx;
> >  	struct pmu *pmu;
> > -	int idx;
> > 
> > -	idx = srcu_read_lock(&pmus_srcu);
> > -	list_for_each_entry_rcu(pmu, &pmus, entry) {
> > +	mutex_lock(&pmus_lock);
> 
> If the state change checked for by perf_pmu_register() needs to be also
> guarded by ctx->mutex, this looks right to me.
> 
> Just for completeness, the other style is to maintain separate per-CPU
> state, in which case you would instead acquire pmus_lock, mark this
> CPU off-limits to more perf_pmu_register() usage, release pmus_lock,
> then clean up any old usage.
> 
> The approach you have here seems to work best when the cleanup
> and initialization naturally mark the CPU as off limits and ready,
> respectively.  The other style seems to work best when you need a separate
> indication of which CPUs are off limits and usable.
> 
> RCU is an example of the other style, with the rcu_node structure's
> ->qsmaskinitnext mask serving to mark which CPUs usable.  One reason
> that the other style works so well for RCU is that a CPU coming online
> has no effect on the current grace period, so rcu_cpu_starting() just
> sets the CPU's bit in ->qsmaskinitnext, which takes effect only once
> the the next grace period starts.
> 
> It is quite possible that many of the other use cases instead need to
> use something like what you have here.  I suspect that the common case
> is that a CPU appearing or disappearing must have some immediate effect.
> 
> > +	list_for_each_entry(pmu, &pmus, entry) {
> >  		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
> >  		ctx = &cpuctx->ctx;
> > 
> > @@ -11014,7 +11011,7 @@ static void perf_event_exit_cpu_context(
> >  		cpuctx->online = 0;
> >  		mutex_unlock(&ctx->mutex);
> >  	}
> > -	srcu_read_unlock(&pmus_srcu, idx);
> > +	mutex_unlock(&pmus_lock);
> >  }
> >  #else
> > 
> > @@ -11027,12 +11024,11 @@ int perf_event_init_cpu(unsigned int cpu
> >  	struct perf_cpu_context *cpuctx;
> >  	struct perf_event_context *ctx;
> >  	struct pmu *pmu;
> > -	int idx;
> > 
> >  	perf_swevent_init_cpu(cpu);
> > 
> > -	idx = srcu_read_lock(&pmus_srcu);
> > -	list_for_each_entry_rcu(pmu, &pmus, entry) {
> > +	mutex_lock(&pmus_lock);
> > +	list_for_each_entry(pmu, &pmus, entry) {
> >  		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
> >  		ctx = &cpuctx->ctx;
> > 
> > @@ -11040,7 +11036,7 @@ int perf_event_init_cpu(unsigned int cpu
> >  		cpuctx->online = 1;
> >  		mutex_unlock(&ctx->mutex);
> >  	}
> > -	srcu_read_unlock(&pmus_srcu, idx);
> > +	mutex_unlock(&pmus_lock);
> 
> And same here.
> 
> Again for completeness, the other style would be to mark this CPU
> as ready for perf usage at the very end, protected by pmus_lock.
> 
> 							Thanx, Paul
> 
> >  	return 0;
> >  }
> > 

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