Date:   Mon, 15 May 2017 11:40:43 -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 Mon, May 15, 2017 at 11:03:18AM +0200, Peter Zijlstra wrote:
> On Sat, May 13, 2017 at 06:40:03AM -0700, Paul E. McKenney wrote:
> > On Fri, May 12, 2017 at 05:34:48PM -0400, Steven Rostedt wrote:
> > > On Fri, 12 May 2017 21:49:56 +0200
> > 
> > [ . . . ]
> > 
> > > This means that text_mutex, which was taken by the alternative code, no
> > > longer is taken in cpu hotplug code. That means there's no longer a
> > > deadlock scenario, as we don't have anyplace(*) that grabs
> > > get_online_cpus() and takes the text_mutex. Removing that will
> > > simplify things tremendously!
> > > 
> > > (*) with one exception: perf.
> > > 
> > > Currently perf does a get_online_cpu() at a high level. Will it be
> > > possible to move that down, such that we don't have it taken when we do
> > > any software events?
> > 
> > Can perf get rid of get_online_cpus(), perhaps using the mutexes acquired
> > by perf_event_init_cpu() or by perf_event_exit_cpu()?
> 
> 
> Best I could come up with is something like that below that moves the
> get_online_cpus() to a possibly less onerous place.
> 
> Compile tested only..

I freely confess that I don't fully understand this code, but...

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?

							Thanx, Paul

> ---
>  include/linux/perf_event.h |  2 ++
>  kernel/events/core.c       | 85 ++++++++++++++++++++++++++++++++++------------
>  2 files changed, 66 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 24a635887f28..7d6aa29094b2 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -801,6 +801,8 @@ struct perf_cpu_context {
> 
>  	struct list_head		sched_cb_entry;
>  	int				sched_cb_usage;
> +
> +	int				online;
>  };
> 
>  struct perf_output_handle {
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 13f5b942580b..f9a595641d32 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3812,14 +3812,6 @@ find_get_context(struct pmu *pmu, struct task_struct *task,
>  		if (perf_paranoid_cpu() && !capable(CAP_SYS_ADMIN))
>  			return ERR_PTR(-EACCES);
> 
> -		/*
> -		 * We could be clever and allow to attach a event to an
> -		 * offline CPU and activate it when the CPU comes up, but
> -		 * that's for later.
> -		 */
> -		if (!cpu_online(cpu))
> -			return ERR_PTR(-ENODEV);
> -
>  		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
>  		ctx = &cpuctx->ctx;
>  		get_ctx(ctx);
> @@ -9005,6 +8997,7 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
>  {
>  	int cpu, ret;
> 
> +	get_online_cpus();
>  	mutex_lock(&pmus_lock);
>  	ret = -ENOMEM;
>  	pmu->pmu_disable_count = alloc_percpu(int);
> @@ -9064,6 +9057,7 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
>  		lockdep_set_class(&cpuctx->ctx.mutex, &cpuctx_mutex);
>  		lockdep_set_class(&cpuctx->ctx.lock, &cpuctx_lock);
>  		cpuctx->ctx.pmu = pmu;
> +		cpuctx->online = cpu_online(cpu);
> 
>  		__perf_mux_hrtimer_init(cpuctx, cpu);
>  	}
> @@ -9099,6 +9093,7 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
>  	ret = 0;
>  unlock:
>  	mutex_unlock(&pmus_lock);
> +	put_online_cpus();
> 
>  	return ret;
> 
> @@ -9908,13 +9903,11 @@ SYSCALL_DEFINE5(perf_event_open,
>  	if (flags & PERF_FLAG_PID_CGROUP)
>  		cgroup_fd = pid;
> 
> -	get_online_cpus();
> -
>  	event = perf_event_alloc(&attr, cpu, task, group_leader, NULL,
>  				 NULL, NULL, cgroup_fd);
>  	if (IS_ERR(event)) {
>  		err = PTR_ERR(event);
> -		goto err_cpus;
> +		goto err_cred;
>  	}
> 
>  	if (is_sampling_event(event)) {
> @@ -10078,6 +10071,23 @@ SYSCALL_DEFINE5(perf_event_open,
>  		goto err_locked;
>  	}
> 
> +	if (!task) {
> +		/*
> +		 * Check if the @cpu we're creating an event for is online.
> +		 *
> +		 * We use the perf_cpu_context::ctx::mutex to serialize against
> +		 * the hotplug notifiers. See perf_event_{init,exit}_cpu().
> +		 */
> +		struct perf_cpu_context *cpuctx =
> +			container_of(ctx, struct perf_cpu_context, ctx);
> +
> +		if (!cpuctx->online) {
> +			err = -ENODEV;
> +			goto err_locked;
> +		}
> +	}
> +
> +
>  	/*
>  	 * Must be under the same ctx::mutex as perf_install_in_context(),
>  	 * because we need to serialize with concurrent event creation.
> @@ -10162,8 +10172,6 @@ SYSCALL_DEFINE5(perf_event_open,
>  		perf_event_ctx_unlock(group_leader, gctx);
>  	mutex_unlock(&ctx->mutex);
> 
> -	put_online_cpus();
> -
>  	if (task) {
>  		mutex_unlock(&task->signal->cred_guard_mutex);
>  		put_task_struct(task);
> @@ -10205,8 +10213,6 @@ SYSCALL_DEFINE5(perf_event_open,
>  	 */
>  	if (!event_file)
>  		free_event(event);
> -err_cpus:
> -	put_online_cpus();
>  err_cred:
>  	if (task)
>  		mutex_unlock(&task->signal->cred_guard_mutex);
> @@ -10237,7 +10243,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
>  	struct perf_event *event;
>  	int err;
> 
> -	get_online_cpus();
>  	/*
>  	 * Get the target context (task or percpu):
>  	 */
> @@ -10264,6 +10269,21 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
>  		goto err_unlock;
>  	}
> 
> +	if (!task) {
> +		/*
> +		 * Check if the @cpu we're creating an event for is online.
> +		 *
> +		 * We use the perf_cpu_context::ctx::mutex to serialize against
> +		 * the hotplug notifiers. See perf_event_{init,exit}_cpu().
> +		 */
> +		struct perf_cpu_context *cpuctx =
> +			container_of(ctx, struct perf_cpu_context, ctx);
> +		if (!cpuctx->online) {
> +			err = -ENODEV;
> +			goto err_unlock;
> +		}
> +	}
> +
>  	if (!exclusive_event_installable(event, ctx)) {
>  		err = -EBUSY;
>  		goto err_unlock;
> @@ -10272,7 +10292,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
>  	perf_install_in_context(ctx, event, cpu);
>  	perf_unpin_context(ctx);
>  	mutex_unlock(&ctx->mutex);
> -	put_online_cpus();
>  	return event;
> 
>  err_unlock:
> @@ -10282,7 +10301,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
>  err_free:
>  	free_event(event);
>  err:
> -	put_online_cpus();
>  	return ERR_PTR(err);
>  }
>  EXPORT_SYMBOL_GPL(perf_event_create_kernel_counter);
> @@ -10951,7 +10969,7 @@ static void __init perf_event_init_all_cpus(void)
>  	}
>  }
> 
> -int perf_event_init_cpu(unsigned int cpu)
> +void perf_swevent_init_cpu(unsigned int cpu)
>  {
>  	struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);
> 
> @@ -10964,7 +10982,6 @@ int perf_event_init_cpu(unsigned int cpu)
>  		rcu_assign_pointer(swhash->swevent_hlist, hlist);
>  	}
>  	mutex_unlock(&swhash->hlist_mutex);
> -	return 0;
>  }
> 
>  #if defined CONFIG_HOTPLUG_CPU || defined CONFIG_KEXEC_CORE
> @@ -10982,16 +10999,19 @@ static void __perf_event_exit_context(void *__info)
> 
>  static void perf_event_exit_cpu_context(int cpu)
>  {
> +	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) {
> -		ctx = &per_cpu_ptr(pmu->pmu_cpu_context, cpu)->ctx;
> +		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
> +		ctx = &cpuctx->ctx;
> 
>  		mutex_lock(&ctx->mutex);
>  		smp_call_function_single(cpu, __perf_event_exit_context, ctx, 1);
> +		cpuctx->online = 0;
>  		mutex_unlock(&ctx->mutex);
>  	}
>  	srcu_read_unlock(&pmus_srcu, idx);
> @@ -11002,6 +11022,29 @@ static void perf_event_exit_cpu_context(int cpu) { }
> 
>  #endif
> 
> +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) {
> +		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
> +		ctx = &cpuctx->ctx;
> +
> +		mutex_lock(&ctx->mutex);
> +		cpuctx->online = 1;
> +		mutex_unlock(&ctx->mutex);
> +	}
> +	srcu_read_unlock(&pmus_srcu, idx);
> +
> +	return 0;
> +}
> +
>  int perf_event_exit_cpu(unsigned int cpu)
>  {
>  	perf_event_exit_cpu_context(cpu);
> 

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