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Message-Id: <20170515184043.GU3956@linux.vnet.ibm.com>
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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