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Message-ID: <20170515090318.amup4tbqujg27nrl@hirez.programming.kicks-ass.net>
Date: Mon, 15 May 2017 11:03:18 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 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..
---
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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