perf: Add a pmu capability for "exclusive" events

Usually, pmus that do, for example, instruction tracing, would only ever
be able to have one event per task per cpu (or per perf_event_context). For
such pmus it makes sense to disallow creating conflicting events early on,
so as to provide consistent behavior for the user.

This patch adds a pmu capability that indicates such constraint on event
creation.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kaixu Xia <kaixu.xia@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Robert Richter <rric@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: acme@infradead.org
Cc: adrian.hunter@intel.com
Cc: kan.liang@intel.com
Cc: markus.t.metzger@intel.com
Cc: mathieu.poirier@linaro.org
Link: http://lkml.kernel.org/r/1422613866-113186-1-git-send-email-alexander.shishkin@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 117 insertions, 2 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index da51128c337a..6d9fdaef7b57 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3459,6 +3459,91 @@ static void unaccount_event(struct perf_event *event)
         unaccount_event_cpu(event, event->cpu);
 }
 
+/*
+ * The following implement mutual exclusion of events on "exclusive" pmus
+ * (PERF_PMU_CAP_EXCLUSIVE). Such pmus can only have one event scheduled
+ * at a time, so we disallow creating events that might conflict, namely:
+ *
+ *  1) cpu-wide events in the presence of per-task events,
+ *  2) per-task events in the presence of cpu-wide events,
+ *  3) two matching events on the same context.
+ *
+ * The former two cases are handled in the allocation path (perf_event_alloc(),
+ * __free_event()), the latter -- before the first perf_install_in_context().
+ */
+static int exclusive_event_init(struct perf_event *event)
+{
+        struct pmu *pmu = event->pmu;
+
+        if (!(pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE))
+                return 0;
+
+        /*
+         * Prevent co-existence of per-task and cpu-wide events on the
+         * same exclusive pmu.
+         *
+         * Negative pmu::exclusive_cnt means there are cpu-wide
+         * events on this "exclusive" pmu, positive means there are
+         * per-task events.
+         *
+         * Since this is called in perf_event_alloc() path, event::ctx
+         * doesn't exist yet; it is, however, safe to use PERF_ATTACH_TASK
+         * to mean "per-task event", because unlike other attach states it
+         * never gets cleared.
+         */
+        if (event->attach_state & PERF_ATTACH_TASK) {
+                if (!atomic_inc_unless_negative(&pmu->exclusive_cnt))
+                        return -EBUSY;
+        } else {
+                if (!atomic_dec_unless_positive(&pmu->exclusive_cnt))
+                        return -EBUSY;
+        }
+
+        return 0;
+}
+
+static void exclusive_event_destroy(struct perf_event *event)
+{
+        struct pmu *pmu = event->pmu;
+
+        if (!(pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE))
+                return;
+
+        /* see comment in exclusive_event_init() */
+        if (event->attach_state & PERF_ATTACH_TASK)
+                atomic_dec(&pmu->exclusive_cnt);
+        else
+                atomic_inc(&pmu->exclusive_cnt);
+}
+
+static bool exclusive_event_match(struct perf_event *e1, struct perf_event *e2)
+{
+        if ((e1->pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE) &&
+            (e1->cpu == e2->cpu ||
+             e1->cpu == -1 ||
+             e2->cpu == -1))
+                return true;
+        return false;
+}
+
+/* Called under the same ctx::mutex as perf_install_in_context() */
+static bool exclusive_event_installable(struct perf_event *event,
+                                        struct perf_event_context *ctx)
+{
+        struct perf_event *iter_event;
+        struct pmu *pmu = event->pmu;
+
+        if (!(pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE))
+                return true;
+
+        list_for_each_entry(iter_event, &ctx->event_list, event_entry) {
+                if (exclusive_event_match(iter_event, event))
+                        return false;
+        }
+
+        return true;
+}
+
 static void __free_event(struct perf_event *event)
 {
         if (!event->parent) {
@@ -3472,8 +3557,10 @@ static void __free_event(struct perf_event *event)
         if (event->ctx)
                 put_ctx(event->ctx);
 
-        if (event->pmu)
+        if (event->pmu) {
+                exclusive_event_destroy(event);
                 module_put(event->pmu->module);
+        }
 
         call_rcu(&event->rcu_head, free_event_rcu);
 }
@@ -7150,6 +7237,7 @@ got_cpu_context:
                 pmu->event_idx = perf_event_idx_default;
 
         list_add_rcu(&pmu->entry, &pmus);
+        atomic_set(&pmu->exclusive_cnt, 0);
         ret = 0;
 unlock:
         mutex_unlock(&pmus_lock);
@@ -7405,16 +7493,23 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
                 goto err_ns;
         }
 
+        err = exclusive_event_init(event);
+        if (err)
+                goto err_pmu;
+
         if (!event->parent) {
                 if (event->attr.sample_type & PERF_SAMPLE_CALLCHAIN) {
                         err = get_callchain_buffers();
                         if (err)
-                                goto err_pmu;
+                                goto err_per_task;
                 }
         }
 
         return event;
 
+err_per_task:
+        exclusive_event_destroy(event);
+
 err_pmu:
         if (event->destroy)
                 event->destroy(event);
@@ -7819,6 +7914,11 @@ SYSCALL_DEFINE5(perf_event_open,
                 goto err_alloc;
         }
 
+        if ((pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE) && group_leader) {
+                err = -EBUSY;
+                goto err_context;
+        }
+
         if (task) {
                 put_task_struct(task);
                 task = NULL;
@@ -7941,6 +8041,13 @@ SYSCALL_DEFINE5(perf_event_open,
                 get_ctx(ctx);
         }
 
+        if (!exclusive_event_installable(event, ctx)) {
+                err = -EBUSY;
+                mutex_unlock(&ctx->mutex);
+                fput(event_file);
+                goto err_context;
+        }
+
         perf_install_in_context(ctx, event, event->cpu);
         perf_unpin_context(ctx);
 
@@ -8032,6 +8139,14 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
 
         WARN_ON_ONCE(ctx->parent_ctx);
         mutex_lock(&ctx->mutex);
+        if (!exclusive_event_installable(event, ctx)) {
+                mutex_unlock(&ctx->mutex);
+                perf_unpin_context(ctx);
+                put_ctx(ctx);
+                err = -EBUSY;
+                goto err_free;
+        }
+
         perf_install_in_context(ctx, event, cpu);
         perf_unpin_context(ctx);
         mutex_unlock(&ctx->mutex);