perf: add the necessary core perf APIs when accessing events counters in eBPF programs

This patch add three core perf APIs:
 - perf_event_attrs(): export the struct perf_event_attr from struct
   perf_event;
 - perf_event_get(): get the struct perf_event from the given fd;
 - perf_event_read_local(): read the events counters active on the
   current CPU;
These APIs are needed when accessing events counters in eBPF programs.

The API perf_event_read_local() comes from Peter and I add the
corresponding SOB.

Signed-off-by: Kaixu Xia <xiakaixu@huawei.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 78 insertions, 0 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index d3dae3419b99..e2c6a8886d4d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3212,6 +3212,59 @@ static inline u64 perf_event_count(struct perf_event *event)
         return __perf_event_count(event);
 }
 
+/*
+ * NMI-safe method to read a local event, that is an event that
+ * is:
+ *   - either for the current task, or for this CPU
+ *   - does not have inherit set, for inherited task events
+ *     will not be local and we cannot read them atomically
+ *   - must not have a pmu::count method
+ */
+u64 perf_event_read_local(struct perf_event *event)
+{
+        unsigned long flags;
+        u64 val;
+
+        /*
+         * Disabling interrupts avoids all counter scheduling (context
+         * switches, timer based rotation and IPIs).
+         */
+        local_irq_save(flags);
+
+        /* If this is a per-task event, it must be for current */
+        WARN_ON_ONCE((event->attach_state & PERF_ATTACH_TASK) &&
+                     event->hw.target != current);
+
+        /* If this is a per-CPU event, it must be for this CPU */
+        WARN_ON_ONCE(!(event->attach_state & PERF_ATTACH_TASK) &&
+                     event->cpu != smp_processor_id());
+
+        /*
+         * It must not be an event with inherit set, we cannot read
+         * all child counters from atomic context.
+         */
+        WARN_ON_ONCE(event->attr.inherit);
+
+        /*
+         * It must not have a pmu::count method, those are not
+         * NMI safe.
+         */
+        WARN_ON_ONCE(event->pmu->count);
+
+        /*
+         * If the event is currently on this CPU, its either a per-task event,
+         * or local to this CPU. Furthermore it means its ACTIVE (otherwise
+         * oncpu == -1).
+         */
+        if (event->oncpu == smp_processor_id())
+                event->pmu->read(event);
+
+        val = local64_read(&event->count);
+        local_irq_restore(flags);
+
+        return val;
+}
+
 static u64 perf_event_read(struct perf_event *event)
 {
         /*
@@ -8574,6 +8627,31 @@ void perf_event_delayed_put(struct task_struct *task)
                 WARN_ON_ONCE(task->perf_event_ctxp[ctxn]);
 }
 
+struct perf_event *perf_event_get(unsigned int fd)
+{
+        int err;
+        struct fd f;
+        struct perf_event *event;
+
+        err = perf_fget_light(fd, &f);
+        if (err)
+                return ERR_PTR(err);
+
+        event = f.file->private_data;
+        atomic_long_inc(&event->refcount);
+        fdput(f);
+
+        return event;
+}
+
+const struct perf_event_attr *perf_event_attrs(struct perf_event *event)
+{
+        if (!event)
+                return ERR_PTR(-EINVAL);
+
+        return &event->attr;
+}
+
 /*
  * inherit a event from parent task to child task:
  */