perf_counter: Allow sharing of output channels

Provide the ability to configure a counter to send its output
to another (already existing) counter's output stream.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: stephane eranian <eranian@googlemail.com>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <20090819092023.980284148@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 85 insertions, 3 deletions

diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index b53f7006cc4e..e022b847c90d 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -216,6 +216,7 @@ struct perf_counter_attr {
 #define PERF_COUNTER_IOC_REFRESH        _IO ('$', 2)
 #define PERF_COUNTER_IOC_RESET          _IO ('$', 3)
 #define PERF_COUNTER_IOC_PERIOD         _IOW('$', 4, u64)
+#define PERF_COUNTER_IOC_SET_OUTPUT     _IO ('$', 5)
 
 enum perf_counter_ioc_flags {
         PERF_IOC_FLAG_GROUP             = 1U << 0,
@@ -415,6 +416,9 @@ enum perf_callchain_context {
         PERF_CONTEXT_MAX                = (__u64)-4095,
 };
 
+#define PERF_FLAG_FD_NO_GROUP   (1U << 0)
+#define PERF_FLAG_FD_OUTPUT     (1U << 1)
+
 #ifdef __KERNEL__
 /*
  * Kernel-internal data types and definitions:
@@ -536,6 +540,7 @@ struct perf_counter {
         struct list_head                sibling_list;
         int                             nr_siblings;
         struct perf_counter             *group_leader;
+        struct perf_counter             *output;
         const struct pmu                *pmu;
 
         enum perf_counter_active_state  state;
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 06bf6a4f2608..53abcbefa0bf 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -1692,6 +1692,11 @@ static void free_counter(struct perf_counter *counter)
                         atomic_dec(&nr_task_counters);
         }
 
+        if (counter->output) {
+                fput(counter->output->filp);
+                counter->output = NULL;
+        }
+
         if (counter->destroy)
                 counter->destroy(counter);
 
@@ -1977,6 +1982,8 @@ unlock:
         return ret;
 }
 
+int perf_counter_set_output(struct perf_counter *counter, int output_fd);
+
 static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 {
         struct perf_counter *counter = file->private_data;
@@ -2000,6 +2007,9 @@ static long perf_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
         case PERF_COUNTER_IOC_PERIOD:
                 return perf_counter_period(counter, (u64 __user *)arg);
 
+        case PERF_COUNTER_IOC_SET_OUTPUT:
+                return perf_counter_set_output(counter, arg);
+
         default:
                 return -ENOTTY;
         }
@@ -2270,6 +2280,11 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 
         WARN_ON_ONCE(counter->ctx->parent_ctx);
         mutex_lock(&counter->mmap_mutex);
+        if (counter->output) {
+                ret = -EINVAL;
+                goto unlock;
+        }
+
         if (atomic_inc_not_zero(&counter->mmap_count)) {
                 if (nr_pages != counter->data->nr_pages)
                         ret = -EINVAL;
@@ -2655,6 +2670,7 @@ static int perf_output_begin(struct perf_output_handle *handle,
                              struct perf_counter *counter, unsigned int size,
                              int nmi, int sample)
 {
+        struct perf_counter *output_counter;
         struct perf_mmap_data *data;
         unsigned int offset, head;
         int have_lost;
@@ -2664,13 +2680,17 @@ static int perf_output_begin(struct perf_output_handle *handle,
                 u64                      lost;
         } lost_event;
 
+        rcu_read_lock();
         /*
          * For inherited counters we send all the output towards the parent.
          */
         if (counter->parent)
                 counter = counter->parent;
 
-        rcu_read_lock();
+        output_counter = rcu_dereference(counter->output);
+        if (output_counter)
+                counter = output_counter;
+
         data = rcu_dereference(counter->data);
         if (!data)
                 goto out;
@@ -4218,6 +4238,57 @@ err_size:
         goto out;
 }
 
+int perf_counter_set_output(struct perf_counter *counter, int output_fd)
+{
+        struct perf_counter *output_counter = NULL;
+        struct file *output_file = NULL;
+        struct perf_counter *old_output;
+        int fput_needed = 0;
+        int ret = -EINVAL;
+
+        if (!output_fd)
+                goto set;
+
+        output_file = fget_light(output_fd, &fput_needed);
+        if (!output_file)
+                return -EBADF;
+
+        if (output_file->f_op != &perf_fops)
+                goto out;
+
+        output_counter = output_file->private_data;
+
+        /* Don't chain output fds */
+        if (output_counter->output)
+                goto out;
+
+        /* Don't set an output fd when we already have an output channel */
+        if (counter->data)
+                goto out;
+
+        atomic_long_inc(&output_file->f_count);
+
+set:
+        mutex_lock(&counter->mmap_mutex);
+        old_output = counter->output;
+        rcu_assign_pointer(counter->output, output_counter);
+        mutex_unlock(&counter->mmap_mutex);
+
+        if (old_output) {
+                /*
+                 * we need to make sure no existing perf_output_*()
+                 * is still referencing this counter.
+                 */
+                synchronize_rcu();
+                fput(old_output->filp);
+        }
+
+        ret = 0;
+out:
+        fput_light(output_file, fput_needed);
+        return ret;
+}
+
 /**
  * sys_perf_counter_open - open a performance counter, associate it to a task/cpu
  *
@@ -4240,7 +4311,7 @@ SYSCALL_DEFINE5(perf_counter_open,
         int ret;
 
         /* for future expandability... */
-        if (flags)
+        if (flags & ~(PERF_FLAG_FD_NO_GROUP | PERF_FLAG_FD_OUTPUT))
                 return -EINVAL;
 
         ret = perf_copy_attr(attr_uptr, &attr);
@@ -4268,7 +4339,7 @@ SYSCALL_DEFINE5(perf_counter_open,
          * Look up the group leader (we will attach this counter to it):
          */
         group_leader = NULL;
-        if (group_fd != -1) {
+        if (group_fd != -1 && !(flags & PERF_FLAG_FD_NO_GROUP)) {
                 ret = -EINVAL;
                 group_file = fget_light(group_fd, &fput_needed);
                 if (!group_file)
@@ -4310,6 +4381,12 @@ SYSCALL_DEFINE5(perf_counter_open,
         if (!counter_file)
                 goto err_free_put_context;
 
+        if (flags & PERF_FLAG_FD_OUTPUT) {
+                ret = perf_counter_set_output(counter, group_fd);
+                if (ret)
+                        goto err_free_put_context;
+        }
+
         counter->filp = counter_file;
         WARN_ON_ONCE(ctx->parent_ctx);
         mutex_lock(&ctx->mutex);