perf/ring-buffer: Use regular variables for nesting

While the IRQ/NMI will nest, the nest-count will be invariant over the
actual exception, since it will decrement equal to increment.

This means we can -- carefully -- use a regular variable since the
typical LOAD-STORE race doesn't exist (similar to preempt_count).

This optimizes the ring-buffer for all LOAD-STORE architectures, since
they need to use atomic ops to implement local_t.

Suggested-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: acme@kernel.org
Cc: mark.rutland@arm.com
Cc: namhyung@kernel.org
Cc: yabinc@google.com
Link: http://lkml.kernel.org/r/20190517115418.481392777@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 79c4707..3aef419 100644 (file)
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -24,7 +24,7 @@ struct ring_buffer {
        atomic_t                        poll;           /* POLL_ for wakeups */
 
        local_t                         head;           /* write position    */
-       local_t                         nest;           /* nested writers    */
+       unsigned int                    nest;           /* nested writers    */
        local_t                         events;         /* event limit       */
        local_t                         wakeup;         /* wakeup stamp      */
        local_t                         lost;           /* nr records lost   */
@@ -41,7 +41,7 @@ struct ring_buffer {
 
        /* AUX area */
        long                            aux_head;
-       local_t                         aux_nest;
+       unsigned int                    aux_nest;
        long                            aux_wakeup;     /* last aux_watermark boundary crossed by aux_head */
        unsigned long                   aux_pgoff;
        int                             aux_nr_pages;

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index 7a0c73e..ffb59a4 100644 (file)
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -38,7 +38,12 @@ static void perf_output_get_handle(struct perf_output_handle *handle)
        struct ring_buffer *rb = handle->rb;
 
        preempt_disable();
-       local_inc(&rb->nest);
+
+       /*
+        * Avoid an explicit LOAD/STORE such that architectures with memops
+        * can use them.
+        */
+       (*(volatile unsigned int *)&rb->nest)++;
        handle->wakeup = local_read(&rb->wakeup);
 }
 
@@ -46,6 +51,17 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
 {
        struct ring_buffer *rb = handle->rb;
        unsigned long head;
+       unsigned int nest;
+
+       /*
+        * If this isn't the outermost nesting, we don't have to update
+        * @rb->user_page->data_head.
+        */
+       nest = READ_ONCE(rb->nest);
+       if (nest > 1) {
+               WRITE_ONCE(rb->nest, nest - 1);
+               goto out;
+       }
 
 again:
        /*
@@ -65,15 +81,6 @@ again:
         */
 
        /*
-        * If this isn't the outermost nesting, we don't have to update
-        * @rb->user_page->data_head.
-        */
-       if (local_read(&rb->nest) > 1) {
-               local_dec(&rb->nest);
-               goto out;
-       }
-
-       /*
         * Since the mmap() consumer (userspace) can run on a different CPU:
         *
         *   kernel                             user
@@ -108,7 +115,7 @@ again:
         * write will (temporarily) publish a stale value.
         */
        barrier();
-       local_set(&rb->nest, 0);
+       WRITE_ONCE(rb->nest, 0);
 
        /*
         * Ensure we decrement @rb->nest before we validate the @rb->head.
@@ -116,7 +123,7 @@ again:
         */
        barrier();
        if (unlikely(head != local_read(&rb->head))) {
-               local_inc(&rb->nest);
+               WRITE_ONCE(rb->nest, 1);
                goto again;
        }
 
@@ -355,6 +362,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
        struct perf_event *output_event = event;
        unsigned long aux_head, aux_tail;
        struct ring_buffer *rb;
+       unsigned int nest;
 
        if (output_event->parent)
                output_event = output_event->parent;
@@ -385,13 +393,16 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
        if (!refcount_inc_not_zero(&rb->aux_refcount))
                goto err;
 
+       nest = READ_ONCE(rb->aux_nest);
        /*
         * Nesting is not supported for AUX area, make sure nested
         * writers are caught early
         */
-       if (WARN_ON_ONCE(local_xchg(&rb->aux_nest, 1)))
+       if (WARN_ON_ONCE(nest))
                goto err_put;
 
+       WRITE_ONCE(rb->aux_nest, nest + 1);
+
        aux_head = rb->aux_head;
 
        handle->rb = rb;
@@ -419,7 +430,7 @@ void *perf_aux_output_begin(struct perf_output_handle *handle,
                if (!handle->size) { /* A, matches D */
                        event->pending_disable = smp_processor_id();
                        perf_output_wakeup(handle);
-                       local_set(&rb->aux_nest, 0);
+                       WRITE_ONCE(rb->aux_nest, 0);
                        goto err_put;
                }
        }
@@ -508,7 +519,7 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size)
 
        handle->event = NULL;
 
-       local_set(&rb->aux_nest, 0);
+       WRITE_ONCE(rb->aux_nest, 0);
        /* can't be last */
        rb_free_aux(rb);
        ring_buffer_put(rb);