perf/ring-buffer: Use regular variables for nesting [Linux 5.2]

perf/ring-buffer: Use regular variables for nesting [Linux 5.2]

This Linux kernel change "perf/ring-buffer: Use regular variables for nesting" is included in the Linux 5.2 release. This change is authored by Peter Zijlstra <peterz [at] infradead.org> on Fri May 17 13:52:34 2019 +0200. The commit for this change in Linux stable tree is 5322ea5 (patch).

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 <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vince Weaver <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

There are 45 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 kernel/events/internal.h    |  4 ++--
 kernel/events/ring_buffer.c | 41 ++++++++++++++++++++++++++---------------
 2 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 79c4707..3aef419 100644
--- 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
--- 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 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
     */

    /*
-    * 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 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
     * 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 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
     */
    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);

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