perf: Fix the perf context switch optimization

Currently we only optimize the context switch between two
contexts that have the same parent; this forgoes the
optimization between parent and child context, even though these
contexts could be equivalent too.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Shishkin, Alexander <alexander.shishkin@intel.com>
Link: http://lkml.kernel.org/r/20131007164257.GH3081@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 85a8bbde64814bdf4ab15356b8d59468811e87b0..17b3c6cf16066b47b55af7ee8e49bab41d94da7c 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -899,6 +899,7 @@ static void unclone_ctx(struct perf_event_context *ctx)
                put_ctx(ctx->parent_ctx);
                ctx->parent_ctx = NULL;
        }
+       ctx->generation++;
 }
 
 static u32 perf_event_pid(struct perf_event *event, struct task_struct *p)
@@ -1136,6 +1137,8 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)
        ctx->nr_events++;
        if (event->attr.inherit_stat)
                ctx->nr_stat++;
+
+       ctx->generation++;
 }
 
 /*
@@ -1313,6 +1316,8 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
         */
        if (event->state > PERF_EVENT_STATE_OFF)
                event->state = PERF_EVENT_STATE_OFF;
+
+       ctx->generation++;
 }
 
 static void perf_group_detach(struct perf_event *event)
@@ -2149,22 +2154,38 @@ static void ctx_sched_out(struct perf_event_context *ctx,
 }
 
 /*
- * Test whether two contexts are equivalent, i.e. whether they
- * have both been cloned from the same version of the same context
- * and they both have the same number of enabled events.
- * If the number of enabled events is the same, then the set
- * of enabled events should be the same, because these are both
- * inherited contexts, therefore we can't access individual events
- * in them directly with an fd; we can only enable/disable all
- * events via prctl, or enable/disable all events in a family
- * via ioctl, which will have the same effect on both contexts.
+ * Test whether two contexts are equivalent, i.e. whether they have both been
+ * cloned from the same version of the same context.
+ *
+ * Equivalence is measured using a generation number in the context that is
+ * incremented on each modification to it; see unclone_ctx(), list_add_event()
+ * and list_del_event().
  */
 static int context_equiv(struct perf_event_context *ctx1,
                         struct perf_event_context *ctx2)
 {
-       return ctx1->parent_ctx && ctx1->parent_ctx == ctx2->parent_ctx
-               && ctx1->parent_gen == ctx2->parent_gen
-               && !ctx1->pin_count && !ctx2->pin_count;
+       /* Pinning disables the swap optimization */
+       if (ctx1->pin_count || ctx2->pin_count)
+               return 0;
+
+       /* If ctx1 is the parent of ctx2 */
+       if (ctx1 == ctx2->parent_ctx && ctx1->generation == ctx2->parent_gen)
+               return 1;
+
+       /* If ctx2 is the parent of ctx1 */
+       if (ctx1->parent_ctx == ctx2 && ctx1->parent_gen == ctx2->generation)
+               return 1;
+
+       /*
+        * If ctx1 and ctx2 have the same parent; we flatten the parent
+        * hierarchy, see perf_event_init_context().
+        */
+       if (ctx1->parent_ctx && ctx1->parent_ctx == ctx2->parent_ctx &&
+                       ctx1->parent_gen == ctx2->parent_gen)
+               return 1;
+
+       /* Unmatched */
+       return 0;
 }
 
 static void __perf_event_sync_stat(struct perf_event *event,
@@ -2247,7 +2268,7 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
 {
        struct perf_event_context *ctx = task->perf_event_ctxp[ctxn];
        struct perf_event_context *next_ctx;
-       struct perf_event_context *parent;
+       struct perf_event_context *parent, *next_parent;
        struct perf_cpu_context *cpuctx;
        int do_switch = 1;
 
@@ -2259,10 +2280,18 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
                return;
 
        rcu_read_lock();
-       parent = rcu_dereference(ctx->parent_ctx);
        next_ctx = next->perf_event_ctxp[ctxn];
-       if (parent && next_ctx &&
-           rcu_dereference(next_ctx->parent_ctx) == parent) {
+       if (!next_ctx)
+               goto unlock;
+
+       parent = rcu_dereference(ctx->parent_ctx);
+       next_parent = rcu_dereference(next_ctx->parent_ctx);
+
+       /* If neither context have a parent context; they cannot be clones. */
+       if (!parent && !next_parent)
+               goto unlock;
+
+       if (next_parent == ctx || next_ctx == parent || next_parent == parent) {
                /*
                 * Looks like the two contexts are clones, so we might be
                 * able to optimize the context switch.  We lock both
@@ -2290,6 +2319,7 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
                raw_spin_unlock(&next_ctx->lock);
                raw_spin_unlock(&ctx->lock);
        }
+unlock:
        rcu_read_unlock();
 
        if (do_switch) {
@@ -7136,7 +7166,6 @@ SYSCALL_DEFINE5(perf_event_open,
        }
 
        perf_install_in_context(ctx, event, event->cpu);
-       ++ctx->generation;
        perf_unpin_context(ctx);
        mutex_unlock(&ctx->mutex);
 
@@ -7219,7 +7248,6 @@ perf_event_create_kernel_counter(struct perf_event_attr *attr, int cpu,
        WARN_ON_ONCE(ctx->parent_ctx);
        mutex_lock(&ctx->mutex);
        perf_install_in_context(ctx, event, cpu);
-       ++ctx->generation;
        perf_unpin_context(ctx);
        mutex_unlock(&ctx->mutex);