perf_counter: Simplify and fix task migration counting

The task migrations counter was causing rare and hard to decypher
memory corruptions under load. After a day of debugging and bisection
we found that the problem was introduced with:

  3f731ca: perf_counter: Fix cpu migration counter

Turning them off fixes the crashes. Incidentally, the whole
perf_counter_task_migration() logic can be done simpler as well,
by injecting a proper sw-counter event.

This cleanup also fixed the crashes. The precise failure mode is
not completely clear yet, but we are clearly not unhappy about
having a fix ;-)

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 3 insertions, 23 deletions

diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index 7e9108efd305..8d4f0dd41c22 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -124,7 +124,7 @@ void perf_enable(void)
 
 static void get_ctx(struct perf_counter_context *ctx)
 {
-        atomic_inc(&ctx->refcount);
+        WARN_ON(!atomic_inc_not_zero(&ctx->refcount));
 }
 
 static void free_ctx(struct rcu_head *head)
@@ -3467,27 +3467,6 @@ static const struct pmu perf_ops_task_clock = {
         .read           = task_clock_perf_counter_read,
 };
 
-/*
- * Software counter: cpu migrations
- */
-void perf_counter_task_migration(struct task_struct *task, int cpu)
-{
-        struct perf_cpu_context *cpuctx = &per_cpu(perf_cpu_context, cpu);
-        struct perf_counter_context *ctx;
-
-        perf_swcounter_ctx_event(&cpuctx->ctx, PERF_TYPE_SOFTWARE,
-                                 PERF_COUNT_SW_CPU_MIGRATIONS,
-                                 1, 1, NULL, 0);
-
-        ctx = perf_pin_task_context(task);
-        if (ctx) {
-                perf_swcounter_ctx_event(ctx, PERF_TYPE_SOFTWARE,
-                                         PERF_COUNT_SW_CPU_MIGRATIONS,
-                                         1, 1, NULL, 0);
-                perf_unpin_context(ctx);
-        }
-}
-
 #ifdef CONFIG_EVENT_PROFILE
 void perf_tpcounter_event(int event_id)
 {
diff --git a/kernel/sched.c b/kernel/sched.c
index 8fb88a906aaa..f46540b359c0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1978,7 +1978,8 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
                 if (task_hot(p, old_rq->clock, NULL))
                         schedstat_inc(p, se.nr_forced2_migrations);
 #endif
-                perf_counter_task_migration(p, new_cpu);
+                perf_swcounter_event(PERF_COUNT_SW_CPU_MIGRATIONS,
+                                     1, 1, NULL, 0);
         }
         p->se.vruntime -= old_cfsrq->min_vruntime -
                                          new_cfsrq->min_vruntime;