perf_counters: make software counters work as per-cpu counters

Impact: kernel crash fix

Yanmin Zhang reported that using a PERF_COUNT_TASK_CLOCK software
counter as a per-cpu counter would reliably crash the system, because
it calls __task_delta_exec with a null pointer.  The page fault,
context switch and cpu migration counters also won't function
correctly as per-cpu counters since they reference the current task.

This fixes the problem by redirecting the task_clock counter to the
cpu_clock counter when used as a per-cpu counter, and by implementing
per-cpu page fault, context switch and cpu migration counters.

Along the way, this:

- Initializes counter->ctx earlier, in perf_counter_alloc, so that
  sw_perf_counter_init can use it
- Adds code to kernel/sched.c to count task migrations into each
  cpu, in rq->nr_migrations_in
- Exports the per-cpu context switch and task migration counts
  via new functions added to kernel/sched.c
- Makes sure that if sw_perf_counter_init fails, we don't try to
  initialize the counter as a hardware counter.  Since the user has
  passed a negative, non-raw event type, they clearly don't intend
  for it to be interpreted as a hardware event.

Reported-by: "Zhang Yanmin" <yanmin_zhang@linux.intel.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 64 insertions, 33 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b85b10abf770..1e5f70062a9c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -137,6 +137,8 @@ extern unsigned long nr_running(void);
 extern unsigned long nr_uninterruptible(void);
 extern unsigned long nr_active(void);
 extern unsigned long nr_iowait(void);
+extern u64 cpu_nr_switches(int cpu);
+extern u64 cpu_nr_migrations(int cpu);
 
 struct seq_file;
 struct cfs_rq;
diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
index f27a7e9f3c41..544193cbc478 100644
--- a/kernel/perf_counter.c
+++ b/kernel/perf_counter.c
@@ -20,6 +20,8 @@
 #include <linux/anon_inodes.h>
 #include <linux/kernel_stat.h>
 #include <linux/perf_counter.h>
+#include <linux/mm.h>
+#include <linux/vmstat.h>
 
 /*
  * Each CPU has a list of per CPU counters:
@@ -502,7 +504,6 @@ perf_install_in_context(struct perf_counter_context *ctx,
 {
         struct task_struct *task = ctx->task;
 
-        counter->ctx = ctx;
         if (!task) {
                 /*
                  * Per cpu counters are installed via an smp call and
@@ -1417,11 +1418,19 @@ static const struct hw_perf_counter_ops perf_ops_task_clock = {
         .read           = task_clock_perf_counter_read,
 };
 
-static u64 get_page_faults(void)
+#ifdef CONFIG_VM_EVENT_COUNTERS
+#define cpu_page_faults()       __get_cpu_var(vm_event_states).event[PGFAULT]
+#else
+#define cpu_page_faults()       0
+#endif
+
+static u64 get_page_faults(struct perf_counter *counter)
 {
-        struct task_struct *curr = current;
+        struct task_struct *curr = counter->ctx->task;
 
-        return curr->maj_flt + curr->min_flt;
+        if (curr)
+                return curr->maj_flt + curr->min_flt;
+        return cpu_page_faults();
 }
 
 static void page_faults_perf_counter_update(struct perf_counter *counter)
@@ -1430,7 +1439,7 @@ static void page_faults_perf_counter_update(struct perf_counter *counter)
         s64 delta;
 
         prev = atomic64_read(&counter->hw.prev_count);
-        now = get_page_faults();
+        now = get_page_faults(counter);
 
         atomic64_set(&counter->hw.prev_count, now);
 
@@ -1446,11 +1455,7 @@ static void page_faults_perf_counter_read(struct perf_counter *counter)
 
 static int page_faults_perf_counter_enable(struct perf_counter *counter)
 {
-        /*
-         * page-faults is a per-task value already,
-         * so we dont have to clear it on switch-in.
-         */
-
+        atomic64_set(&counter->hw.prev_count, get_page_faults(counter));
         return 0;
 }
 
@@ -1465,11 +1470,13 @@ static const struct hw_perf_counter_ops perf_ops_page_faults = {
         .read           = page_faults_perf_counter_read,
 };
 
-static u64 get_context_switches(void)
+static u64 get_context_switches(struct perf_counter *counter)
 {
-        struct task_struct *curr = current;
+        struct task_struct *curr = counter->ctx->task;
 
-        return curr->nvcsw + curr->nivcsw;
+        if (curr)
+                return curr->nvcsw + curr->nivcsw;
+        return cpu_nr_switches(smp_processor_id());
 }
 
 static void context_switches_perf_counter_update(struct perf_counter *counter)
@@ -1478,7 +1485,7 @@ static void context_switches_perf_counter_update(struct perf_counter *counter)
         s64 delta;
 
         prev = atomic64_read(&counter->hw.prev_count);
-        now = get_context_switches();
+        now = get_context_switches(counter);
 
         atomic64_set(&counter->hw.prev_count, now);
 
@@ -1494,11 +1501,7 @@ static void context_switches_perf_counter_read(struct perf_counter *counter)
 
 static int context_switches_perf_counter_enable(struct perf_counter *counter)
 {
-        /*
-         * ->nvcsw + curr->nivcsw is a per-task value already,
-         * so we dont have to clear it on switch-in.
-         */
-
+        atomic64_set(&counter->hw.prev_count, get_context_switches(counter));
         return 0;
 }
 
@@ -1513,9 +1516,13 @@ static const struct hw_perf_counter_ops perf_ops_context_switches = {
         .read           = context_switches_perf_counter_read,
 };
 
-static inline u64 get_cpu_migrations(void)
+static inline u64 get_cpu_migrations(struct perf_counter *counter)
 {
-        return current->se.nr_migrations;
+        struct task_struct *curr = counter->ctx->task;
+
+        if (curr)
+                return curr->se.nr_migrations;
+        return cpu_nr_migrations(smp_processor_id());
 }
 
 static void cpu_migrations_perf_counter_update(struct perf_counter *counter)
@@ -1524,7 +1531,7 @@ static void cpu_migrations_perf_counter_update(struct perf_counter *counter)
         s64 delta;
 
         prev = atomic64_read(&counter->hw.prev_count);
-        now = get_cpu_migrations();
+        now = get_cpu_migrations(counter);
 
         atomic64_set(&counter->hw.prev_count, now);
 
@@ -1540,11 +1547,7 @@ static void cpu_migrations_perf_counter_read(struct perf_counter *counter)
 
 static int cpu_migrations_perf_counter_enable(struct perf_counter *counter)
 {
-        /*
-         * se.nr_migrations is a per-task value already,
-         * so we dont have to clear it on switch-in.
-         */
-
+        atomic64_set(&counter->hw.prev_count, get_cpu_migrations(counter));
         return 0;
 }
 
@@ -1569,7 +1572,14 @@ sw_perf_counter_init(struct perf_counter *counter)
                 hw_ops = &perf_ops_cpu_clock;
                 break;
         case PERF_COUNT_TASK_CLOCK:
-                hw_ops = &perf_ops_task_clock;
+                /*
+                 * If the user instantiates this as a per-cpu counter,
+                 * use the cpu_clock counter instead.
+                 */
+                if (counter->ctx->task)
+                        hw_ops = &perf_ops_task_clock;
+                else
+                        hw_ops = &perf_ops_cpu_clock;
                 break;
         case PERF_COUNT_PAGE_FAULTS:
                 hw_ops = &perf_ops_page_faults;
@@ -1592,6 +1602,7 @@ sw_perf_counter_init(struct perf_counter *counter)
 static struct perf_counter *
 perf_counter_alloc(struct perf_counter_hw_event *hw_event,
                    int cpu,
+                   struct perf_counter_context *ctx,
                    struct perf_counter *group_leader,
                    gfp_t gfpflags)
 {
@@ -1623,6 +1634,7 @@ perf_counter_alloc(struct perf_counter_hw_event *hw_event,
         counter->wakeup_pending         = 0;
         counter->group_leader           = group_leader;
         counter->hw_ops                 = NULL;
+        counter->ctx                    = ctx;
 
         counter->state = PERF_COUNTER_STATE_INACTIVE;
         if (hw_event->disabled)
@@ -1631,7 +1643,7 @@ perf_counter_alloc(struct perf_counter_hw_event *hw_event,
         hw_ops = NULL;
         if (!hw_event->raw && hw_event->type < 0)
                 hw_ops = sw_perf_counter_init(counter);
-        if (!hw_ops)
+        else
                 hw_ops = hw_perf_counter_init(counter);
 
         if (!hw_ops) {
@@ -1707,7 +1719,8 @@ sys_perf_counter_open(struct perf_counter_hw_event *hw_event_uptr __user,
         }
 
         ret = -EINVAL;
-        counter = perf_counter_alloc(&hw_event, cpu, group_leader, GFP_KERNEL);
+        counter = perf_counter_alloc(&hw_event, cpu, ctx, group_leader,
+                                     GFP_KERNEL);
         if (!counter)
                 goto err_put_context;
 
@@ -1777,15 +1790,14 @@ inherit_counter(struct perf_counter *parent_counter,
                 parent_counter = parent_counter->parent;
 
         child_counter = perf_counter_alloc(&parent_counter->hw_event,
-                                            parent_counter->cpu, group_leader,
-                                            GFP_KERNEL);
+                                           parent_counter->cpu, child_ctx,
+                                           group_leader, GFP_KERNEL);
         if (!child_counter)
                 return NULL;
 
         /*
          * Link it up in the child's context:
          */
-        child_counter->ctx = child_ctx;
         child_counter->task = child;
         list_add_counter(child_counter, child_ctx);
         child_ctx->nr_counters++;
diff --git a/kernel/sched.c b/kernel/sched.c
index 8db1a4cf2082..173768f142ad 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -558,6 +558,7 @@ struct rq {
         struct load_weight load;
         unsigned long nr_load_updates;
         u64 nr_switches;
+        u64 nr_migrations_in;
 
         struct cfs_rq cfs;
         struct rt_rq rt;
@@ -1908,6 +1909,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
 #endif
         if (old_cpu != new_cpu) {
                 p->se.nr_migrations++;
+                new_rq->nr_migrations_in++;
 #ifdef CONFIG_SCHEDSTATS
                 if (task_hot(p, old_rq->clock, NULL))
                         schedstat_inc(p, se.nr_forced2_migrations);
@@ -2811,6 +2813,21 @@ unsigned long nr_active(void)
 }
 
 /*
+ * Externally visible per-cpu scheduler statistics:
+ * cpu_nr_switches(cpu) - number of context switches on that cpu
+ * cpu_nr_migrations(cpu) - number of migrations into that cpu
+ */
+u64 cpu_nr_switches(int cpu)
+{
+        return cpu_rq(cpu)->nr_switches;
+}
+
+u64 cpu_nr_migrations(int cpu)
+{
+        return cpu_rq(cpu)->nr_migrations_in;
+}
+
+/*
  * Update rq->cpu_load[] statistics. This function is usually called every
  * scheduler tick (TICK_NSEC).
  */