[PATCH v2 1/10] cpufreq: Reduce cpufreq_update_util() overhead a bit

Rafael J. Wysocki
2016-03-04T02:58:22Z

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Use the observation that cpufreq_update_util() is only called
by the scheduler with rq->lock held, so the callers of
cpufreq_set_update_util_data() can use synchronize_sched()
instead of synchronize_rcu() to wait for cpufreq_update_util()
to complete. Moreover, if they are updated to do that,
rcu_read_(un)lock() calls in cpufreq_update_util() might be
replaced with rcu_read_(un)lock_sched(), respectively, but
those aren't really necessary, because the scheduler calls
that function from RCU-sched read-side critical sections
already.

In addition to that, if cpufreq_set_update_util_data() checks
the func field in the struct update_util_data before setting
the per-CPU pointer to it, the data->func check may be dropped
from cpufreq_update_util() as well.

Make the above changes to reduce the overhead from
cpufreq_update_util() in the scheduler paths invoking it
and to make the cleanup after removing its callbacks less
heavy-weight somewhat.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
---

Changes from the previous version:
- Use rcu_dereference_sched() in cpufreq_update_util().

---
drivers/cpufreq/cpufreq.c | 25 +++++++++++++++++--------
drivers/cpufreq/cpufreq_governor.c | 2 +-
drivers/cpufreq/intel_pstate.c | 4 ++--
3 files changed, 20 insertions(+), 11 deletions(-)

Index: linux-pm/drivers/cpufreq/cpufreq.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq.c
+++ linux-pm/drivers/cpufreq/cpufreq.c
@@ -77,12 +77,15 @@ static DEFINE_PER_CPU(struct update_util
* to call from cpufreq_update_util(). That function will be called from an RCU
* read-side critical section, so it must not sleep.
*
- * Callers must use RCU callbacks to free any memory that might be accessed
- * via the old update_util_data pointer or invoke synchronize_rcu() right after
- * this function to avoid use-after-free.
+ * Callers must use RCU-sched callbacks to free any memory that might be
+ * accessed via the old update_util_data pointer or invoke synchronize_sched()
+ * right after this function to avoid use-after-free.
*/
void cpufreq_set_update_util_data(int cpu, struct update_util_data *data)
{
+ if (WARN_ON(data && !data->func))
+ return;
+
rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
}
EXPORT_SYMBOL_GPL(cpufreq_set_update_util_data);
@@ -95,18 +98,24 @@ EXPORT_SYMBOL_GPL(cpufreq_set_update_uti
*
* This function is called by the scheduler on every invocation of
* update_load_avg() on the CPU whose utilization is being updated.
+ *
+ * It can only be called from RCU-sched read-side critical sections.
*/
void cpufreq_update_util(u64 time, unsigned long util, unsigned long max)
{
struct update_util_data *data;

- rcu_read_lock();
+#ifdef CONFIG_LOCKDEP
+ WARN_ON(debug_locks && !rcu_read_lock_sched_held());
+#endif

- data = rcu_dereference(*this_cpu_ptr(&cpufreq_update_util_data));
- if (data && data->func)
+ data = rcu_dereference_sched(*this_cpu_ptr(&cpufreq_update_util_data));
+ /*
+ * If this isn't inside of an RCU-sched read-side critical section, data
+ * may become NULL after the check below.
+ */
+ if (data)
data->func(data, time, util, max);
-
- rcu_read_unlock();
}

/* Flag to suspend/resume CPUFreq governors */
Index: linux-pm/drivers/cpufreq/cpufreq_governor.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.c
+++ linux-pm/drivers/cpufreq/cpufreq_governor.c
@@ -280,7 +280,7 @@ static inline void gov_clear_update_util
for_each_cpu(i, policy->cpus)
cpufreq_set_update_util_data(i, NULL);

- synchronize_rcu();
+ synchronize_sched();
}

static void gov_cancel_work(struct cpufreq_policy *policy)
Index: linux-pm/drivers/cpufreq/intel_pstate.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/intel_pstate.c
+++ linux-pm/drivers/cpufreq/intel_pstate.c
@@ -1174,7 +1174,7 @@ static void intel_pstate_stop_cpu(struct
pr_debug("intel_pstate: CPU %d exiting\n", cpu_num);

cpufreq_set_update_util_data(cpu_num, NULL);
- synchronize_rcu();
+ synchronize_sched();

if (hwp_active)
return;
@@ -1442,7 +1442,7 @@ out:
for_each_online_cpu(cpu) {
if (all_cpu_data[cpu]) {
cpufreq_set_update_util_data(cpu, NULL);
- synchronize_rcu();
+ synchronize_sched();
kfree(all_cpu_data[cpu]);
}
}