cpufreq: Reduce cpufreq_update_util() overhead a bit

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>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
This commit is contained in:
Rafael J. Wysocki 2016-03-04 03:58:22 +01:00
parent e6f036571e
commit 08f511fd41
3 changed files with 20 additions and 11 deletions

View File

@ -115,12 +115,15 @@ static DEFINE_PER_CPU(struct update_util_data *, cpufreq_update_util_data);
* 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);
@ -133,18 +136,24 @@ EXPORT_SYMBOL_GPL(cpufreq_set_update_util_data);
*
* 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 */

View File

@ -280,7 +280,7 @@ static inline void gov_clear_update_util(struct cpufreq_policy *policy)
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)

View File

@ -1168,7 +1168,7 @@ static void intel_pstate_stop_cpu(struct cpufreq_policy *policy)
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;
@ -1426,7 +1426,7 @@ 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]);
}
}