On Wed, Mar 9, 2016 at 1:39 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Fri, Mar 04, 2016 at 03:58:22AM +0100, Rafael J. Wysocki wrote: >> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> >> >> 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@xxxxxxxxx> >> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx> >> --- >> >> Changes from the previous version: >> - Use rcu_dereference_sched() in cpufreq_update_util(). > > Which I think also shows the WARN_ON I insisted upon is redundant. > > In any case, I cannot object to reducing overhead, esp. as this whole > patch was suggested by me in the first place, so: > > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Thanks! > That said, how about the below? It avoids a function call. That is fine by me. What about taking it a bit further, though, and moving the definition of cpufreq_update_util_data to somewhere under kernel/sched/ (like kernel/sched/cpufreq.c maybe)? Then, the whole static inline void cpufreq_update_util() definition can go into kernel/sched/sched.h (it doesn't have to be visible anywhere beyond kernel/sched/) and the only thing that needs to be exported to cpufreq will be a helper (or two), to set/clear the cpufreq_update_util_data pointers. I'll try to cut a patch doing that later today for illustration. -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
