Re: [PATCH v5] cpufreq: split the cpufreq_driver_lock and use thercu (was cpufreq: cpufreq_driver_lock is hot on large systems)

[Nathan Zimmer]

On 04/01/2013 11:28 AM, Viresh Kumar wrote:
> Hi Nathan,
>
> Welcome back :)
>
> On 1 April 2013 21:03, Nathan Zimmer <nzimmer@xxxxxxx> wrote:
>
> You need to resent this patch as we don't want current mail subject as commit
> subject.. You could have used the area after three dashes "-" inside the
> commit for logs which you don't want to commit.
Ok.
> > The cpufreq_driver_lock is hot with some configs.
> > This lock covers both cpufreq_driver and cpufreq_cpu_data so part one of the
> s/ so/, so/
>
> > proposed fix is to split up the lock abit.
> s/abit/a bit/
>
> What's the other part?
>
> > cpufreq_cpu_data is now covered by the cpufreq_data_lock.
> > cpufreq_driver is now covered by the cpufreq_driver lock and the rcu.
> >
> > This means that the cpufreq_driver_lock is no longer hot.
> > There remains some measurable heat on the cpufreq_data_lock it is significantly
> s/it/but it/

> > less then previous measured though.
> >
> > Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> > Cc: "Rafael J. Wysocki" <rjw@xxxxxxx>
> > Signed-off-by: Nathan Zimmer <nzimmer@xxxxxxx>
> > ---
> >   drivers/cpufreq/cpufreq.c | 305 +++++++++++++++++++++++++++++++++-------------
> >   1 file changed, 222 insertions(+), 83 deletions(-)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > @@ -329,11 +339,23 @@ static int cpufreq_parse_governor(char *str_governor, unsigned int *policy,
> >                                  struct cpufreq_governor **governor)
> >   {
> >          int err = -EINVAL;
> > -
> > -       if (!cpufreq_driver)
> > +       struct cpufreq_driver *driver;
> > +       int (*setpolicy)(struct cpufreq_policy *policy);
> > +       int (*target)(struct cpufreq_policy *policy,
> > +                     unsigned int target_freq,
> > +                     unsigned int relation);
> You can keep bools here instead of complex function pointers.
> setpolicy_supported and target_supported
Good point.  In a few places I needed the function pointer but not here.
I'll convert the unneeded ones to bools and resend.

> > +       rcu_read_lock();
> > +       driver = rcu_dereference(cpufreq_driver);
> > +       if (!driver) {
> > +               rcu_read_unlock();
> >                  goto out;
> > +       }
> > +       setpolicy = driver->setpolicy;
> > +       target = driver->target;
> > +       rcu_read_unlock();
> >
> > -       if (cpufreq_driver->setpolicy) {
> > +       if (setpolicy) {
> >                  if (!strnicmp(str_governor, "performance", CPUFREQ_NAME_LEN)) {
> >                          *policy = CPUFREQ_POLICY_PERFORMANCE;
> >                          err = 0;
> > @@ -342,7 +364,7 @@ static int cpufreq_parse_governor(char *str_governor, unsigned int *policy,
> >                          *policy = CPUFREQ_POLICY_POWERSAVE;
> >                          err = 0;
> >                  }
> > -       } else if (cpufreq_driver->target) {
> > +       } else if (target) {
> >                  struct cpufreq_governor *t;
> >
> >                  mutex_lock(&cpufreq_governor_mutex);
> > @@ -731,6 +766,8 @@ static int cpufreq_add_dev_interface(unsigned int cpu,
> >   {
> >          struct cpufreq_policy new_policy;
> >          struct freq_attr **drv_attr;
> > +       struct cpufreq_driver *driver;
> > +       int (*exit)(struct cpufreq_policy *policy);
> Declare it in the block which used it.
>
> >          if (ret) {
> >                  pr_debug("setting policy failed\n");
> > -               if (cpufreq_driver->exit)
> > -                       cpufreq_driver->exit(policy);
> > +               rcu_read_lock();
> > +               exit = rcu_dereference(cpufreq_driver)->exit;
> > +               if (exit)
> > +                       exit(policy);
> > +               rcu_read_unlock();
> > +
> >          }
> > @@ -1002,32 +1059,42 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
> >          unsigned int cpu = dev->id, ret, cpus;
> >          unsigned long flags;
> >          struct cpufreq_policy *data;
> > +       struct cpufreq_driver *driver;
> >          struct kobject *kobj;
> >          struct completion *cmp;That
> >          struct device *cpu_dev;
> > +       int (*target)(struct cpufreq_policy *policy,
> > +                      unsigned int target_freq,
> > +                      unsigned int relation);
> can be bool?
>
> > +       int (*exit)(struct cpufreq_policy *policy);
>
> One more generic comment: What about a reader-writer lock for
> cpufreq_data_lock??
I had been looking for ways to use the rcu but wasn't having much success.
Let me try a rwlock and grab some numbers after lunch.
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