On 03/10/2014 04:40 PM, Gautham R. Shenoy wrote:
> From: "Srivatsa S. Bhat" <srivatsa.b...@linux.vnet.ibm.com>
>
> On POWER systems, the CPU frequency is controlled at a core-level and
> hence we need to serialize so that only one of the threads in the core
> switches the core's frequency at a time.
>
> Using a global mutex lock would needlessly serialize _all_ frequency
> transitions in the system (across all cores). So introduce per-core
> locking to enable finer-grained synchronization and thereby enhance
> the speed and responsiveness of the cpufreq driver to varying workload
> demands.
>
> The design of per-core locking is very simple and straight-forward: we
> first define a Per-CPU lock and use the ones that belongs to the first
> thread sibling of the core.
>
> cpu_first_thread_sibling() macro is used to find the *common* lock for
> all thread siblings belonging to a core.
>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.b...@linux.vnet.ibm.com>
> Signed-off-by: Vaidyanathan Srinivasan <sva...@linux.vnet.ibm.com>
> Signed-off-by: Gautham R. Shenoy <e...@linux.vnet.ibm.com>
> ---
> drivers/cpufreq/powernv-cpufreq.c | 21 ++++++++++++++++-----
> 1 file changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/cpufreq/powernv-cpufreq.c
> b/drivers/cpufreq/powernv-cpufreq.c
> index 4cad727..4c2e8ca 100644
> --- a/drivers/cpufreq/powernv-cpufreq.c
> +++ b/drivers/cpufreq/powernv-cpufreq.c
> @@ -24,8 +24,15 @@
> #include <linux/of.h>
> #include <asm/cputhreads.h>
>
> -/* FIXME: Make this per-core */
> -static DEFINE_MUTEX(freq_switch_mutex);
> +/* Per-Core locking for frequency transitions */
> +static DEFINE_PER_CPU(struct mutex, freq_switch_lock);
> +
> +#define lock_core_freq(cpu) \
> + mutex_lock(&per_cpu(freq_switch_lock,\
> + cpu_first_thread_sibling(cpu)));
> +#define unlock_core_freq(cpu) \
> + mutex_unlock(&per_cpu(freq_switch_lock,\
> + cpu_first_thread_sibling(cpu)));
>
> #define POWERNV_MAX_PSTATES 256
>
> @@ -233,7 +240,7 @@ static int powernv_cpufreq_target(struct cpufreq_policy
> *policy,
> freqs.new = powernv_freqs[new_index].frequency;
> freqs.cpu = policy->cpu;
>
> - mutex_lock(&freq_switch_mutex);
> + lock_core_freq(policy->cpu);
> cpufreq_notify_transition(policy, &freqs, CPUFREQ_PRECHANGE);
>
> pr_debug("setting frequency for cpu %d to %d kHz index %d pstate %d",
> @@ -245,7 +252,7 @@ static int powernv_cpufreq_target(struct cpufreq_policy
> *policy,
> rc = powernv_set_freq(policy->cpus, new_index);
>
> cpufreq_notify_transition(policy, &freqs, CPUFREQ_POSTCHANGE);
> - mutex_unlock(&freq_switch_mutex);
> + unlock_core_freq(policy->cpu);
>
> return rc;
> }
> @@ -262,7 +269,7 @@ static struct cpufreq_driver powernv_cpufreq_driver = {
>
> static int __init powernv_cpufreq_init(void)
> {
> - int rc = 0;
> + int cpu, rc = 0;
>
> /* Discover pstates from device tree and init */
>
> @@ -272,6 +279,10 @@ static int __init powernv_cpufreq_init(void)
> pr_info("powernv-cpufreq disabled\n");
> return rc;
> }
> + /* Init per-core mutex */
> + for_each_possible_cpu(cpu) {
> + mutex_init(&per_cpu(freq_switch_lock, cpu));
> + }
>
> rc = cpufreq_register_driver(&powernv_cpufreq_driver);
> return rc;
>
Reviewed-by: Preeti U Murthy <pre...@linux.vnet.ibm.com>
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