cpufreq: pcc: Fix the potentinal scheduling delays in target_index()

pcc_cpufreq_target():
	cpufreq_freq_transition_begin();
	spin_lock(&pcc_lock);
	[critical section]
	cpufreq_freq_transition_end();
	spin_unlock(&pcc_lock);

Above code has a performance issue, consider that Task0 executes
'cpufreq_freq_transition_end()' to wake Task1 and preempted imediatedly
without releasing 'pcc_lock', then Task1 needs to wait for Task0 to
release 'pcc_lock'. In the worst case, this locking order can result in
Task1 wasting two scheduling rounds before it can enter the critical
section.

Signed-off-by: Liao Chang <liaochang1@huawei.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/drivers/cpufreq/pcc-cpufreq.c b/drivers/cpufreq/pcc-cpufreq.c
index 84fe37d..6f8b5ea 100644 (file)
--- a/drivers/cpufreq/pcc-cpufreq.c
+++ b/drivers/cpufreq/pcc-cpufreq.c
@@ -232,8 +232,8 @@ static int pcc_cpufreq_target(struct cpufreq_policy *policy,
        status = ioread16(&pcch_hdr->status);
        iowrite16(0, &pcch_hdr->status);
 
-       cpufreq_freq_transition_end(policy, &freqs, status != CMD_COMPLETE);
        spin_unlock(&pcc_lock);
+       cpufreq_freq_transition_end(policy, &freqs, status != CMD_COMPLETE);
 
        if (status != CMD_COMPLETE) {
                pr_debug("target: FAILED for cpu %d, with status: 0x%x\n",