[PATCH] sched/cpufreq: calculate util / max firstly in get_next_freq()

[Chunyan Zhang]

From: Vincent Wang <vincent.wang@xxxxxxxxxx>

When a task that is in_iowait state is enqueued, cpufreq_update_util() will
be invoked with SCHED_CPUFREQ_IOWAIT flag. In this case,the value of util
and max, which are parameters used in get_next_freq(), will be cpu
frequency, instead of cpu util or capactiy.

For some 32bit architectures, the size of unsigned long is 32. When
calculating freq, there may be an overflow error in this expression:

freq = (freq + (freq >> 2)) * util / max;

This patch will fix this overflow risk by calulating util / max firstly,
whether they be frequency or util.

Signed-off-by: Vincent Wang <vincent.wang@xxxxxxxxxx>
Signed-off-by: Chunyan Zhang <chunyan.zhang@xxxxxxxxxx>
---
 kernel/sched/cpufreq_schedutil.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 3fffad3..7c372db1 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -166,8 +166,9 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
 	struct cpufreq_policy *policy = sg_policy->policy;
 	unsigned int freq = arch_scale_freq_invariant() ?
 				policy->cpuinfo.max_freq : policy->cur;
+	unsigned int ratio = util * 100 / max;
 
-	freq = (freq + (freq >> 2)) * util / max;
+	freq = (freq + (freq >> 2)) * ratio / 100;
 
 	if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
 		return sg_policy->next_freq;
-- 
2.7.4