On Fri, Jun 9, 2017 at 3:15 AM, Viresh Kumar viresh.kumar@linaro.org wrote:
While reducing frequency if there are no frequencies available between "current" and "next" calculated frequency, then the core will never select the "next" frequency.
For example, consider the possible range of frequencies as 900 MHz, 1 GHz, 1.1 GHz, and 1.2 GHz. If the current frequency is 1.1 GHz and the next frequency (based on current utilization) is 1 GHz, then the schedutil governor will try to set the average of these as the next frequency (i.e. 1.05 GHz).
Because we always try to find the lowest frequency greater than equal to the target frequency, cpufreq_driver_resolve_freq() will end up returning 1.1 GHz only. And we will not be able to reduce the frequency eventually. The worst hit is the policy->min frequency as that will never get selected after the frequency is increased once.
But once utilization goes to 0, it will select the min frequency (because it selects lowest frequency >= target)?
This affects all the drivers that provide ->target() or ->target_index() callbacks.
Though for cpufreq drivers, like intel_pstate, which provide ->target() but not ->resolve_freq() (i.e. cpufreq_driver_resolve_freq() simply returns the next frequency), sg_policy->next_freq gets updated with the average frequency. And so we will finally select the min frequency when the next_freq is 1 more than the min frequency as the average then will be equal to the min frequency. But that will also take lots of iterations of the schedutil update callbacks to happen.
Fix that by not using the average value for the next_freq in such cases.
Note that this still doesn't fix the drivers which provide ->target() but don't provide ->resolve_freq() (e.g. intel_pstate) and such drivers need to be updated to provide the ->resolve_freq() callbacks as well in order to fix this.
Fixes: 39b64aa1c007 ("cpufreq: schedutil: Reduce frequencies slower") Signed-off-by: Viresh Kumar viresh.kumar@linaro.org
kernel/sched/cpufreq_schedutil.c | 33 ++++++++++++++++++++++++++++----- 1 file changed, 28 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c index 1852bd73d903..30e6a62d227c 100644 --- a/kernel/sched/cpufreq_schedutil.c +++ b/kernel/sched/cpufreq_schedutil.c @@ -117,6 +117,17 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time, } }
+static unsigned int resolve_freq(struct sugov_policy *sg_policy,
unsigned int freq)+{
if (freq == sg_policy->cached_raw_freq &&sg_policy->next_freq != UINT_MAX)return sg_policy->next_freq;sg_policy->cached_raw_freq = freq;return cpufreq_driver_resolve_freq(sg_policy->policy, freq);+}
/**
- get_next_freq - Compute a new frequency for a given cpufreq policy.
- @sg_policy: schedutil policy object to compute the new frequency for.
@@ -145,6 +156,7 @@ 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 target, original = 0; freq = (freq + (freq >> 2)) * util / max;@@ -156,13 +168,24 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, if (freq < policy->min) freq = policy->min;
if (sg_policy->next_freq > freq)
if (sg_policy->next_freq > freq) {original = freq; freq = (sg_policy->next_freq + freq) >> 1;}
if (freq == sg_policy->cached_raw_freq && sg_policy->next_freq != UINT_MAX)return sg_policy->next_freq;sg_policy->cached_raw_freq = freq;return cpufreq_driver_resolve_freq(policy, freq);
target = resolve_freq(sg_policy, freq);/** While reducing frequency if there are no frequencies available* between "original" and "next_freq", resolve_freq() will return* next_freq because we always try to find the lowest frequency greater* than equal to the "freq". Fix that by going directly to the* "original" frequency in that case.*/if (unlikely(original && target == sg_policy->next_freq))target = resolve_freq(sg_policy, original);return target;}
I thought its confusing to have a special case like this. On one hand we're saying we'd like to select next frequency to be lowest frequency
= target, on the other hand we're saying if the target wasn't low
enough to trigger an OPP change, then we'd just rather drop the frequency to the lower OPP. I get why you'd like to do that, because with patch 1/3 you're lowering frequency more slower before doing the cpufreq_resolve, but what if the reduction in utilization was really small to begin with and not because the "reduce frequencies more slowly" stuff that you moved in patch 1/3? Then in that case you'd be falsely dropping frequency when the right thing to do would be to select the lowest freq >= target?
Thanks, Joel