Re: [PATCH v2 1/3] cppc_cpufreq: Return desired perf in ->get() if feedback counters are 0

Jie Zhan
2024-09-18T02:05:53Z

On 17/09/2024 18:36, Ionela Voinescu wrote:

...

>>> @@ -747,19 +750,22 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
>>> cpufreq_cpu_put(policy);
>>>
>>> ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t0);
>>> - if (ret)
>>> - return 0;
>>> -
>>> - udelay(2); /* 2usec delay between sampling */
>>> -
>>> - ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
>>> - if (ret)
>>> - return 0;
>>> + if (!ret) {
>>> + udelay(2); /* 2usec delay between sampling */
>>> + ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
>>> + }
>>> + if (!ret)
>>> + delivered_perf = cppc_perf_from_fbctrs(cpu_data, &fb_ctrs_t0,
>>> + &fb_ctrs_t1);
>>
>> TBH, 'if (!ret)' style looks very strange to me.
>> We haven't done so anywhere in cppc_cpufreq, so let's keep consistency and make
>> it easier for people to read and maintain?
>
> I agree it's a bit of a difficult read, that's why I only sent my code
> as a suggestion. I did like the benefit of not having to have two
> different calls to cppc_perf_to_khz() and making the code below common
> for the error and non-error paths. But it's up to you.

Yeah understood. I did try minimizing duplicate code, but ended up with either
duplicate 'get desired perf' stuff or duplicate cppc_perf_to_khz().

...
>>
>> delivered_perf = cppc_perf_from_fbctrs(cpu_data, &fb_ctrs_t0,
>> &fb_ctrs_t1);
>
> You need a check here for !delivered_perf (when at least one of the
> deltas is 0) in which case it would be good to take the same error path
> below. Something like:
>
> if(delivered_perf)
> return cppc_perf_to_khz(&cpu_data->perf_caps, delivered_perf);
> else
> ret = -EFAULT;
>
> That's why I did the tricky if/else dance above as we need to take the
> error path below for multiple cases.
>
> Thanks,
> Ionela.
>

Sure, thanks for reminding this.

...

How does this look? I think this should have the least duplicate code except for
two cppc_perf_to_khz() calls, while keeping the logic easy to follow.

diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index bafa32dd375d..6070444ed098 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -118,6 +118,9 @@ static void cppc_scale_freq_workfn(struct kthread_work *work)

perf = cppc_perf_from_fbctrs(cpu_data, &cppc_fi->prev_perf_fb_ctrs,
&fb_ctrs);
+ if (!perf)
+ return;
+
cppc_fi->prev_perf_fb_ctrs = fb_ctrs;

perf <<= SCHED_CAPACITY_SHIFT;
@@ -726,11 +729,27 @@ static int cppc_perf_from_fbctrs(struct cppc_cpudata *cpu_data,

/* Check to avoid divide-by zero and invalid delivered_perf */
if (!delta_reference || !delta_delivered)
- return cpu_data->perf_ctrls.desired_perf;
+ return 0;

return (reference_perf * delta_delivered) / delta_reference;
}

+static int cppc_get_perf_ctrs_sample(int cpu,
+ struct cppc_perf_fb_ctrs *fb_ctrs_t0,
+ struct cppc_perf_fb_ctrs *fb_ctrs_t1)
+{
+ int ret;
+
+ ret = cppc_get_perf_ctrs(cpu, fb_ctrs_t0);
+ if (ret)
+ return ret;
+
+ udelay(2); /* 2usec delay between sampling */
+
+ ret = cppc_get_perf_ctrs(cpu, fb_ctrs_t1);
+ return ret;
+}
+
static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
{
struct cppc_perf_fb_ctrs fb_ctrs_t0 = {0}, fb_ctrs_t1 = {0};
@@ -746,20 +765,30 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)

cpufreq_cpu_put(policy);

- ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t0);
- if (ret)
- return 0;
-
- udelay(2); /* 2usec delay between sampling */
-
- ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
- if (ret)
- return 0;
+ ret = cppc_get_perf_ctrs_sample(cpu, &fb_ctrs_t0, &fb_ctrs_t1);
+ if (ret) {
+ if (ret == -EFAULT)
+ goto out_invalid_counters;
+ else
+ return 0;
+ }

delivered_perf = cppc_perf_from_fbctrs(cpu_data, &fb_ctrs_t0,
&fb_ctrs_t1);
+ if (!delivered_perf)
+ goto out_invalid_counters;

return cppc_perf_to_khz(&cpu_data->perf_caps, delivered_perf);
+
+out_invalid_counters:
+ /*
+ * Feedback counters could be unchanged or 0 when a cpu enters a
+ * low-power idle state, e.g. clock-gated or power-gated.
+ * Get the lastest or cached desired perf for reflecting frequency.
+ */
+ if (cppc_get_desired_perf(cpu, &delivered_perf))
+ delivered_perf = cpu_data->perf_ctrls.desired_perf;
+ return cppc_perf_to_khz(&cpu_data->perf_caps, delivered_perf);
}

static int cppc_cpufreq_set_boost(struct cpufreq_policy *policy, int state)

Thanks!
Jie