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

Ionela Voinescu
2024-09-18T10:15:54Z

Hi,

On Wednesday 18 Sep 2024 at 10:05:13 (+0800), Jie Zhan wrote:
>
>
> 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);

nit: white line before return.

> + 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;

nit: same white line before return here :).

Looks good, thanks for the changes.

Reviewed-by: Ionela Voinescu <ionela.voinescu@arm.com>

Ionela.

> + 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