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Date: Fri, 1 Apr 2016 16:06:34 +0200
From: Jörg Otte <jrg.otte@...il.com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux PM list <linux-pm@...r.kernel.org>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
Doug Smythies <dsmythies@...us.net>
Subject: Re: [intel-pstate driver regression] processor frequency very high
even if in idle
2016-04-01 14:40 GMT+02:00 Rafael J. Wysocki <rjw@...ysocki.net>:
> On Friday, April 01, 2016 11:20:42 AM Jörg Otte wrote:
>> 2016-03-31 17:43 GMT+02:00 Rafael J. Wysocki <rjw@...ysocki.net>:
>> > On Thursday, March 31, 2016 05:25:18 PM Jörg Otte wrote:
>> >> 2016-03-31 13:42 GMT+02:00 Rafael J. Wysocki <rjw@...ysocki.net>:
>> >> > On Thursday, March 31, 2016 11:05:56 AM Jörg Otte wrote:
>> >> >
>> >> > [cut]
>> >> >
>> >> >> >
>> >> >>
>> >> >> Yes, works for me.
>> >> >>
>> >> >> CPUID(7): No-SGX
>> >> >> CPU Avg_MHz Busy% Bzy_MHz TSC_MHz
>> >> >> - 11 0.66 1682 2494
>> >> >> 0 11 0.60 1856 2494
>> >> >> 1 6 0.34 1898 2494
>> >> >> 2 13 0.82 1628 2494
>> >> >> 3 13 0.87 1528 2494
>> >> >> CPU Avg_MHz Busy% Bzy_MHz TSC_MHz
>> >> >> - 6 0.58 963 2494
>> >> >> 0 8 0.83 957 2494
>> >> >> 1 1 0.08 984 2494
>> >> >> 2 10 1.04 975 2494
>> >> >> 3 3 0.35 934 2494
>> >> >>
>> >> >
>> >
>> > [cut]
>> >
>> >> >
>> >>
>> >> No, this patch doesn't help.
>> >
>> > Well, more work to do then.
>> >
>> > I've just noticed a bug in this patch, which is not relevant for the results,
>> > but below is a new version.
>> >
>> >> CPUID(7): No-SGX
>> >> CPU Avg_MHz Busy% Bzy_MHz TSC_MHz
>> >> - 8 0.32 2507 2495
>> >> 0 13 0.53 2505 2495
>> >> 1 3 0.11 2523 2495
>> >> 2 1 0.06 2555 2495
>> >> 3 15 0.59 2500 2495
>> >> CPU Avg_MHz Busy% Bzy_MHz TSC_MHz
>> >> - 8 0.33 2486 2495
>> >> 0 12 0.50 2482 2495
>> >> 1 5 0.22 2489 2495
>> >> 2 1 0.04 2492 2495
>> >> 3 15 0.59 2487 2495
>> >
>
> [cut]
>
>>
>> here they are.
>>
>
> Thanks!
>
> First of all, the sampling mechanics works as expected in the failing case,
> which is the most important thing I wanted to know. However, there are anomalies
> in the failing case trace. The core_busy column is clearly suspicious and it
> looks like CPUs 2 and 3 never really go idle. I guess we'll need to find out
> why they don't go idle to get to the bottom of this, but it firmly falls into
> the weird stuff territory already.
>
> In the meantime, below is one more patch to test, on top of the previous one
> (that is, https://patchwork.kernel.org/patch/8714401/).
>
> Again, this is a change I'd like to make regardless, so it would be good to
> know if anything more has to be done before we go further.
>
> ---
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Subject: [PATCH] intel_pstate: Avoid extra invocation of intel_pstate_sample()
>
> The initialization of intel_pstate for a given CPU involves populating
> the fields of its struct cpudata that represent the previous sample,
> but currently that is done in a problematic way.
>
> Namely, intel_pstate_init_cpu() makes an extra call to
> intel_pstate_sample() so it reads the current register values that
> will be used to populate the "previous sample" record during the
> next invocation of intel_pstate_sample(). However, after commit
> a4675fbc4a7a (cpufreq: intel_pstate: Replace timers with utilization
> update callbacks) that doesn't work for last_sample_time, because
> the time value is passed to intel_pstate_sample() as an argument now.
> Passing 0 to it from intel_pstate_init_cpu() is problematic, because
> that causes cpu->last_sample_time == 0 to be visible in
> get_target_pstate_use_performance() (and hence the extra
> cpu->last_sample_time > 0 check in there) and effectively allows
> the first invocation of intel_pstate_sample() from
> intel_pstate_update_util() to happen immediately after the
> initialization which may lead to a significant "turn on"
> effect in the governor algorithm.
>
> To mitigate that issue, rework the initialization to avoid the
> extra intel_pstate_sample() call from intel_pstate_init_cpu().
> Instead, make intel_pstate_sample() return false if it has been
> called with cpu->sample.time equal to zero, which will make
> intel_pstate_update_util() skip the sample in that case, and
> reset cpu->sample.time from intel_pstate_set_update_util_hook()
> to make the algorithm start properly every time the hook is set.
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> drivers/cpufreq/intel_pstate.c | 21 +++++++++++++++------
> 1 file changed, 15 insertions(+), 6 deletions(-)
>
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -910,7 +910,14 @@ static inline bool intel_pstate_sample(s
> cpu->prev_aperf = aperf;
> cpu->prev_mperf = mperf;
> cpu->prev_tsc = tsc;
> - return true;
> + /*
> + * First time this function is invoked in a given cycle, all of the
> + * previous sample data fields are equal to zero or stale and they must
> + * be populated with meaningful numbers for things to work, so assume
> + * that sample.time will always be reset before setting the utilization
> + * update hook and make the caller skip the sample then.
> + */
> + return !!cpu->last_sample_time;
> }
>
> static inline int32_t get_avg_frequency(struct cpudata *cpu)
> @@ -984,8 +991,7 @@ static inline int32_t get_target_pstate_
> * enough period of time to adjust our busyness.
> */
> duration_ns = cpu->sample.time - cpu->last_sample_time;
> - if ((s64)duration_ns > pid_params.sample_rate_ns * 3
> - && cpu->last_sample_time > 0) {
> + if ((s64)duration_ns > pid_params.sample_rate_ns * 3) {
> sample_ratio = div_fp(int_tofp(pid_params.sample_rate_ns),
> int_tofp(duration_ns));
> core_busy = mul_fp(core_busy, sample_ratio);
> @@ -1100,7 +1106,6 @@ static int intel_pstate_init_cpu(unsigne
> intel_pstate_get_cpu_pstates(cpu);
>
> intel_pstate_busy_pid_reset(cpu);
> - intel_pstate_sample(cpu, 0);
>
> cpu->update_util.func = intel_pstate_update_util;
>
> @@ -1121,9 +1126,13 @@ static unsigned int intel_pstate_get(uns
> return get_avg_frequency(cpu);
> }
>
> -static void intel_pstate_set_update_util_hook(unsigned int cpu)
> +static void intel_pstate_set_update_util_hook(unsigned int cpu_num)
> {
> - cpufreq_set_update_util_data(cpu, &all_cpu_data[cpu]->update_util);
> + struct cpudata *cpu = all_cpu_data[cpu_num];
> +
> + /* Prevent intel_pstate_update_util() from using stale data. */
> + cpu->sample.time = 0;
> + cpufreq_set_update_util_data(cpu_num, &cpu->update_util);
> }
>
> static void intel_pstate_clear_update_util_hook(unsigned int cpu)
>
Done. Attached the tracer.
For me it looks like the previous one of the failing case.
Thanks, Jörg
Download attachment "intel_pstate_tracer_3.xz" of type "application/x-xz" (8868 bytes)
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