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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Wed, 16 May 2018 11:45:32 +0200
Message-ID: <CAJZ5v0jSfZyGoKJKHEQCZRgQtiNt+_D0_sY+K2p3nQ9xdg3qKg@mail.gmail.com>
Subject: Re: [RFC/RFT] [PATCH 05/10] cpufreq: intel_pstate: HWP boost
 performance on IO Wake
To:     Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@suse.de>, Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Mel Gorman <mgorman@techsingularity.net>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Juri Lelli <juri.lelli@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, May 16, 2018 at 6:49 AM, Srinivas Pandruvada
<srinivas.pandruvada@linux.intel.com> wrote:
> When a task is woken up from IO wait, boost HWP prformance to max. This
> helps IO workloads on servers with per core P-states. But changing limits
> has extra over head of issuing new HWP Request MSR, which takes 1000+
> cycles. So this change limits setting HWP Request MSR. Also request can
> be for a remote CPU.
> Rate control in setting HWP Requests:
> - If the current performance is around P1, simply ignore IO flag.
> - Once set wait till hold time, till remove boost. While the boost
>   is on, another IO flags is notified, it will prolong boost.
> - If the IO flags are notified multiple ticks apart, this may not be
> IO bound task. Othewise idle system gets periodic boosts for one
> IO wake.
>
> Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
> ---
>  drivers/cpufreq/intel_pstate.c | 75 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 75 insertions(+)
>
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index e200887..d418265 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -20,6 +20,7 @@
>  #include <linux/tick.h>
>  #include <linux/slab.h>
>  #include <linux/sched/cpufreq.h>
> +#include <linux/sched/topology.h>
>  #include <linux/list.h>
>  #include <linux/cpu.h>
>  #include <linux/cpufreq.h>
> @@ -224,6 +225,8 @@ struct global_params {
>   * @hwp_req_cached:    Cached value of the last HWP request MSR
>   * @csd:               A structure used to issue SMP async call, which
>   *                     defines callback and arguments
> + * @hwp_boost_active:  HWP performance is boosted on this CPU
> + * @last_io_update:    Last time when IO wake flag was set
>   *
>   * This structure stores per CPU instance data for all CPUs.
>   */
> @@ -258,6 +261,8 @@ struct cpudata {
>         s16 epp_saved;
>         u64 hwp_req_cached;
>         call_single_data_t csd;
> +       bool hwp_boost_active;
> +       u64 last_io_update;
>  };
>
>  static struct cpudata **all_cpu_data;
> @@ -1421,10 +1426,80 @@ static void csd_init(struct cpudata *cpu)
>         cpu->csd.info = cpu;
>  }
>
> +/*
> + * Long hold time will keep high perf limits for long time,
> + * which negatively impacts perf/watt for some workloads,
> + * like specpower. 3ms is based on experiements on some
> + * workoads.
> + */
> +static int hwp_boost_hold_time_ms = 3;
> +
> +/* Default: This will roughly around P1 on SKX */
> +#define BOOST_PSTATE_THRESHOLD (SCHED_CAPACITY_SCALE / 2)
> +static int hwp_boost_pstate_threshold = BOOST_PSTATE_THRESHOLD;
> +
> +static inline bool intel_pstate_check_boost_threhold(struct cpudata *cpu)
> +{
> +       /*
> +        * If the last performance is above threshold, then return false,
> +        * so that caller can ignore boosting.
> +        */
> +       if (arch_scale_freq_capacity(cpu->cpu) > hwp_boost_pstate_threshold)
> +               return false;
> +
> +       return true;
> +}
> +
>  static inline void intel_pstate_update_util_hwp(struct update_util_data *data,
>                                                 u64 time, unsigned int flags)
>  {
> +       struct cpudata *cpu = container_of(data, struct cpudata, update_util);
> +
> +       if (flags & SCHED_CPUFREQ_IOWAIT) {
> +               /*
> +                * Set iowait_boost flag and update time. Since IO WAIT flag
> +                * is set all the time, we can't just conclude that there is
> +                * some IO bound activity is scheduled on this CPU with just
> +                * one occurrence. If we receive at least two in two
> +                * consecutive ticks, then we start treating as IO. So
> +                * there will be one tick latency.
> +                */
> +               if (time_before64(time, cpu->last_io_update + 2 * TICK_NSEC) &&
> +                   intel_pstate_check_boost_threhold(cpu))
> +                       cpu->iowait_boost = true;
> +
> +               cpu->last_io_update = time;
> +               cpu->last_update = time;

This is a shared data structure and it gets updated without
synchronization, unless I'm missing something.

How much does the cross-CPU case matter?

> +       }
>
> +       /*
> +        * If the boost is active, we will remove it after timeout on local
> +        * CPU only.
> +        */
> +       if (cpu->hwp_boost_active) {
> +               if (smp_processor_id() == cpu->cpu) {
> +                       bool expired;
> +
> +                       expired = time_after64(time, cpu->last_update +
> +                                              (hwp_boost_hold_time_ms * NSEC_PER_MSEC));
> +                       if (expired) {
> +                               intel_pstate_hwp_boost_down(cpu);
> +                               cpu->hwp_boost_active = false;
> +                               cpu->iowait_boost = false;
> +                       }
> +               }
> +               return;
> +       }
> +
> +       cpu->last_update = time;
> +
> +       if (cpu->iowait_boost) {
> +               cpu->hwp_boost_active = true;
> +               if (smp_processor_id() == cpu->cpu)
> +                       intel_pstate_hwp_boost_up(cpu);
> +               else
> +                       smp_call_function_single_async(cpu->cpu, &cpu->csd);
> +       }
>  }
>
>  static inline void intel_pstate_calc_avg_perf(struct cpudata *cpu)
> --
> 2.9.5
>