Re: [RFC/RFT] [PATCH 05/10] cpufreq: intel_pstate: HWP boost performance on IO Wake

Rafael J. Wysocki
2018-05-16T10:02:46Z

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
>