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Subject[RFC/RFT] [PATCH 05/10] cpufreq: intel_pstate: HWP boost performance on IO Wake
Date
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;
+ }

+ /*
+ * 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
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