cpufreq: intel_pstate: Improve IO performance with per-core P-states
In the current implementation, the response latency between seeing
SCHED_CPUFREQ_IOWAIT set and the actual P-state adjustment can be up
to 10ms. It can be reduced by bumping up the P-state to the max at
the time SCHED_CPUFREQ_IOWAIT is passed to intel_pstate_update_util().
With this change, the IO performance improves significantly.
For a simple "grep -r . linux" (Here linux is the kernel source
folder) with caches dropped every time on a Broadwell Xeon workstation
with per-core P-states, the user and system time is shorter by as much
as 30% - 40%.
The same performance difference was not observed on clients that don't
support per-core P-state.
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
[ rjw: Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index 2eac2ad..532e261 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -1526,6 +1526,15 @@ static void intel_pstate_update_util(struct update_util_data *data, u64 time,
if (flags & SCHED_CPUFREQ_IOWAIT) {
cpu->iowait_boost = int_tofp(1);
+ cpu->last_update = time;
+ /*
+ * The last time the busy was 100% so P-state was max anyway
+ * so avoid overhead of computation.
+ */
+ if (fp_toint(cpu->sample.busy_scaled) == 100)
+ return;
+
+ goto set_pstate;
} else if (cpu->iowait_boost) {
/* Clear iowait_boost if the CPU may have been idle. */
delta_ns = time - cpu->last_update;
@@ -1537,6 +1546,7 @@ static void intel_pstate_update_util(struct update_util_data *data, u64 time,
if ((s64)delta_ns < INTEL_PSTATE_DEFAULT_SAMPLING_INTERVAL)
return;
+set_pstate:
if (intel_pstate_sample(cpu, time)) {
int target_pstate;
