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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>
This commit is contained in:
Srinivas Pandruvada 2017-08-03 19:03:14 -07:00 committed by Rafael J. Wysocki
parent f5c13f44c7
commit 7bde2d5001
1 changed files with 10 additions and 0 deletions
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10
drivers/cpufreq/intel_pstate.c
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@ -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;