linux-kernel - [PATCH] intel_pstate: Do not skip samples partially

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Message-ID: <1608361.vYXCevKAEf@vostro.rjw.lan>
Date:	Thu, 10 Mar 2016 23:45:19 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Linux PM list <linux-pm@...r.kernel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Philippe Longepe <philippe.longepe@...ux.intel.com>
Subject: [PATCH] intel_pstate: Do not skip samples partially

From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

If the current value of MPERF or the current value of TSC is the
same as the previous one, respectively, intel_pstate_sample() bails
out early and skips the sample.

However, intel_pstate_adjust_busy_pstate() is still called in that
case which is not correct, so modify intel_pstate_sample() to
return a bool value indicating whether or not the sample has been
taken and use it to decide whether or not to call
intel_pstate_adjust_busy_pstate().

While at it, remove redundant parentheses from the MPERF/TSC
check in intel_pstate_sample().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---

On top of my linux-next branch.

---
 drivers/cpufreq/intel_pstate.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

Index: linux-pm/drivers/cpufreq/intel_pstate.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/intel_pstate.c
+++ linux-pm/drivers/cpufreq/intel_pstate.c
@@ -890,7 +890,7 @@ static inline void intel_pstate_calc_bus
 	sample->core_pct_busy = (int32_t)core_pct;
 }
 
-static inline void intel_pstate_sample(struct cpudata *cpu, u64 time)
+static inline bool intel_pstate_sample(struct cpudata *cpu, u64 time)
 {
 	u64 aperf, mperf;
 	unsigned long flags;
@@ -900,9 +900,9 @@ static inline void intel_pstate_sample(s
 	rdmsrl(MSR_IA32_APERF, aperf);
 	rdmsrl(MSR_IA32_MPERF, mperf);
 	tsc = rdtsc();
-	if ((cpu->prev_mperf == mperf) || (cpu->prev_tsc == tsc)) {
+	if (cpu->prev_mperf == mperf || cpu->prev_tsc == tsc) {
 		local_irq_restore(flags);
-		return;
+		return false;
 	}
 	local_irq_restore(flags);
 
@@ -920,6 +920,7 @@ static inline void intel_pstate_sample(s
 	cpu->prev_aperf = aperf;
 	cpu->prev_mperf = mperf;
 	cpu->prev_tsc = tsc;
+	return true;
 }
 
 static inline int32_t get_target_pstate_use_cpu_load(struct cpudata *cpu)
@@ -1026,8 +1027,9 @@ static void intel_pstate_update_util(str
 	u64 delta_ns = time - cpu->sample.time;
 
 	if ((s64)delta_ns >= pid_params.sample_rate_ns) {
-		intel_pstate_sample(cpu, time);
-		if (!hwp_active)
+		bool sample_taken = intel_pstate_sample(cpu, time);
+
+		if (sample_taken && !hwp_active)
 			intel_pstate_adjust_busy_pstate(cpu);
 	}
 }