linux-kernel - [PATCH 6/7] cputime: Introduce vtime accounting check for readers

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Date:	Thu, 19 Nov 2015 16:47:33 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org,
	Rik van Riel <riel@...hat.com>
Subject: [PATCH 6/7] cputime: Introduce vtime accounting check for readers

Readers need to know if vtime runs at all on some CPU somewhere, this
is a fast-path check to determine if we need to check further the need
to add up any tickless cputime delta.

This fast path check uses context tracking state because vtime is tied
to context tracking as of now. This check appears to be confusing though
so lets use a vtime function that deals with context tracking details
in vtime implementation instead.

Cc: Christoph Lameter <cl@...ux.com>
Cc: Chris Metcalf <cmetcalf@...hip.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Luiz Capitulino <lcapitulino@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Hiroshi Shimamoto <h-shimamoto@...jp.nec.com>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 include/linux/vtime.h  | 13 ++++++++++++-
 kernel/sched/cputime.c |  6 +++---
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/include/linux/vtime.h b/include/linux/vtime.h
index ca23e83..fa21969 100644
--- a/include/linux/vtime.h
+++ b/include/linux/vtime.h
@@ -17,9 +17,20 @@ static inline bool vtime_accounting_cpu_enabled(void) { return true; }
 #endif /* CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
 
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
+/*
+ * Checks if vtime is enabled on some CPU. Cputime readers want to be careful
+ * in that case and compute the tickless cputime.
+ * For now vtime state is tied to context tracking. We might want to decouple
+ * those later if necessary.
+ */
+static inline bool vtime_accounting_enabled(void)
+{
+	return context_tracking_is_enabled();
+}
+
 static inline bool vtime_accounting_cpu_enabled(void)
 {
-	if (context_tracking_is_enabled()) {
+	if (vtime_accounting_enabled()) {
 		if (context_tracking_cpu_is_enabled())
 			return true;
 	}
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 5727217..9989c3f 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -788,7 +788,7 @@ cputime_t task_gtime(struct task_struct *t)
 	unsigned int seq;
 	cputime_t gtime;
 
-	if (!context_tracking_is_enabled())
+	if (!vtime_accounting_enabled())
 		return t->gtime;
 
 	do {
@@ -853,7 +853,7 @@ void task_cputime(struct task_struct *t, cputime_t *utime, cputime_t *stime)
 {
 	cputime_t udelta, sdelta;
 
-	if (!context_tracking_is_enabled()) {
+	if (!vtime_accounting_enabled()) {
 		if (utime)
 			*utime = t->utime;
 		if (stime)
@@ -874,7 +874,7 @@ void task_cputime_scaled(struct task_struct *t,
 {
 	cputime_t udelta, sdelta;
 
-	if (!context_tracking_is_enabled()) {
+	if (!vtime_accounting_enabled()) {
 		if (utimescaled)
 			*utimescaled = t->utimescaled;
 		if (stimescaled)
-- 
2.5.3

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