sched/cputime: Rename vtime fields

The current "snapshot" based naming on vtime fields suggests we record
some past event but that's a low level picture of their actual purpose
which comes out blurry. The real point of these fields is to run a basic
state machine that tracks down cputime entry while switching between
contexts.

So lets reflect that with more meaningful names.

Tested-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wanpeng Li <kernellwp@gmail.com>
Link: http://lkml.kernel.org/r/1498756511-11714-4-git-send-email-fweisbec@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/init_task.h

@@ -171,8 +171,8 @@ extern struct cred init_cred;
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
 # define INIT_VTIME(tsk)						\
 	.vtime_seqcount = SEQCNT_ZERO(tsk.vtime_seqcount),	\
-	.vtime_snap = 0,				\
-	.vtime_snap_whence = VTIME_SYS,
+	.vtime_starttime = 0,				\
+	.vtime_state = VTIME_SYS,
 #else
 # define INIT_VTIME(tsk)
 #endif

include/linux/sched.h

@@ -689,7 +689,7 @@ struct task_struct {
 	struct prev_cputime		prev_cputime;
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
 	seqcount_t			vtime_seqcount;
-	unsigned long long		vtime_snap;
+	unsigned long long		vtime_starttime;
 	enum {
 		/* Task is sleeping or running in a CPU with VTIME inactive: */
 		VTIME_INACTIVE = 0,
@@ -697,7 +697,7 @@ struct task_struct {
 		VTIME_USER,
 		/* Task runs in kernelspace in a CPU with VTIME active: */
 		VTIME_SYS,
-	} vtime_snap_whence;
+	} vtime_state;
 #endif
 
 #ifdef CONFIG_NO_HZ_FULL

kernel/fork.c

@@ -1638,8 +1638,8 @@ static __latent_entropy struct task_struct *copy_process(
 
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
 	seqcount_init(&p->vtime_seqcount);
-	p->vtime_snap = 0;
-	p->vtime_snap_whence = VTIME_INACTIVE;
+	p->vtime_starttime = 0;
+	p->vtime_state = VTIME_INACTIVE;
 #endif
 
 #if defined(SPLIT_RSS_COUNTING)

kernel/sched/cputime.c

@@ -683,10 +683,10 @@ static u64 vtime_delta(struct task_struct *tsk)
 {
 	unsigned long now = READ_ONCE(jiffies);
 
-	if (time_before(now, (unsigned long)tsk->vtime_snap))
+	if (time_before(now, (unsigned long)tsk->vtime_starttime))
 		return 0;
 
-	return jiffies_to_nsecs(now - tsk->vtime_snap);
+	return jiffies_to_nsecs(now - tsk->vtime_starttime);
 }
 
 static u64 get_vtime_delta(struct task_struct *tsk)
@@ -701,10 +701,10 @@ static u64 get_vtime_delta(struct task_struct *tsk)
 	 * elapsed time. Limit account_other_time to prevent rounding
 	 * errors from causing elapsed vtime to go negative.
 	 */
-	delta = jiffies_to_nsecs(now - tsk->vtime_snap);
+	delta = jiffies_to_nsecs(now - tsk->vtime_starttime);
 	other = account_other_time(delta);
-	WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE);
-	tsk->vtime_snap = now;
+	WARN_ON_ONCE(tsk->vtime_state == VTIME_INACTIVE);
+	tsk->vtime_starttime = now;
 
 	return delta - other;
 }
@@ -746,7 +746,7 @@ void vtime_guest_enter(struct task_struct *tsk)
 {
 	/*
 	 * The flags must be updated under the lock with
-	 * the vtime_snap flush and update.
+	 * the vtime_starttime flush and update.
 	 * That enforces a right ordering and update sequence
 	 * synchronization against the reader (task_gtime())
 	 * that can thus safely catch up with a tickless delta.
@@ -776,12 +776,12 @@ void vtime_account_idle(struct task_struct *tsk)
 void arch_vtime_task_switch(struct task_struct *prev)
 {
 	write_seqcount_begin(&prev->vtime_seqcount);
-	prev->vtime_snap_whence = VTIME_INACTIVE;
+	prev->vtime_state = VTIME_INACTIVE;
 	write_seqcount_end(&prev->vtime_seqcount);
 
 	write_seqcount_begin(&current->vtime_seqcount);
-	current->vtime_snap_whence = VTIME_SYS;
-	current->vtime_snap = jiffies;
+	current->vtime_state = VTIME_SYS;
+	current->vtime_starttime = jiffies;
 	write_seqcount_end(&current->vtime_seqcount);
 }
 
@@ -791,8 +791,8 @@ void vtime_init_idle(struct task_struct *t, int cpu)
 
 	local_irq_save(flags);
 	write_seqcount_begin(&t->vtime_seqcount);
-	t->vtime_snap_whence = VTIME_SYS;
-	t->vtime_snap = jiffies;
+	t->vtime_state = VTIME_SYS;
+	t->vtime_starttime = jiffies;
 	write_seqcount_end(&t->vtime_seqcount);
 	local_irq_restore(flags);
 }
@@ -809,7 +809,7 @@ u64 task_gtime(struct task_struct *t)
 		seq = read_seqcount_begin(&t->vtime_seqcount);
 
 		gtime = t->gtime;
-		if (t->vtime_snap_whence == VTIME_SYS && t->flags & PF_VCPU)
+		if (t->vtime_state == VTIME_SYS && t->flags & PF_VCPU)
 			gtime += vtime_delta(t);
 
 	} while (read_seqcount_retry(&t->vtime_seqcount, seq));
@@ -840,7 +840,7 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
 		*stime = t->stime;
 
 		/* Task is sleeping, nothing to add */
-		if (t->vtime_snap_whence == VTIME_INACTIVE || is_idle_task(t))
+		if (t->vtime_state == VTIME_INACTIVE || is_idle_task(t))
 			continue;
 
 		delta = vtime_delta(t);
@@ -849,9 +849,9 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
 		 * Task runs either in user or kernel space, add pending nohz time to
 		 * the right place.
 		 */
-		if (t->vtime_snap_whence == VTIME_USER || t->flags & PF_VCPU)
+		if (t->vtime_state == VTIME_USER || t->flags & PF_VCPU)
 			*utime += delta;
-		else if (t->vtime_snap_whence == VTIME_SYS)
+		else if (t->vtime_state == VTIME_SYS)
 			*stime += delta;
 	} while (read_seqcount_retry(&t->vtime_seqcount, seq));
 }