[PATCH v17 5/8] sched: Fix runtime accounting w/ split exec & sched contexts

[John Stultz <jstultz@google.com>]

Without proxy-exec, we normally charge the "current" task for
both its vruntime as well as its sum_exec_runtime.

With proxy, however, we have two "current" contexts: the
scheduler context and the execution context. We want to charge
the execution context rq->curr (ie: proxy/lock holder) execution
time to its sum_exec_runtime (so it's clear to userland the
rq->curr task *is* running), as well as its thread group.

However the rest of the time accounting (such a vruntime and
cgroup accounting), we charge against the scheduler context
(rq->donor) task, because it is from that task that the time
is being "donated".

If the donor and curr tasks are the same, then it's the same as
without proxy.

Signed-off-by: John Stultz <jstultz@google.com>
---
v16:
* Renamed update_curr_se to update_se_times, as suggested by
  Steven Rostedt.
* Reworded the commit message as suggested by Steven Rostedt
v17:
* Renamed update_se_times to update_se, as suggested by Peter
* Added comment clarifying cfs_rq->curr being rq->donor.se as
  suggested by Peter
* Reworked to fix issue Peter pointed out with thread group
  accounting being done on the donor, rather than the running
  execution context.
* Slight rewording of the commit message to further clarify
  things.

Cc: Joel Fernandes <joelagnelf@nvidia.com>
Cc: Qais Yousef <qyousef@layalina.io>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Zimuzo Ezeozue <zezeozue@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Will Deacon <will@kernel.org>
Cc: Waiman Long <longman@redhat.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Metin Kaya <Metin.Kaya@arm.com>
Cc: Xuewen Yan <xuewen.yan94@gmail.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: kernel-team@android.com
---
 kernel/sched/fair.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8d067168185f4..1fd4c78314d6e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1143,30 +1143,40 @@ static void update_tg_load_avg(struct cfs_rq *cfs_rq)
 }
 #endif /* CONFIG_SMP */
 
-static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
+static s64 update_se(struct rq *rq, struct sched_entity *se)
 {
 	u64 now = rq_clock_task(rq);
 	s64 delta_exec;
 
-	delta_exec = now - curr->exec_start;
+	delta_exec = now - se->exec_start;
 	if (unlikely(delta_exec <= 0))
 		return delta_exec;
 
-	curr->exec_start = now;
-	curr->sum_exec_runtime += delta_exec;
+	se->exec_start = now;
+	if (entity_is_task(se)) {
+		struct task_struct *donor = task_of(se);
+		struct task_struct *running = rq->curr;
+		/*
+		 * If se is a task, we account the time against the running
+		 * task, as w/ proxy-exec they may not be the same.
+		 */
+		running->se.exec_start = now;
+		running->se.sum_exec_runtime += delta_exec;
 
-	if (entity_is_task(curr)) {
-		struct task_struct *p = task_of(curr);
+		trace_sched_stat_runtime(running, delta_exec);
+		account_group_exec_runtime(running, delta_exec);
 
-		trace_sched_stat_runtime(p, delta_exec);
-		account_group_exec_runtime(p, delta_exec);
-		cgroup_account_cputime(p, delta_exec);
+		/* cgroup time is always accounted against the donor */
+		cgroup_account_cputime(donor, delta_exec);
+	} else {
+		/* If not task, account the time against donor se  */
+		se->sum_exec_runtime += delta_exec;
 	}
 
 	if (schedstat_enabled()) {
 		struct sched_statistics *stats;
 
-		stats = __schedstats_from_se(curr);
+		stats = __schedstats_from_se(se);
 		__schedstat_set(stats->exec_max,
 				max(delta_exec, stats->exec_max));
 	}
@@ -1213,7 +1223,7 @@ s64 update_curr_common(struct rq *rq)
 {
 	struct task_struct *donor = rq->donor;
 
-	return update_curr_se(rq, &donor->se);
+	return update_se(rq, &donor->se);
 }
 
 /*
@@ -1221,6 +1231,12 @@ s64 update_curr_common(struct rq *rq)
  */
 static void update_curr(struct cfs_rq *cfs_rq)
 {
+	/*
+	 * Note: cfs_rq->curr corresponds to the task picked to
+	 * run (ie: rq->donor.se) which due to proxy-exec may
+	 * not necessarily be the actual task running
+	 * (rq->curr.se). This is easy to confuse!
+	 */
 	struct sched_entity *curr = cfs_rq->curr;
 	struct rq *rq = rq_of(cfs_rq);
 	s64 delta_exec;
@@ -1229,7 +1245,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
 	if (unlikely(!curr))
 		return;
 
-	delta_exec = update_curr_se(rq, curr);
+	delta_exec = update_se(rq, curr);
 	if (unlikely(delta_exec <= 0))
 		return;
 
-- 
2.49.0.1101.gccaa498523-goog