[tip: sched/core] sched: Fix runtime accounting w/ split exec & sched contexts

[tip-bot2 for John Stultz]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     aa4f74dfd42ba4399f785fb9c460a11bd1756f0a
Gitweb:        https://git.kernel.org/tip/aa4f74dfd42ba4399f785fb9c460a11bd1756f0a
Author:        John Stultz <jstultz@xxxxxxxxxx>
AuthorDate:    Sat, 12 Jul 2025 03:33:46 
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 14 Jul 2025 17:16:32 +02:00

sched: Fix runtime accounting w/ split exec & sched contexts

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Link: https://lkml.kernel.org/r/20250712033407.2383110-6-jstultz@xxxxxxxxxx
---
 kernel/sched/fair.c | 42 ++++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8334580..9717645 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1152,30 +1152,40 @@ void post_init_entity_util_avg(struct task_struct *p)
 	sa->runnable_avg = sa->util_avg;
 }
 
-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));
 	}
@@ -1188,9 +1198,7 @@ static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
  */
 s64 update_curr_common(struct rq *rq)
 {
-	struct task_struct *donor = rq->donor;
-
-	return update_curr_se(rq, &donor->se);
+	return update_se(rq, &rq->donor->se);
 }
 
 /*
@@ -1198,6 +1206,12 @@ 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;
@@ -1206,7 +1220,7 @@ 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;