[PATCH v2] sched/uclamp: Fix uclamp_tg_restrict()

* [PATCH v2] sched/uclamp: Fix uclamp_tg_restrict()
@ 2021-06-17 16:51 Qais Yousef
  2021-06-22 13:13 ` Peter Zijlstra
  2021-06-23  8:19 ` [tip: sched/core] " tip-bot2 for Qais Yousef
  0 siblings, 2 replies; 3+ messages in thread
From: Qais Yousef @ 2021-06-17 16:51 UTC (permalink / raw)
  To: Peter Zijlstra (Intel), Ingo Molnar
  Cc: Vincent Guittot, Dietmar Eggemann, Patrick Bellasi, Tejun Heo,
	Quentin Perret, Wei Wang, Yun Hsiang, Xuewen Yan, linux-kernel,
	Qais Yousef

Now cpu.uclamp.min acts as a protection, we need to make sure that the
uclamp request of the task is within the allowed range of the cgroup,
that is it is clamp()'ed correctly by tg->uclamp[UCLAMP_MIN] and
tg->uclamp[UCLAMP_MAX].

As reported by Xuewen [1] we can have some corner cases where there's
inversion between uclamp requested by task (p) and the uclamp values of
the taskgroup it's attached to (tg). Following table demonstrates
2 corner cases:

	           |  p  |  tg  |  effective
	-----------+-----+------+-----------
	CASE 1
	-----------+-----+------+-----------
	uclamp_min | 60% | 0%   |  60%
	-----------+-----+------+-----------
	uclamp_max | 80% | 50%  |  50%
	-----------+-----+------+-----------
	CASE 2
	-----------+-----+------+-----------
	uclamp_min | 0%  | 30%  |  30%
	-----------+-----+------+-----------
	uclamp_max | 20% | 50%  |  20%
	-----------+-----+------+-----------

With this fix we get:

	           |  p  |  tg  |  effective
	-----------+-----+------+-----------
	CASE 1
	-----------+-----+------+-----------
	uclamp_min | 60% | 0%   |  50%
	-----------+-----+------+-----------
	uclamp_max | 80% | 50%  |  50%
	-----------+-----+------+-----------
	CASE 2
	-----------+-----+------+-----------
	uclamp_min | 0%  | 30%  |  30%
	-----------+-----+------+-----------
	uclamp_max | 20% | 50%  |  30%
	-----------+-----+------+-----------

Additionally uclamp_update_active_tasks() must now unconditionally
update both UCLAMP_MIN/MAX because changing the tg's UCLAMP_MAX for
instance could have an impact on the effective UCLAMP_MIN of the tasks.

	           |  p  |  tg  |  effective
	-----------+-----+------+-----------
	old
	-----------+-----+------+-----------
	uclamp_min | 60% | 0%   |  50%
	-----------+-----+------+-----------
	uclamp_max | 80% | 50%  |  50%
	-----------+-----+------+-----------
	*new*
	-----------+-----+------+-----------
	uclamp_min | 60% | 0%   | *60%*
	-----------+-----+------+-----------
	uclamp_max | 80% |*70%* | *70%*
	-----------+-----+------+-----------

[1] https://lore.kernel.org/lkml/CAB8ipk_a6VFNjiEnHRHkUMBKbA+qzPQvhtNjJ_YNzQhqV_o8Zw@mail.gmail.com/

Reported-by: Xuewen Yan <xuewen.yan94@gmail.com>
Fixes: 0c18f2ecfcc2 ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min")
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---

Changes in v2:

	* Fixed a typo in commit message (Diemtar).
	* Renamed uc_eff back to uc_req (Dietmar).
	* Move the loop to update clamp_id to be done once inside the
	  task_rq_lock() (Dietmar).


 kernel/sched/core.c | 49 +++++++++++++++++----------------------------
 1 file changed, 18 insertions(+), 31 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9e9a5be35cde..fdb9a109fd68 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1403,8 +1403,10 @@ static void uclamp_sync_util_min_rt_default(void)
 static inline struct uclamp_se
 uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
 {
+	/* Copy by value as we could modify it */
 	struct uclamp_se uc_req = p->uclamp_req[clamp_id];
 #ifdef CONFIG_UCLAMP_TASK_GROUP
+	unsigned int tg_min, tg_max, value;
 
 	/*
 	 * Tasks in autogroups or root task group will be
@@ -1415,23 +1417,11 @@ uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
 	if (task_group(p) == &root_task_group)
 		return uc_req;
 
-	switch (clamp_id) {
-	case UCLAMP_MIN: {
-		struct uclamp_se uc_min = task_group(p)->uclamp[clamp_id];
-		if (uc_req.value < uc_min.value)
-			return uc_min;
-		break;
-	}
-	case UCLAMP_MAX: {
-		struct uclamp_se uc_max = task_group(p)->uclamp[clamp_id];
-		if (uc_req.value > uc_max.value)
-			return uc_max;
-		break;
-	}
-	default:
-		WARN_ON_ONCE(1);
-		break;
-	}
+	tg_min = task_group(p)->uclamp[UCLAMP_MIN].value;
+	tg_max = task_group(p)->uclamp[UCLAMP_MAX].value;
+	value = uc_req.value;
+	value = clamp(value, tg_min, tg_max);
+	uclamp_se_set(&uc_req, value, false);
 #endif
 
 	return uc_req;
@@ -1630,8 +1620,9 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
 }
 
 static inline void
-uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
+uclamp_update_active(struct task_struct *p)
 {
+	enum uclamp_id clamp_id;
 	struct rq_flags rf;
 	struct rq *rq;
 
@@ -1651,9 +1642,11 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
 	 * affecting a valid clamp bucket, the next time it's enqueued,
 	 * it will already see the updated clamp bucket value.
 	 */
-	if (p->uclamp[clamp_id].active) {
-		uclamp_rq_dec_id(rq, p, clamp_id);
-		uclamp_rq_inc_id(rq, p, clamp_id);
+	for_each_clamp_id(clamp_id) {
+		if (p->uclamp[clamp_id].active) {
+			uclamp_rq_dec_id(rq, p, clamp_id);
+			uclamp_rq_inc_id(rq, p, clamp_id);
+		}
 	}
 
 	task_rq_unlock(rq, p, &rf);
@@ -1661,20 +1654,14 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
 
 #ifdef CONFIG_UCLAMP_TASK_GROUP
 static inline void
-uclamp_update_active_tasks(struct cgroup_subsys_state *css,
-			   unsigned int clamps)
+uclamp_update_active_tasks(struct cgroup_subsys_state *css)
 {
-	enum uclamp_id clamp_id;
 	struct css_task_iter it;
 	struct task_struct *p;
 
 	css_task_iter_start(css, 0, &it);
-	while ((p = css_task_iter_next(&it))) {
-		for_each_clamp_id(clamp_id) {
-			if ((0x1 << clamp_id) & clamps)
-				uclamp_update_active(p, clamp_id);
-		}
-	}
+	while ((p = css_task_iter_next(&it)))
+		uclamp_update_active(p);
 	css_task_iter_end(&it);
 }
 
@@ -9626,7 +9613,7 @@ static void cpu_util_update_eff(struct cgroup_subsys_state *css)
 		}
 
 		/* Immediately update descendants RUNNABLE tasks */
-		uclamp_update_active_tasks(css, clamps);
+		uclamp_update_active_tasks(css);
 	}
 }
 
-- 
2.25.1


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