A couple of uclamp fixes

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

Commit-ID:     ca4984a7dd863f3e1c0df775ae3e744bff24c303
Gitweb:        https://git.kernel.org/tip/ca4984a7dd863f3e1c0df775ae3e744bff24c303
Author:        Quentin Perret <[email protected]>
AuthorDate:    Thu, 05 Aug 2021 11:21:53 +01:00
Committer:     Peter Zijlstra <[email protected]>
CommitterDate: Fri, 06 Aug 2021 14:25:25 +02:00

sched: Fix UCLAMP_FLAG_IDLE setting

The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last uclamp active task (that is, when buckets.tasks reaches 0 for all buckets) to maintain the last uclamp.max and prevent blocked util from suddenly becoming visible.

However, there is an asymmetry in how the flag is set and cleared which can lead to having the flag set whilst there are active tasks on the rq. Specifically, the flag is cleared in the uclamp_rq_inc() path, which is called at enqueue time, but set in uclamp_rq_dec_id() which is called both when dequeueing a task _and_ in the update_uclamp_active() path. As a result, when both uclamp_rq_{dec,ind}_id() are called from update_uclamp_active(), the flag ends up being set but not cleared, hence leaving the runqueue in a broken state.

Fix this by clearing the flag in update_uclamp_active() as well.

---
 kernel/sched/core.c | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

index 314f70d..df0480a 100644
@@ -1621,6 +1621,23 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
 		uclamp_rq_dec_id(rq, p, clamp_id);
 }
 
+static inline void uclamp_rq_reinc_id(struct rq *rq, struct task_struct *p,
+				      enum uclamp_id clamp_id)
+{
+	if (!p->uclamp[clamp_id].active)
+		return;
+
+	uclamp_rq_dec_id(rq, p, clamp_id);
+	uclamp_rq_inc_id(rq, p, clamp_id);
+
+	/*
+	 * Make sure to clear the idle flag if we've transiently reached 0
+	 * active tasks on rq.
+	 */
+	if (clamp_id == UCLAMP_MAX && (rq->uclamp_flags & UCLAMP_FLAG_IDLE))
+		rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
+}
+
 static inline void
 uclamp_update_active(struct task_struct *p)
 {

@@ -1644,12 +1661,8 @@ uclamp_update_active(struct task_struct *p)
 	 * affecting a valid clamp bucket, the next time it's enqueued,
 	 * it will already see the updated clamp bucket value.
 	 */
-	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);
-		}
-	}
+	for_each_clamp_id(clamp_id)
+		uclamp_rq_reinc_id(rq, p, clamp_id);
 
 	task_rq_unlock(rq, p, &rf);
 }

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

Commit-ID:     f4dddf90d58d77b48492b775868af4041a217f4c
Gitweb:        https://git.kernel.org/tip/f4dddf90d58d77b48492b775868af4041a217f4c
Author:        Quentin Perret <[email protected]>
AuthorDate:    Thu, 05 Aug 2021 11:21:54 +01:00
Committer:     Peter Zijlstra <[email protected]>
CommitterDate: Fri, 06 Aug 2021 14:25:25 +02:00

sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS

SCHED_FLAG_KEEP_PARAMS can be passed to sched_setattr to specify that the call must not touch scheduling parameters (nice or priority). This is particularly handy for uclamp when used in conjunction with SCHED_FLAG_KEEP_POLICY as that allows to issue a syscall that only impacts uclamp values.

However, sched_setattr always checks whether the priorities and nice values passed in sched_attr are valid first, even if those never get used down the line. This is useless at best since userspace can trivially bypass this check to set the uclamp values by specifying low priorities. However, it is cumbersome to do so as there is no single expression of this that skips both RT and CFS checks at once. As such, userspace needs to query the task policy first with e.g. sched_getattr and then set sched_attr.sched_priority accordingly. This is racy and slower than a single call.

As the priority and nice checks are useless when SCHED_FLAG_KEEP_PARAMS is specified, simply inherit them in this case to match the policy inheritance of SCHED_FLAG_KEEP_POLICY.

---
 kernel/sched/core.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

index df0480a..433b400 100644
@@ -7328,6 +7328,16 @@ err_size:
 	return -E2BIG;
 }
 
+static void get_params(struct task_struct *p, struct sched_attr *attr)
+{
+	if (task_has_dl_policy(p))
+		__getparam_dl(p, attr);
+	else if (task_has_rt_policy(p))
+		attr->sched_priority = p->rt_priority;
+	else
+		attr->sched_nice = task_nice(p);
+}
+
 /**
  * sys_sched_setscheduler - set/change the scheduler policy and RT priority
  * @pid: the pid in question.

@@ -7389,6 +7399,8 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
 	rcu_read_unlock();
 
 	if (likely(p)) {
+		if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS)
+			get_params(p, &attr);
 		retval = sched_setattr(p, &attr);
 		put_task_struct(p);
 	}

@@ -7537,12 +7549,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 	kattr.sched_policy = p->policy;
 	if (p->sched_reset_on_fork)
 		kattr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
-	if (task_has_dl_policy(p))
-		__getparam_dl(p, &kattr);
-	else if (task_has_rt_policy(p))
-		kattr.sched_priority = p->rt_priority;
-	else
-		kattr.sched_nice = task_nice(p);
+	get_params(p, &kattr);
 	kattr.sched_flags &= SCHED_FLAG_ALL;
 
 #ifdef CONFIG_UCLAMP_TASK