linux-kernel - [PATCH v2 2/4] sched: Remove mm_cid_next

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Message-ID: <20241213095407.271357-3-gmonaco@redhat.com>
Date: Fri, 13 Dec 2024 10:54:05 +0100
From: Gabriele Monaco <gmonaco@...hat.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Cc: Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Mel Gorman <mgorman@...e.de>,
	Shuah Khan <shuah@...nel.org>,
	linux-kselftest@...r.kernel.org,
	Gabriele Monaco <gmonaco@...hat.com>
Subject: [PATCH v2 2/4] sched: Remove mm_cid_next_scan as obsolete

The checks for the scan time in task_mm_cid_work are now superfluous
since the task runs in a delayed_work and the minimum periodicity is
already implied.

This patch removes those checks and the field from the mm_struct.

Additionally, we include a simple check to quickly terminate the
function if we have no work to be done (i.e. no mm_cid is allocated).
This is helpful for tasks that sleep for a long time, but also for
terminated task. We are no longer following the process' state, hence
the function continues to run after a process terminates but before its
mm is freed.

Signed-off-by: Gabriele Monaco <gmonaco@...hat.com>
---
 include/linux/mm_types.h |  7 -------
 kernel/sched/core.c      | 19 +++----------------
 2 files changed, 3 insertions(+), 23 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 92acb827fee4..8a76a1c09234 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -829,12 +829,6 @@ struct mm_struct {
 		 * runqueue locks.
 		 */
 		struct mm_cid __percpu *pcpu_cid;
-		/*
-		 * @mm_cid_next_scan: Next mm_cid scan (in jiffies).
-		 *
-		 * When the next mm_cid scan is due (in jiffies).
-		 */
-		unsigned long mm_cid_next_scan;
 		/**
 		 * @nr_cpus_allowed: Number of CPUs allowed for mm.
 		 *
@@ -1228,7 +1222,6 @@ static inline int mm_alloc_cid_noprof(struct mm_struct *mm, struct task_struct *
 		return -ENOMEM;
 	mm_init_cid(mm, p);
 	INIT_DELAYED_WORK(&mm->mm_cid_work, task_mm_cid_work);
-	mm->mm_cid_next_scan = jiffies + msecs_to_jiffies(MM_CID_SCAN_DELAY);
 	schedule_delayed_work(&mm->mm_cid_work,
 			      msecs_to_jiffies(MM_CID_SCAN_DELAY));
 	return 0;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e3b27b73301c..30d78fe14eff 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10520,28 +10520,15 @@ static void sched_mm_cid_remote_clear_weight(struct mm_struct *mm, int cpu,
 
 void task_mm_cid_work(struct work_struct *work)
 {
-	unsigned long now = jiffies, old_scan, next_scan;
 	struct cpumask *cidmask;
 	struct delayed_work *delayed_work = container_of(work, struct delayed_work, work);
 	struct mm_struct *mm = container_of(delayed_work, struct mm_struct, mm_cid_work);
 	int weight, cpu;
 
-	old_scan = READ_ONCE(mm->mm_cid_next_scan);
-	next_scan = now + msecs_to_jiffies(MM_CID_SCAN_DELAY);
-	if (!old_scan) {
-		unsigned long res;
-
-		res = cmpxchg(&mm->mm_cid_next_scan, old_scan, next_scan);
-		if (res != old_scan)
-			old_scan = res;
-		else
-			old_scan = next_scan;
-	}
-	if (time_before(now, old_scan))
-		goto out;
-	if (!try_cmpxchg(&mm->mm_cid_next_scan, &old_scan, next_scan))
-		goto out;
 	cidmask = mm_cidmask(mm);
+	/* Nothing to clear for now */
+	if (cpumask_empty(cidmask))
+		goto out;
 	/* Clear cids that were not recently used. */
 	for_each_possible_cpu(cpu)
 		sched_mm_cid_remote_clear_old(mm, cpu);
-- 
2.47.1