linux-kernel - [RFC PATCH v1 3/9] freezer: Add per-priority layered freeze logic

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Message-Id: <20250807121418.139765-4-zhangzihuan@kylinos.cn>
Date: Thu,  7 Aug 2025 20:14:12 +0800
From: Zihuan Zhang <zhangzihuan@...inos.cn>
To: "Rafael J . Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	David Hildenbrand <david@...hat.com>,
	Michal Hocko <mhocko@...e.com>,
	Jonathan Corbet <corbet@....net>
Cc: Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	len brown <len.brown@...el.com>,
	pavel machek <pavel@...nel.org>,
	Kees Cook <kees@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Nico Pache <npache@...hat.com>,
	xu xin <xu.xin16@....com.cn>,
	wangfushuai <wangfushuai@...du.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Christian Brauner <brauner@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jeff Layton <jlayton@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Adrian Ratiu <adrian.ratiu@...labora.com>,
	linux-pm@...r.kernel.org,
	linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Zihuan Zhang <zhangzihuan@...inos.cn>
Subject: [RFC PATCH v1 3/9] freezer: Add per-priority layered freeze logic

The current freezer traverses all user tasks in a single pass, without
distinguishing between tasks that are easier or harder to freeze. This
uniform treatment may cause suboptimal behavior when certain newly created
tasks, service daemons, or system threads block the progress of freeze due
to dependency ordering issues.

This patch introduces a simple multi-pass traversal model in
try_to_freeze_tasks(), where user tasks are grouped and frozen by their
freeze_priority in descending order. Tasks marked with higher priority
are attempted earlier, which can help break dependency cycles earlier
and reduce retry iterations.

Specifically:
 - A new loop iterates over priority levels.
 - In each round, only tasks with freeze_priority < current priority are visited.
 - The behavior applies only to user task freezing (when user_only == true).

This approach preserves compatibility with the current logic, while enabling
fine-grained control via future enhancements (e.g., dynamic priority tuning).

Signed-off-by: Zihuan Zhang <zhangzihuan@...inos.cn>
---
 kernel/power/process.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/power/process.c b/kernel/power/process.c
index dc0dfc349f22..06eafdb32abb 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -32,10 +32,12 @@ static int try_to_freeze_tasks(bool user_only)
 	struct task_struct *g, *p;
 	unsigned long end_time;
 	unsigned int todo;
+	unsigned int round = 0;
 	bool wq_busy = false;
 	ktime_t start, end, elapsed;
 	unsigned int elapsed_msecs;
 	bool wakeup = false;
+	bool has_freezable_task;
 	int sleep_usecs = USEC_PER_MSEC;
 
 	pr_info("Freezing %s\n", what);
@@ -47,13 +49,18 @@ static int try_to_freeze_tasks(bool user_only)
 	if (!user_only)
 		freeze_workqueues_begin();
 
-	while (true) {
+	while (round < FREEZE_PRIORITY_NEVER) {
 		todo = 0;
+		has_freezable_task = false;
 		read_lock(&tasklist_lock);
 		for_each_process_thread(g, p) {
+			if (user_only && !(p->flags & PF_KTHREAD) && round < p->freeze_priority)
+				continue;
+
 			if (p == current || !freeze_task(p))
 				continue;
 
+			has_freezable_task = true;
 			todo++;
 		}
 		read_unlock(&tasklist_lock);
@@ -63,6 +70,12 @@ static int try_to_freeze_tasks(bool user_only)
 			todo += wq_busy;
 		}
 
+		round++;
+
+		/* sleep only if need to freeze tasks */
+		if (user_only && !has_freezable_task)
+			continue;
+
 		if (!todo || time_after(jiffies, end_time))
 			break;
 
-- 
2.25.1