[SRU][F:linux-bluefield][PATCH 1/1] tick/rcu: Stop allowing RCU_SOFTIRQ in idle

[Bodong Wang]

Hi Tim,

Hope you had a great holiday. I just found out this backport is defective and missing the local_bh_blocked definition from a whole series. https://patchwork.kernel.org/project/linux-sh/cover/20201113140207.499353218@linutronix.de/

Given our release timeframe, could you please revert this patch from Focal? It is fine for J.

Bodong

-----Original Message-----
From: Bodong Wang <bodong at nvidia.com> 
Sent: Thursday, December 8, 2022 5:53 PM
To: kernel-team at lists.ubuntu.com
Cc: Vladimir Sokolovsky <vlad at nvidia.com>; Bodong Wang <bodong at nvidia.com>
Subject: [SRU][F:linux-bluefield][PATCH 1/1] tick/rcu: Stop allowing RCU_SOFTIRQ in idle

From: Frederic Weisbecker <frederic at kernel.org>

BugLink: https://bugs.launchpad.net/bugs/1998857

RCU_SOFTIRQ used to be special in that it could be raised on purpose within the idle path to prevent from stopping the tick. Some code still prevents from unnecessary warnings related to this specific behaviour while entering in dynticks-idle mode.

However the nohz layout has changed quite a bit in ten years, and the removal of CONFIG_RCU_FAST_NO_HZ has been the final straw to this safe-conduct. Now the RCU_SOFTIRQ vector is expected to be raised from sane places.

A remaining corner case is admitted though when the vector is invoked in fragile hotplug path.

Signed-off-by: Frederic Weisbecker <frederic at kernel.org>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Paul E. McKenney <paulmck at kernel.org>
Cc: Paul Menzel <pmenzel at molgen.mpg.de>
(Backported from commit 0345691b24c076655ce8f0f4bfd24cba3467ccbd)
[Bodong: Use the pr_warn from new commit]
Signed-off-by: Bodong Wang <bodong at nvidia.com>
---
 include/linux/interrupt.h |  8 ++++++-
 kernel/time/tick-sched.c  | 50 +++++++++++++++++++++++++++++++--------
 2 files changed, 47 insertions(+), 11 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h index 89fc59dab57d..ffc65ce13b02 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -527,7 +527,13 @@ enum
 	NR_SOFTIRQS
 };
 
-#define SOFTIRQ_STOP_IDLE_MASK (~(1 << RCU_SOFTIRQ))
+/*
+ * Ignoring the RCU vector after ksoftirqd is parked is fine
+ * because:
+ * 	1) rcutree_migrate_callbacks() takes care of the queue.
+ * 	2) rcu_report_dead() reports the final quiescent states.
+ */
+#define SOFTIRQ_HOTPLUG_SAFE_MASK (BIT(RCU_SOFTIRQ))
 
 /* map softirq index to softirq name. update 'softirq_to_name' in
  * kernel/softirq.c when adding a new softirq.
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 5eb04bb59802..642e8608bd6a 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -871,6 +871,45 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)  #endif  }
 
+/*
+ * A pending softirq outside an IRQ (or softirq disabled section) 
+context
+ * should be waiting for ksoftirqd to handle it. Therefore we shouldn't
+ * reach here due to the need_resched() early check in can_stop_idle_tick().
+ *
+ * However if we are between CPUHP_AP_SMPBOOT_THREADS and 
+CPU_TEARDOWN_CPU on the
+ * cpu_down() process, softirqs can still be raised while ksoftirqd is 
+parked,
+ * triggering the below since wakep_softirqd() is ignored.
+ *
+ */
+static bool report_idle_softirq(void)
+{
+	static int ratelimit;
+	unsigned int pending = local_softirq_pending();
+
+	if (likely(!pending))
+		return false;
+
+	/* Some softirqs claim to be safe against hotplug and ksoftirqd parking */
+	if (!cpu_active(smp_processor_id())) {
+		pending &= ~SOFTIRQ_HOTPLUG_SAFE_MASK;
+		if (!pending)
+			return false;
+	}
+
+	if (ratelimit < 10)
+		return false;
+
+	/* On RT, softirqs handling may be waiting on some lock */
+	if (!local_bh_blocked())
+		return false;
+
+	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
+		pending);
+	ratelimit++;
+
+	return true;
+}
+
 static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)  {
 	/*
@@ -897,17 +936,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 	if (need_resched())
 		return false;
 
-	if (unlikely(local_softirq_pending())) {
-		static int ratelimit;
-
-		if (ratelimit < 10 &&
-		    (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
-			pr_warn("NOHZ: local_softirq_pending %02x\n",
-				(unsigned int) local_softirq_pending());
-			ratelimit++;
-		}
+	if (unlikely(report_idle_softirq()))
 		return false;
-	}
 
 	if (tick_nohz_full_enabled()) {
 		/*
--
2.27.0