[PATCH 0/2] RCU offloading vs scheduler latency(for 5.10 stable)

[PATCH 0/2] RCU offloading vs scheduler latency(for 5.10 stable)

[Uladzislau Rezki (Sony)]

This port is for 5.10 kernel. See the motivation and explanation in the cover
letter for the 5.15 port.

Frederic Weisbecker (2):
  rcu: [for 5.10 stable] Fix callbacks processing time limit retaining cond_resched()
  rcu: [for 5.10 stable] Apply callbacks processing time limit only on softirq

 kernel/rcu/tree.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

-- 
2.30.2

[PATCH 1/2] rcu: [for 5.10 stable] Fix callbacks processing time limit retaining cond_resched()

[Uladzislau Rezki (Sony)]

From: Frederic Weisbecker <frederic@kernel.org>

commit 3e61e95e2d095e308616cba4ffb640f95a480e01 upstream.

The callbacks processing time limit makes sure we are not exceeding a
given amount of time executing the queue.

However its "continue" clause bypasses the cond_resched() call on
rcuc and NOCB kthreads, delaying it until we reach the limit, which can
be very long...

Make sure the scheduler has a higher priority than the time limit.

Motivation of backport:
-----------------------

1. The cfcdef5e30469 ("rcu: Allow rcu_do_batch() to dynamically adjust batch sizes")
broke the default behaviour of "offloading rcu callbacks" setup. In that scenario
after each callback the caller context was used to check if it has to be rescheduled
giving a CPU time for others. After that change an "offloaded" setup can switch to
time-based RCU callbacks processing, what can be long for latency sensitive workloads
and SCHED_FIFO processes, i.e. callbacks are invoked for a long time with keeping
preemption off and without checking cond_resched().

2. Our devices which run Android and 5.10 kernel have some critical areas which
are sensitive to latency. It is a low latency audio, 8k video, UI stack and so on.
For example below is a trace that illustrates a delay of "irq/396-5-0072" RT task
to complete IRQ processing:

<snip>
 rcuop/6-54  [000] d.h2  183.752989: irq_handler_entry:    irq=85 name=i2c_geni
 rcuop/6-54  [000] d.h5  183.753007: sched_waking:         comm=irq/396-5-0072 pid=12675 prio=49 target_cpu=000
 rcuop/6-54  [000] dNh6  183.753014: sched_wakeup:         irq/396-5-0072:12675 [49] success=1 CPU:000
 rcuop/6-54  [000] dNh2  183.753015: irq_handler_exit:     irq=85 ret=handled
 rcuop/6-54  [000] .N..  183.753018: rcu_invoke_callback:  rcu_preempt rhp=0xffffff88ffd440b0 func=__d_free.cfi_jt
 rcuop/6-54  [000] .N..  183.753020: rcu_invoke_callback:  rcu_preempt rhp=0xffffff892ffd8400 func=inode_free_by_rcu.cfi_jt
 rcuop/6-54  [000] .N..  183.753021: rcu_invoke_callback:  rcu_preempt rhp=0xffffff89327cd708 func=i_callback.cfi_jt
 ...
 rcuop/6-54  [000] .N..  183.755941: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c5a968 func=i_callback.cfi_jt
 rcuop/6-54  [000] .N..  183.755942: rcu_invoke_callback:  rcu_preempt rhp=0xffffff8993c4bd20 func=__d_free.cfi_jt
 rcuop/6-54  [000] dN..  183.755944: rcu_batch_end:        rcu_preempt CBs-invoked=2112 idle=>c<>c<>c<>c<
 rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      Start context switch
 rcuop/6-54  [000] dN..  183.755946: rcu_utilization:      End context switch
 rcuop/6-54  [000] d..2  183.755959: sched_switch:         rcuop/6:54 [120] R ==> migration/0:16 [0]
 ...
 migratio-16 [000] d..2  183.756021: sched_switch:         migration/0:16 [0] S ==> irq/396-5-0072:12675 [49]
<snip>

The "irq/396-5-0072:12675" was delayed for ~3 milliseconds due to introduced side effect.
Please note, on our Android devices we get ~70 000 callbacks registered to be invoked by
the "rcuop/x" workers. This is during 1 seconds time interval and regular handset usage.
Latencies bigger that 3 milliseconds affect our high-resolution audio streaming over the
LDAC/Bluetooth stack.

Fixes: cfcdef5e30469 ("rcu: Allow rcu_do_batch() to dynamically adjust batch sizes")
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
[UR: backport to 5.10-stable + commit update]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 844c35803739..f340df6ebd86 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2490,10 +2490,22 @@ static void rcu_do_batch(struct rcu_data *rdp)
 		 * Stop only if limit reached and CPU has something to do.
 		 * Note: The rcl structure counts down from zero.
 		 */
-		if (-rcl.len >= bl && !offloaded &&
-		    (need_resched() ||
-		     (!is_idle_task(current) && !rcu_is_callbacks_kthread())))
-			break;
+		if (in_serving_softirq()) {
+			if (-rcl.len >= bl && (need_resched() ||
+					(!is_idle_task(current) && !rcu_is_callbacks_kthread())))
+				break;
+		} else {
+			local_bh_enable();
+			lockdep_assert_irqs_enabled();
+			cond_resched_tasks_rcu_qs();
+			lockdep_assert_irqs_enabled();
+			local_bh_disable();
+		}
+
+		/*
+		 * Make sure we don't spend too much time here and deprive other
+		 * softirq vectors of CPU cycles.
+		 */
 		if (unlikely(tlimit)) {
 			/* only call local_clock() every 32 callbacks */
 			if (likely((-rcl.len & 31) || local_clock() < tlimit))
@@ -2501,14 +2513,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
 			/* Exceeded the time limit, so leave. */
 			break;
 		}
-		if (offloaded) {
-			WARN_ON_ONCE(in_serving_softirq());
-			local_bh_enable();
-			lockdep_assert_irqs_enabled();
-			cond_resched_tasks_rcu_qs();
-			lockdep_assert_irqs_enabled();
-			local_bh_disable();
-		}
 	}
 
 	local_irq_save(flags);
-- 
2.30.2

[PATCH 2/2] rcu: [for 5.10 stable] Apply callbacks processing time limit only on softirq

[Uladzislau Rezki (Sony)]

From: Frederic Weisbecker <frederic@kernel.org>

commit a554ba288845fd3f6f12311fd76a51694233458a upstream.

Time limit only makes sense when callbacks are serviced in softirq mode
because:

_ In case we need to get back to the scheduler,
  cond_resched_tasks_rcu_qs() is called after each callback.

_ In case some other softirq vector needs the CPU, the call to
  local_bh_enable() before cond_resched_tasks_rcu_qs() takes care about
  them via a call to do_softirq().

Therefore, make sure the time limit only applies to softirq mode.

Fixes: cfcdef5e30469 ("rcu: Allow rcu_do_batch() to dynamically adjust batch sizes")
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
[UR: backport to 5.10-stable]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index f340df6ebd86..b41009a283ca 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2456,7 +2456,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
 	div = READ_ONCE(rcu_divisor);
 	div = div < 0 ? 7 : div > sizeof(long) * 8 - 2 ? sizeof(long) * 8 - 2 : div;
 	bl = max(rdp->blimit, pending >> div);
-	if (unlikely(bl > 100)) {
+	if (in_serving_softirq() && unlikely(bl > 100)) {
 		long rrn = READ_ONCE(rcu_resched_ns);
 
 		rrn = rrn < NSEC_PER_MSEC ? NSEC_PER_MSEC : rrn > NSEC_PER_SEC ? NSEC_PER_SEC : rrn;
@@ -2494,6 +2494,18 @@ static void rcu_do_batch(struct rcu_data *rdp)
 			if (-rcl.len >= bl && (need_resched() ||
 					(!is_idle_task(current) && !rcu_is_callbacks_kthread())))
 				break;
+
+			/*
+			 * Make sure we don't spend too much time here and deprive other
+			 * softirq vectors of CPU cycles.
+			 */
+			if (unlikely(tlimit)) {
+				/* only call local_clock() every 32 callbacks */
+				if (likely((-rcl.len & 31) || local_clock() < tlimit))
+					continue;
+				/* Exceeded the time limit, so leave. */
+				break;
+			}
 		} else {
 			local_bh_enable();
 			lockdep_assert_irqs_enabled();
@@ -2501,18 +2513,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
 			lockdep_assert_irqs_enabled();
 			local_bh_disable();
 		}
-
-		/*
-		 * Make sure we don't spend too much time here and deprive other
-		 * softirq vectors of CPU cycles.
-		 */
-		if (unlikely(tlimit)) {
-			/* only call local_clock() every 32 callbacks */
-			if (likely((-rcl.len & 31) || local_clock() < tlimit))
-				continue;
-			/* Exceeded the time limit, so leave. */
-			break;
-		}
 	}
 
 	local_irq_save(flags);
-- 
2.30.2