netdev - [PATCH RT 03/12] Revert "softirq: Let ksoftirqd do its job"

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Message-Id: <20231018193707.650413-4-joseph.salisbury@canonical.com>
Date: Wed, 18 Oct 2023 15:36:58 -0400
From: Joseph Salisbury <joseph.salisbury@...onical.com>
To: Joseph Salisbury <joseph.salisbury@...onical.com>
Cc: Paolo Abeni <pabeni@...hat.com>,
	Jason Xing <kerneljasonxing@...il.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	netdev@...r.kernel.org
Subject: [PATCH RT 03/12] Revert "softirq: Let ksoftirqd do its job"

From: Paolo Abeni <pabeni@...hat.com>

v5.15.133-rt70-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


This reverts the following commits:

  4cd13c21b207 ("softirq: Let ksoftirqd do its job")
  3c53776e29f8 ("Mark HI and TASKLET softirq synchronous")
  1342d8080f61 ("softirq: Don't skip softirq execution when softirq thread is parking")

in a single change to avoid known bad intermediate states introduced by a
patch series reverting them individually.

Due to the mentioned commit, when the ksoftirqd threads take charge of
softirq processing, the system can experience high latencies.

In the past a few workarounds have been implemented for specific
side-effects of the initial ksoftirqd enforcement commit:

commit 1ff688209e2e ("watchdog: core: make sure the watchdog_worker is not deferred")
commit 8d5755b3f77b ("watchdog: softdog: fire watchdog even if softirqs do not get to run")
commit 217f69743681 ("net: busy-poll: allow preemption in sk_busy_loop()")
commit 3c53776e29f8 ("Mark HI and TASKLET softirq synchronous")

But the latency problem still exists in real-life workloads, see the link
below.

The reverted commit intended to solve a live-lock scenario that can now be
addressed with the NAPI threaded mode, introduced with commit 29863d41bb6e
("net: implement threaded-able napi poll loop support"), which is nowadays
in a pretty stable status.

While a complete solution to put softirq processing under nice resource
control would be preferable, that has proven to be a very hard task. In
the short term, remove the main pain point, and also simplify a bit the
current softirq implementation.

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Jason Xing <kerneljasonxing@...il.com>
Reviewed-by: Jakub Kicinski <kuba@...nel.org>
Reviewed-by: Eric Dumazet <edumazet@...gle.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: netdev@...r.kernel.org
Link: https://lore.kernel.org/netdev/305d7742212cbe98621b16be782b0562f1012cb6.camel@redhat.com
Link: https://lore.kernel.org/r/57e66b364f1b6f09c9bc0316742c3b14f4ce83bd.1683526542.git.pabeni@redhat.com
(cherry picked from commit d15121be7485655129101f3960ae6add40204463)
Signed-off-by: Joseph Salisbury <joseph.salisbury@...onical.com>
---
 kernel/softirq.c | 22 ++--------------------
 1 file changed, 2 insertions(+), 20 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index 41f470929e99..398951403331 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -80,21 +80,6 @@ static void wakeup_softirqd(void)
 		wake_up_process(tsk);
 }
 
-/*
- * If ksoftirqd is scheduled, we do not want to process pending softirqs
- * right now. Let ksoftirqd handle this at its own rate, to get fairness,
- * unless we're doing some of the synchronous softirqs.
- */
-#define SOFTIRQ_NOW_MASK ((1 << HI_SOFTIRQ) | (1 << TASKLET_SOFTIRQ))
-static bool ksoftirqd_running(unsigned long pending)
-{
-	struct task_struct *tsk = __this_cpu_read(ksoftirqd);
-
-	if (pending & SOFTIRQ_NOW_MASK)
-		return false;
-	return tsk && task_is_running(tsk) && !__kthread_should_park(tsk);
-}
-
 #ifdef CONFIG_TRACE_IRQFLAGS
 DEFINE_PER_CPU(int, hardirqs_enabled);
 DEFINE_PER_CPU(int, hardirq_context);
@@ -236,7 +221,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 		goto out;
 
 	pending = local_softirq_pending();
-	if (!pending || ksoftirqd_running(pending))
+	if (!pending)
 		goto out;
 
 	/*
@@ -419,9 +404,6 @@ static inline bool should_wake_ksoftirqd(void)
 
 static inline void invoke_softirq(void)
 {
-	if (ksoftirqd_running(local_softirq_pending()))
-		return;
-
 	if (!force_irqthreads() || !__this_cpu_read(ksoftirqd)) {
 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 		/*
@@ -455,7 +437,7 @@ asmlinkage __visible void do_softirq(void)
 
 	pending = local_softirq_pending();
 
-	if (pending && !ksoftirqd_running(pending))
+	if (pending)
 		do_softirq_own_stack();
 
 	local_irq_restore(flags);
-- 
2.34.1