Commit d15121be authored by Paolo Abeni's avatar Paolo Abeni Committed by Thomas Gleixner
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Revert "softirq: Let ksoftirqd do its job" 



This reverts the following commits:

  4cd13c21 ("softirq: Let ksoftirqd do its job")
  3c53776e ("Mark HI and TASKLET softirq synchronous")
  1342d808 ("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 1ff68820 ("watchdog: core: make sure the watchdog_worker is not deferred")
commit 8d5755b3 ("watchdog: softdog: fire watchdog even if softirqs do not get to run")
commit 217f6974 ("net: busy-poll: allow preemption in sk_busy_loop()")
commit 3c53776e ("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 29863d41
("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: default avatarPaolo Abeni <pabeni@redhat.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarJason Xing <kerneljasonxing@gmail.com>
Reviewed-by: default avatarJakub Kicinski <kuba@kernel.org>
Reviewed-by: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/netdev/305d7742212cbe98621b16be782b0562f1012cb6.camel@redhat.com
Link: https://lore.kernel.org/r/57e66b364f1b6f09c9bc0316742c3b14f4ce83bd.1683526542.git.pabeni@redhat.com
parent ac9a7868

kernel/softirq.c

+2 −20
Original line number Diff line number Diff line
@@ -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;



	/*
@@ -432,9 +417,6 @@ 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

		/*
@@ -468,7 +450,7 @@ 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);