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Subject[PATCH v5 3/3] softirq: defer softirq processing to ksoftirqd if CPU is busy with RT
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From: Pavankumar Kondeti <pkondeti@codeaurora.org>

Defer the softirq processing to ksoftirqd if a RT task is
running or queued on the current CPU. This complements the RT
task placement algorithm which tries to find a CPU that is not
currently busy with softirqs.

Currently NET_TX, NET_RX, BLOCK and IRQ_POLL softirqs are only
deferred as they can potentially run for long time.

Additionally, this patch stubs out ksoftirqd_running() logic,
in the CONFIG_RT_SOFTIRQ_AWARE_SCHED case, as deferring
potentially long-running softirqs will cause the logic to not
process shorter-running softirqs immediately. By stubbing it out
the potentially long running softirqs are deferred, but the
shorter running ones can still run immediately.

This patch includes folded-in fixes by:
Lingutla Chandrasekhar <clingutla@codeaurora.org>
Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>
J. Avila <elavila@google.com>

Cc: John Dias <joaodias@google.com>
Cc: Connor O'Brien <connoro@google.com>
Cc: Rick Yiu <rickyiu@google.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Qais Yousef <qyousef@google.com>
Cc: Chris Redpath <chris.redpath@arm.com>
Cc: Abhijeet Dharmapurikar <adharmap@quicinc.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: kernel-team@android.com
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
[satyap@codeaurora.org: trivial merge conflict resolution.]
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>
[elavila: Port to mainline, squash with bugfix]
Signed-off-by: J. Avila <elavila@google.com>
[jstultz: Rebase to linus/HEAD, minor rearranging of code,
included bug fix Reported-by: Qais Yousef <qais.yousef@arm.com> ]
Signed-off-by: John Stultz <jstultz@google.com>
---
v4:
* Fix commit message to accurately note long-running softirqs
(suggested by Qais)
* Switch to using rt_task(current) (suggested by Qais)
v5:
* Switch to using CONFIG_RT_SOFTIRQ_AWARE_SCHED (suggested by
Joel Fernandes <joel@joelfernandes.org>)
---
kernel/softirq.c | 29 +++++++++++++++++++++++++++--
1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index dd92ce8f771b..5db2afd0be68 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -95,6 +95,7 @@ static void wakeup_softirqd(void)
wake_up_process(tsk);
}

+#ifndef CONFIG_RT_SOFTIRQ_AWARE_SCHED
/*
* 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,
@@ -109,6 +110,9 @@ static bool ksoftirqd_running(unsigned long pending)
return false;
return tsk && task_is_running(tsk) && !__kthread_should_park(tsk);
}
+#else
+#define ksoftirqd_running(pending) (false)
+#endif /* CONFIG_RT_SOFTIRQ_AWARE_SCHED */

#ifdef CONFIG_TRACE_IRQFLAGS
DEFINE_PER_CPU(int, hardirqs_enabled);
@@ -540,6 +544,21 @@ static inline bool lockdep_softirq_start(void) { return false; }
static inline void lockdep_softirq_end(bool in_hardirq) { }
#endif

+#ifdef CONFIG_RT_SOFTIRQ_AWARE_SCHED
+static __u32 softirq_deferred_for_rt(__u32 *pending)
+{
+ __u32 deferred = 0;
+
+ if (rt_task(current)) {
+ deferred = *pending & LONG_SOFTIRQ_MASK;
+ *pending &= ~LONG_SOFTIRQ_MASK;
+ }
+ return deferred;
+}
+#else
+#define softirq_deferred_for_rt(x) (0)
+#endif
+
asmlinkage __visible void __softirq_entry __do_softirq(void)
{
unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
@@ -547,6 +566,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
int max_restart = MAX_SOFTIRQ_RESTART;
struct softirq_action *h;
bool in_hardirq;
+ __u32 deferred;
__u32 pending;
int softirq_bit;

@@ -558,14 +578,16 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
current->flags &= ~PF_MEMALLOC;

pending = local_softirq_pending();
+ deferred = softirq_deferred_for_rt(&pending);

softirq_handle_begin();
+
in_hardirq = lockdep_softirq_start();
account_softirq_enter(current);

restart:
/* Reset the pending bitmask before enabling irqs */
- set_softirq_pending(0);
+ set_softirq_pending(deferred);
set_active_softirqs(pending);

local_irq_enable();
@@ -604,13 +626,16 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
local_irq_disable();

pending = local_softirq_pending();
+ deferred = softirq_deferred_for_rt(&pending);
+
if (pending) {
if (time_before(jiffies, end) && !need_resched() &&
--max_restart)
goto restart;
+ }

+ if (pending | deferred)
wakeup_softirqd();
- }

account_softirq_exit(current);
lockdep_softirq_end(in_hardirq);
--
2.38.1.431.g37b22c650d-goog
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