linux-kernel - [RFC PATCH v2 4/5] softirq: Unify should_wakeup

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240904111223.1035-5-kprateek.nayak@amd.com>
Date: Wed, 4 Sep 2024 11:12:22 +0000
From: K Prateek Nayak <kprateek.nayak@....com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, "Mel
 Gorman" <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, "Thomas
 Gleixner" <tglx@...utronix.de>
CC: Leonardo Bras <leobras@...hat.com>, "Paul E. McKenney"
	<paulmck@...nel.org>, Rik van Riel <riel@...riel.com>, Thorsten Blum
	<thorsten.blum@...lux.com>, Zqiang <qiang.zhang1211@...il.com>, Tejun Heo
	<tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, Caleb Sander Mateos
	<csander@...estorage.com>, <linux-kernel@...r.kernel.org>, K Prateek Nayak
	<kprateek.nayak@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
	"Chen Yu" <yu.c.chen@...el.com>, Julia Lawall <Julia.Lawall@...ia.fr>,
	"Sebastian Andrzej Siewior" <bigeasy@...utronix.de>
Subject: [RFC PATCH v2 4/5] softirq: Unify should_wakeup_ksoftirqd()

Define softirq_ctrl::cnt for !PREEMPT_RT kernels too and unify
should_wakeup_ksoftirqd() to return based on softirq_ctrl::cnt.
Since these counts can change quite frequently when running interrupt
heavy benchmark, declare per-cpu softirq_ctrl as cacheline aligned.

No functional changes intended since !PREEMPT_RT kernels do not
increment the softirq_ctrl::cnt (yet) and should always return true
mimicking the current behavior.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
v1..v2:

- New patch. Broken off from approach discussed in
  https://lore.kernel.org/lkml/880f13fd-753d-2c5a-488a-d75c99e8dfa3@amd.com/

- Cacheline align softirq_ctrl since benchmark stressing the IPI path
  showed sensitivity to softirq_ctrl being aligned vs unaligned. This
  was also observed on v1 and has been elaborated in
  https://lore.kernel.org/lkml/20240710090210.41856-4-kprateek.nayak@amd.com/
---
 kernel/softirq.c | 55 ++++++++++++++++++++++++------------------------
 1 file changed, 27 insertions(+), 28 deletions(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index e70a51d737ee..d8902fbcdebf 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -88,23 +88,6 @@ EXPORT_PER_CPU_SYMBOL_GPL(hardirqs_enabled);
 EXPORT_PER_CPU_SYMBOL_GPL(hardirq_context);
 #endif
 
-/*
- * SOFTIRQ_OFFSET usage:
- *
- * On !RT kernels 'count' is the preempt counter, on RT kernels this applies
- * to a per CPU counter and to task::softirqs_disabled_cnt.
- *
- * - count is changed by SOFTIRQ_OFFSET on entering or leaving softirq
- *   processing.
- *
- * - count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
- *   on local_bh_disable or local_bh_enable.
- *
- * This lets us distinguish between whether we are currently processing
- * softirq and whether we just have bh disabled.
- */
-#ifdef CONFIG_PREEMPT_RT
-
 /*
  * RT accounts for BH disabled sections in task::softirqs_disabled_cnt and
  * also in per CPU softirq_ctrl::cnt. This is necessary to allow tasks in a
@@ -118,14 +101,40 @@ EXPORT_PER_CPU_SYMBOL_GPL(hardirq_context);
  * the task which is in a softirq disabled section is preempted or blocks.
  */
 struct softirq_ctrl {
+#ifdef CONFIG_PREEMPT_RT
 	local_lock_t	lock;
+#endif
 	int		cnt;
 };
 
-static DEFINE_PER_CPU(struct softirq_ctrl, softirq_ctrl) = {
+static DEFINE_PER_CPU_ALIGNED(struct softirq_ctrl, softirq_ctrl) = {
+#ifdef CONFIG_PREEMPT_RT
 	.lock	= INIT_LOCAL_LOCK(softirq_ctrl.lock),
+#endif
 };
 
+static inline bool should_wake_ksoftirqd(void)
+{
+	return !this_cpu_read(softirq_ctrl.cnt);
+}
+
+/*
+ * SOFTIRQ_OFFSET usage:
+ *
+ * On !RT kernels 'count' is the preempt counter, on RT kernels this applies
+ * to a per CPU counter and to task::softirqs_disabled_cnt.
+ *
+ * - count is changed by SOFTIRQ_OFFSET on entering or leaving softirq
+ *   processing.
+ *
+ * - count is changed by SOFTIRQ_DISABLE_OFFSET (= 2 * SOFTIRQ_OFFSET)
+ *   on local_bh_disable or local_bh_enable.
+ *
+ * This lets us distinguish between whether we are currently processing
+ * softirq and whether we just have bh disabled.
+ */
+#ifdef CONFIG_PREEMPT_RT
+
 /**
  * local_bh_blocked() - Check for idle whether BH processing is blocked
  *
@@ -270,11 +279,6 @@ static inline void ksoftirqd_run_end(void)
 static inline void softirq_handle_begin(void) { }
 static inline void softirq_handle_end(void) { }
 
-static inline bool should_wake_ksoftirqd(void)
-{
-	return !this_cpu_read(softirq_ctrl.cnt);
-}
-
 static inline void invoke_softirq(void)
 {
 	if (should_wake_ksoftirqd())
@@ -419,11 +423,6 @@ static inline void ksoftirqd_run_end(void)
 	local_irq_enable();
 }
 
-static inline bool should_wake_ksoftirqd(void)
-{
-	return true;
-}
-
 static inline void invoke_softirq(void)
 {
 	if (!force_irqthreads() || !__this_cpu_read(ksoftirqd)) {
-- 
2.34.1