netdev - [patch 05/14] tasklets: Replace spin wait in tasklet_unlock

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Date:   Tue, 09 Mar 2021 09:42:08 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Denis Kirjanov <kda@...ux-powerpc.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        ath9k-devel@....qualcomm.com, Kalle Valo <kvalo@...eaurora.org>,
        linux-wireless@...r.kernel.org, Chas Williams <3chas3@...il.com>,
        linux-atm-general@...ts.sourceforge.net,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        Stefan Richter <stefanr@...6.in-berlin.de>,
        linux1394-devel@...ts.sourceforge.net
Subject: [patch 05/14] tasklets: Replace spin wait in tasklet_unlock_wait()

From: Peter Zijlstra <peterz@...radead.org>

tasklet_unlock_wait() spin waits for TASKLET_STATE_RUN to be cleared. This
is wasting CPU cycles in a tight loop which is especially painful in a
guest when the CPU running the tasklet is scheduled out.

tasklet_unlock_wait() is invoked from tasklet_kill() which is used in
teardown paths and not performance critical at all. Replace the spin wait
with wait_var_event().

There are no users of tasklet_unlock_wait() which are invoked from atomic
contexts. The usage in tasklet_disable() has been replaced temporarily with
the spin waiting variant until the atomic users are fixed up and will be
converted to the sleep wait variant later.

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 include/linux/interrupt.h |   13 ++-----------
 kernel/softirq.c          |   18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 11 deletions(-)

--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -664,17 +664,8 @@ static inline int tasklet_trylock(struct
 	return !test_and_set_bit(TASKLET_STATE_RUN, &(t)->state);
 }
 
-static inline void tasklet_unlock(struct tasklet_struct *t)
-{
-	smp_mb__before_atomic();
-	clear_bit(TASKLET_STATE_RUN, &(t)->state);
-}
-
-static inline void tasklet_unlock_wait(struct tasklet_struct *t)
-{
-	while (test_bit(TASKLET_STATE_RUN, &t->state))
-		cpu_relax();
-}
+void tasklet_unlock(struct tasklet_struct *t);
+void tasklet_unlock_wait(struct tasklet_struct *t);
 
 /*
  * Do not use in new code. Waiting for tasklets from atomic contexts is
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -25,6 +25,7 @@
 #include <linux/smpboot.h>
 #include <linux/tick.h>
 #include <linux/irq.h>
+#include <linux/wait_bit.h>
 
 #include <asm/softirq_stack.h>
 
@@ -621,6 +622,23 @@ void tasklet_kill(struct tasklet_struct
 }
 EXPORT_SYMBOL(tasklet_kill);
 
+#ifdef CONFIG_SMP
+void tasklet_unlock(struct tasklet_struct *t)
+{
+	smp_mb__before_atomic();
+	clear_bit(TASKLET_STATE_RUN, &t->state);
+	smp_mb__after_atomic();
+	wake_up_var(&t->state);
+}
+EXPORT_SYMBOL_GPL(tasklet_unlock);
+
+void tasklet_unlock_wait(struct tasklet_struct *t)
+{
+	wait_var_event(&t->state, !test_bit(TASKLET_STATE_RUN, &t->state));
+}
+EXPORT_SYMBOL_GPL(tasklet_unlock_wait);
+#endif
+
 void __init softirq_init(void)
 {
 	int cpu;