[tip: irq/core] tasklets: Replace spin wait in tasklet_unlock_wait()

tip-bot2 for Peter Zijlstra
2021-03-17T15:50:53Z

The following commit has been merged into the irq/core branch of tip:

Commit-ID: da044747401fc16202e223c9da970ed4e84fd84d
Gitweb: https://git.kernel.org/tip/da044747401fc16202e223c9da970ed4e84fd84d
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 09 Mar 2021 09:42:08 +01:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Wed, 17 Mar 2021 16:33:55 +01:00

tasklets: Replace spin wait in tasklet_unlock_wait()

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@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309084241.783936921@linutronix.de

---
include/linux/interrupt.h | 13 ++-----------
kernel/softirq.c | 18 ++++++++++++++++++
2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index b7f0012..b50be4f 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -664,17 +664,8 @@ static inline int tasklet_trylock(struct tasklet_struct *t)
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
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 8d56bbf..ef6429a 100644
--- 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>

@@ -632,6 +633,23 @@ void tasklet_kill(struct tasklet_struct *t)
}
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;