2021-03-09 08:47:06

by Thomas Gleixner

[permalink] [raw]
Subject: [patch 05/14] tasklets: Replace spin wait in tasklet_unlock_wait()

From: Peter Zijlstra <[email protected]>

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 <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
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

+#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);
+void tasklet_unlock_wait(struct tasklet_struct *t)
+ wait_var_event(&t->state, !test_bit(TASKLET_STATE_RUN, &t->state));
void __init softirq_init(void)
int cpu;