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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
This commit is contained in:
Peter Zijlstra 2021-03-09 09:42:08 +01:00 committed by Thomas Gleixner
parent b0cd02c2a9
commit da04474740
2 changed files with 20 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
include/linux/interrupt.h
View File

@ -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

18
kernel/softirq.c
View File

@ -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;