Message-Id: <1182496008.3228.25.camel@dhcp193.mvista.com>
Date:	Fri, 22 Jun 2007 00:06:48 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>,
	john stultz <johnstul@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, matthew.wilcox@...com,
	kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 6/6] Convert tasklets to work queues

On Fri, 2007-06-22 at 00:00 -0400, Steven Rostedt wrote:
> plain text document attachment (tasklets-to-workqueues.patch)
> This patch creates an alternative for drivers from using tasklets.
> It creates a "work_tasklet". When configured to use work_tasklets
> instead of tasklets, instead of creating tasklets, a work queue
> is made in its place.  The API is still the same, and the drivers
> don't know that a work queue is being used.
> 
> This is (for now) a proof of concept approach to using work queues
> instead of tasklets.  More can be done. For one thing, we could make
> individual work queues for each tasklet instead of using the global
> ktasklet_wq.  But for now it's just easier to do this.
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 
> 
> Index: linux-2.6-test/kernel/Kconfig.preempt
> ===================================================================
> --- linux-2.6-test.orig/kernel/Kconfig.preempt
> +++ linux-2.6-test/kernel/Kconfig.preempt
> @@ -63,3 +63,18 @@ config PREEMPT_BKL
>  	  Say Y here if you are building a kernel for a desktop system.
>  	  Say N if you are unsure.
>  
> +config TASKLETS_AS_WORKQUEUES
> +	bool "Treat tasklets as workqueues (EXPERIMENTAL)"
> +	depends on EXPERIMENTAL
> +	help
> +	  Tasklets are an old solution to an old problem with respect
> +	  to SMP.  But today they are not necessary anymore.
> +	  There are better solutions to the problem that tasklets
> +	  are trying to solve.
> +
> +	  This option converts tasklets into workqueues and
> +	  removes the tasklet softirq.  This is a clean up until
> +	  we get rid of all tasklets that are currently in
> +	  the kernel.
> +
> +	  Say Y if you are unsure and brave (not very well tested code!).
> Index: linux-2.6-test/kernel/Makefile
> ===================================================================
> --- linux-2.6-test.orig/kernel/Makefile
> +++ linux-2.6-test/kernel/Makefile
> @@ -21,7 +21,11 @@ obj-$(CONFIG_FUTEX) += futex.o
>  ifeq ($(CONFIG_COMPAT),y)
>  obj-$(CONFIG_FUTEX) += futex_compat.o
>  endif
> +ifeq ($(CONFIG_TASKLETS_AS_WORKQUEUES), y)
> +obj-y += tasklet_work.o
> +else
>  obj-y += tasklet.o
> +endif
>  obj-$(CONFIG_RT_MUTEXES) += rtmutex.o
>  obj-$(CONFIG_DEBUG_RT_MUTEXES) += rtmutex-debug.o
>  obj-$(CONFIG_RT_MUTEX_TESTER) += rtmutex-tester.o
> Index: linux-2.6-test/init/main.c
> ===================================================================
> --- linux-2.6-test.orig/init/main.c
> +++ linux-2.6-test/init/main.c
> @@ -121,6 +121,11 @@ extern void time_init(void);
>  /* Default late time init is NULL. archs can override this later. */
>  void (*late_time_init)(void);
>  extern void softirq_init(void);
> +#ifdef CONFIG_TASKLETS_AS_WORKQUEUES
> +  extern void init_tasklets(void);
> +#else
> +# define init_tasklets() do { } while(0)
> +#endif
>  
>  /* Untouched command line saved by arch-specific code. */
>  char __initdata boot_command_line[COMMAND_LINE_SIZE];
> @@ -706,6 +711,7 @@ static void __init do_basic_setup(void)
>  {
>  	/* drivers will send hotplug events */
>  	init_workqueues();
> +	init_tasklets();
>  	usermodehelper_init();
>  	driver_init();
>  	init_irq_proc();
> Index: linux-2.6-test/include/linux/tasklet.h
> ===================================================================
> --- linux-2.6-test.orig/include/linux/tasklet.h
> +++ linux-2.6-test/include/linux/tasklet.h
> @@ -1,6 +1,10 @@
>  #ifndef _LINUX_TASKLET_H
>  #define _LINUX_TASKLET_H
>  
> -#include <linux/tasklet_softirq.h>
> +#ifdef CONFIG_TASKLETS_AS_WORKQUEUES
> +# include <linux/tasklet_work.h>
> +#else
> +# include <linux/tasklet_softirq.h>
> +#endif
>  
>  #endif
> Index: linux-2.6-test/include/linux/tasklet_work.h
> ===================================================================
> --- /dev/null
> +++ linux-2.6-test/include/linux/tasklet_work.h
> @@ -0,0 +1,62 @@
> +#ifndef _LINUX_WORK_TASKLET_H
> +#define _LINUX_WORK_TASKLET_H
> +
> +#ifndef _LINUX_INTERRUPT_H
> +# error "Do not include this header directly! use linux/interrupt.h"
> +#endif
> +
> +#include <linux/workqueue.h>
> +
> +extern void work_tasklet_exec(struct work_struct *work);
> +
> +struct tasklet_struct
> +{
> +	struct work_struct work;
> +	struct list_head list;
> +	unsigned long state;
> +	atomic_t count;
> +	void (*func)(unsigned long);
> +	unsigned long data;
> +	char *n;
> +};
> +
> +#define DECLARE_TASKLET(name, func, data)				\
> +	struct tasklet_struct name = {					\
> +		__WORK_INITIALIZER((name).work, work_tasklet_exec),	\
> +		LIST_HEAD_INIT((name).list),				\
> +		0,							\
> +		ATOMIC_INIT(0),						\
> +		func,							\
> +		data,							\
> +		#name							\
> +	}
> +
> +#define DECLARE_TASKLET_DISABLED(name, func, data)			\
> +	struct tasklet_struct name = {					\
> +		__WORK_INITIALIZER((name).work, work_tasklet_exec),	\
> +		LIST_HEAD_INIT((name).list),				\
> +		0,							\
> +		ATOMIC_INIT(1),						\
> +		func,							\
> +		data,							\
> + 		#name							\
> +	}
> +
> +void tasklet_schedule(struct tasklet_struct *t);
> +#define tasklet_hi_schedule tasklet_schedule
> +extern fastcall void tasklet_enable(struct tasklet_struct *t);
> +#define tasklet_hi_enable tasklet_enable
> +
> +void tasklet_disable_nosync(struct tasklet_struct *t);
> +void tasklet_disable(struct tasklet_struct *t);
> +
> +extern int tasklet_is_scheduled(struct tasklet_struct *t);
> +
> +extern void tasklet_kill(struct tasklet_struct *t);
> +extern void tasklet_kill_immediate(struct tasklet_struct *t, unsigned int cpu);
> +extern void tasklet_init(struct tasklet_struct *t,
> +			 void (*func)(unsigned long), unsigned long data);
> +void takeover_tasklets(unsigned int cpu);
> +
> +
> +#endif /* _LINUX_WORK_TASKLET_H */
> Index: linux-2.6-test/kernel/tasklet_work.c
> ===================================================================
> --- /dev/null
> +++ linux-2.6-test/kernel/tasklet_work.c
> @@ -0,0 +1,138 @@
> +/*
> + *	linux/kernel/work_tasklet.c
> + *
> + *	Copyright (C) 2007 Steven Rostedt, Red Hat
> + *
> + */
> +
> +#include <linux/interrupt.h>
> +
> +static struct workqueue_struct *ktaskletd_wq;
> +
> +enum
> +{
> +	TASKLET_STATE_SCHED,	/* Tasklet is scheduled for execution */
> +	TASKLET_STATE_RUN,	/* Tasklet is running (SMP only) */
> +	TASKLET_STATE_PENDING	/* Tasklet is pending */
> +};
> +
> +#define TASKLET_STATEF_SCHED	(1 << TASKLET_STATE_SCHED)
> +#define TASKLET_STATEF_RUN	(1 << TASKLET_STATE_RUN)
> +#define TASKLET_STATEF_PENDING	(1 << TASKLET_STATE_PENDING)
> +
> +void tasklet_schedule(struct tasklet_struct *t)
> +{
> +	BUG_ON(!ktaskletd_wq);
> +	pr_debug("scheduling tasklet %s %p\n", t->n, t);

I'd change these pr_debug lines to "tasklet : scheduling %s %p\n" for
readability ..

> +	queue_work(ktaskletd_wq, &t->work);
> +}
> +
> +EXPORT_SYMBOL(tasklet_schedule);
> +
> +int tasklet_is_scheduled(struct tasklet_struct *t)
> +{
> +	int ret;
> +	ret = work_pending(&t->work);
> +	pr_debug("sched %s pending=%d\n", t->n, ret);
> +	return ret;
> +}
> +
> +EXPORT_SYMBOL(tasklet_is_scheduled);
> +
> +void tasklet_disable_nosync(struct tasklet_struct *t)
> +{
> +	pr_debug("disable tasklet %s %p\n", t->n, t);
> +	atomic_inc(&t->count);
> +	smp_mb__after_atomic_inc();
> +}
> +
> +EXPORT_SYMBOL(tasklet_disable_nosync);
> +
> +void tasklet_disable(struct tasklet_struct *t)
> +{
> +	tasklet_disable_nosync(t);
> +	pr_debug("flush tasklet %s %p\n", t->n, t);
> +	flush_workqueue(ktaskletd_wq);
> +	smp_mb();
> +}
> +
> +EXPORT_SYMBOL(tasklet_disable);
> +
> +void work_tasklet_exec(struct work_struct *work)
> +{
> +	struct tasklet_struct *t =
> +		container_of(work, struct tasklet_struct, work);
> +
> +	if (unlikely(atomic_read(&t->count))) {
> +		pr_debug("tasklet disabled %s %p\n", t->n, t);
> +		set_bit(TASKLET_STATE_PENDING, &t->state);
> +		smp_mb();
> +		/* make sure we were not just enabled */
> +		if (likely(atomic_read(&t->count)))
> +			goto out;
> +		clear_bit(TASKLET_STATE_PENDING, &t->state);

smp_mb__before_clear_bit ?

> +	}
> +
> +	local_bh_disable();
> +	pr_debug("run tasklet %s %p\n", t->n, t);
> +	t->func(t->data);
> +	local_bh_enable();
> +
> +out:
> +	return;
> +}
> +
> +EXPORT_SYMBOL(work_tasklet_exec);
> +
> +void __init softirq_init(void)
> +{
> +}

ifdef's ?

> +void init_tasklets(void)
> +{
> +	ktaskletd_wq = create_workqueue("tasklets");
> +	BUG_ON(!ktaskletd_wq);
> +}
> +
> +void takeover_tasklets(unsigned int cpu)
> +{
> +	pr_debug("Implement takeover tasklets??\n");
> +}

hmm .. Looks like it's for migration of tasklets .. I take it your not
sure that's handled ? Try cpu hotplug ..

> +void tasklet_init(struct tasklet_struct *t,
> +		  void (*func)(unsigned long), unsigned long data)
> +{
> +	INIT_WORK(&t->work, work_tasklet_exec);
> +	INIT_LIST_HEAD(&t->list);
> +	t->state = 0;
> +	atomic_set(&t->count, 0);
> +	t->func = func;
> +	t->data = data;
> +	t->n = "anonymous";

Is this "anonymous" just used for debugging ? Is so you could fill it
with a kallsyms lookup with the __builtin_return_address() ..

> +	pr_debug("anonymous tasklet %p set at %p\n",
> +		t, __builtin_return_address(0));
> +}
> +
> +EXPORT_SYMBOL(tasklet_init);
> +
> +void fastcall tasklet_enable(struct tasklet_struct *t)
> +{
> +	pr_debug("enable tasklet %s (count was %d)\n",
> +		 t->n, atomic_read(&t->count));
> +	if (!atomic_dec_and_test(&t->count))
> +		return;
> +	if (test_and_clear_bit(TASKLET_STATE_PENDING, &t->state)) {
> +		pr_debug("tasklet %s was pending\n", t->n);
> +		tasklet_schedule(t);
> +	}
> +}
> +
> +EXPORT_SYMBOL(tasklet_enable);
> +
> +void tasklet_kill(struct tasklet_struct *t)
> +{
> +	pr_debug("kill tasklet %s\n", t->n);
> +	flush_workqueue(ktaskletd_wq);
> +}
> +
> +EXPORT_SYMBOL(tasklet_kill);
> 

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