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rcu: Make TINY_RCU also use softirq for RCU_BOOST=n
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This patch #ifdefs TINY_RCU kthreads out of the kernel unless RCU_BOOST=y,
thus eliminating context-switch overhead if RCU priority boosting has
not been configured.

Signed-off-by: Paul E. McKenney <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
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@paulmck
Paul E. McKenney authored and paulmck committed Sep 29, 2011
1 parent 385680a commit 965a002
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Showing 3 changed files with 97 additions and 91 deletions.
4 changes: 4 additions & 0 deletions include/linux/rcutiny.h
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Original file line number Diff line number Diff line change
Expand Up @@ -27,9 +27,13 @@

#include <linux/cache.h>

#ifdef CONFIG_RCU_BOOST
static inline void rcu_init(void)
{
}
#else /* #ifdef CONFIG_RCU_BOOST */
void rcu_init(void);
#endif /* #else #ifdef CONFIG_RCU_BOOST */

static inline void rcu_barrier_bh(void)
{
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74 changes: 10 additions & 64 deletions kernel/rcutiny.c
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Original file line number Diff line number Diff line change
Expand Up @@ -43,16 +43,11 @@

#include "rcu.h"

/* Controls for rcu_kthread() kthread, replacing RCU_SOFTIRQ used previously. */
static struct task_struct *rcu_kthread_task;
static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
static unsigned long have_rcu_kthread_work;

/* Forward declarations for rcutiny_plugin.h. */
struct rcu_ctrlblk;
static void invoke_rcu_kthread(void);
static void rcu_process_callbacks(struct rcu_ctrlblk *rcp);
static int rcu_kthread(void *arg);
static void invoke_rcu_callbacks(void);
static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp);
static void rcu_process_callbacks(struct softirq_action *unused);
static void __call_rcu(struct rcu_head *head,
void (*func)(struct rcu_head *rcu),
struct rcu_ctrlblk *rcp);
Expand Down Expand Up @@ -101,16 +96,6 @@ static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
return 0;
}

/*
* Wake up rcu_kthread() to process callbacks now eligible for invocation
* or to boost readers.
*/
static void invoke_rcu_kthread(void)
{
have_rcu_kthread_work = 1;
wake_up(&rcu_kthread_wq);
}

/*
* Record an rcu quiescent state. And an rcu_bh quiescent state while we
* are at it, given that any rcu quiescent state is also an rcu_bh
Expand All @@ -123,7 +108,7 @@ void rcu_sched_qs(int cpu)
local_irq_save(flags);
if (rcu_qsctr_help(&rcu_sched_ctrlblk) +
rcu_qsctr_help(&rcu_bh_ctrlblk))
invoke_rcu_kthread();
invoke_rcu_callbacks();
local_irq_restore(flags);
}

Expand All @@ -136,7 +121,7 @@ void rcu_bh_qs(int cpu)

local_irq_save(flags);
if (rcu_qsctr_help(&rcu_bh_ctrlblk))
invoke_rcu_kthread();
invoke_rcu_callbacks();
local_irq_restore(flags);
}

Expand All @@ -160,7 +145,7 @@ void rcu_check_callbacks(int cpu, int user)
* Invoke the RCU callbacks on the specified rcu_ctrlkblk structure
* whose grace period has elapsed.
*/
static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
{
struct rcu_head *next, *list;
unsigned long flags;
Expand Down Expand Up @@ -200,36 +185,11 @@ static void rcu_process_callbacks(struct rcu_ctrlblk *rcp)
RCU_TRACE(trace_rcu_batch_end(rcp->name, cb_count));
}

/*
* This kthread invokes RCU callbacks whose grace periods have
* elapsed. It is awakened as needed, and takes the place of the
* RCU_SOFTIRQ that was used previously for this purpose.
* This is a kthread, but it is never stopped, at least not until
* the system goes down.
*/
static int rcu_kthread(void *arg)
static void rcu_process_callbacks(struct softirq_action *unused)
{
unsigned long work;
unsigned long morework;
unsigned long flags;

for (;;) {
wait_event_interruptible(rcu_kthread_wq,
have_rcu_kthread_work != 0);
morework = rcu_boost();
local_irq_save(flags);
work = have_rcu_kthread_work;
have_rcu_kthread_work = morework;
local_irq_restore(flags);
if (work) {
rcu_process_callbacks(&rcu_sched_ctrlblk);
rcu_process_callbacks(&rcu_bh_ctrlblk);
rcu_preempt_process_callbacks();
}
schedule_timeout_interruptible(1); /* Leave CPU for others. */
}

return 0; /* Not reached, but needed to shut gcc up. */
__rcu_process_callbacks(&rcu_sched_ctrlblk);
__rcu_process_callbacks(&rcu_bh_ctrlblk);
rcu_preempt_process_callbacks();
}

/*
Expand Down Expand Up @@ -291,17 +251,3 @@ void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
__call_rcu(head, func, &rcu_bh_ctrlblk);
}
EXPORT_SYMBOL_GPL(call_rcu_bh);

/*
* Spawn the kthread that invokes RCU callbacks.
*/
static int __init rcu_spawn_kthreads(void)
{
struct sched_param sp;

rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
sp.sched_priority = RCU_BOOST_PRIO;
sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
return 0;
}
early_initcall(rcu_spawn_kthreads);
110 changes: 83 additions & 27 deletions kernel/rcutiny_plugin.h
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Original file line number Diff line number Diff line change
Expand Up @@ -245,6 +245,13 @@ static void show_tiny_preempt_stats(struct seq_file *m)

#include "rtmutex_common.h"

#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO

/* Controls for rcu_kthread() kthread. */
static struct task_struct *rcu_kthread_task;
static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
static unsigned long have_rcu_kthread_work;

/*
* Carry out RCU priority boosting on the task indicated by ->boost_tasks,
* and advance ->boost_tasks to the next task in the ->blkd_tasks list.
Expand Down Expand Up @@ -332,7 +339,7 @@ static int rcu_initiate_boost(void)
if (rcu_preempt_ctrlblk.exp_tasks == NULL)
rcu_preempt_ctrlblk.boost_tasks =
rcu_preempt_ctrlblk.gp_tasks;
invoke_rcu_kthread();
invoke_rcu_callbacks();
} else
RCU_TRACE(rcu_initiate_boost_trace());
return 1;
Expand All @@ -350,14 +357,6 @@ static void rcu_preempt_boost_start_gp(void)

#else /* #ifdef CONFIG_RCU_BOOST */

/*
* If there is no RCU priority boosting, we don't boost.
*/
static int rcu_boost(void)
{
return 0;
}

/*
* If there is no RCU priority boosting, we don't initiate boosting,
* but we do indicate whether there are blocked readers blocking the
Expand Down Expand Up @@ -425,7 +424,7 @@ static void rcu_preempt_cpu_qs(void)

/* If there are done callbacks, cause them to be invoked. */
if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
invoke_rcu_kthread();
invoke_rcu_callbacks();
}

/*
Expand Down Expand Up @@ -646,7 +645,7 @@ static void rcu_preempt_check_callbacks(void)
rcu_preempt_cpu_qs();
if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
rcu_preempt_ctrlblk.rcb.donetail)
invoke_rcu_kthread();
invoke_rcu_callbacks();
if (rcu_preempt_gp_in_progress() &&
rcu_cpu_blocking_cur_gp() &&
rcu_preempt_running_reader())
Expand All @@ -672,7 +671,7 @@ static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
*/
static void rcu_preempt_process_callbacks(void)
{
rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
__rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
}

/*
Expand Down Expand Up @@ -847,15 +846,6 @@ static void show_tiny_preempt_stats(struct seq_file *m)

#endif /* #ifdef CONFIG_RCU_TRACE */

/*
* Because preemptible RCU does not exist, it is never necessary to
* boost preempted RCU readers.
*/
static int rcu_boost(void)
{
return 0;
}

/*
* Because preemptible RCU does not exist, it never has any callbacks
* to check.
Expand All @@ -882,6 +872,78 @@ static void rcu_preempt_process_callbacks(void)

#endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */

#ifdef CONFIG_RCU_BOOST

/*
* Wake up rcu_kthread() to process callbacks now eligible for invocation
* or to boost readers.
*/
static void invoke_rcu_callbacks(void)
{
have_rcu_kthread_work = 1;
wake_up(&rcu_kthread_wq);
}

/*
* This kthread invokes RCU callbacks whose grace periods have
* elapsed. It is awakened as needed, and takes the place of the
* RCU_SOFTIRQ that is used for this purpose when boosting is disabled.
* This is a kthread, but it is never stopped, at least not until
* the system goes down.
*/
static int rcu_kthread(void *arg)
{
unsigned long work;
unsigned long morework;
unsigned long flags;

for (;;) {
wait_event_interruptible(rcu_kthread_wq,
have_rcu_kthread_work != 0);
morework = rcu_boost();
local_irq_save(flags);
work = have_rcu_kthread_work;
have_rcu_kthread_work = morework;
local_irq_restore(flags);
if (work)
rcu_process_callbacks(NULL);
schedule_timeout_interruptible(1); /* Leave CPU for others. */
}

return 0; /* Not reached, but needed to shut gcc up. */
}

/*
* Spawn the kthread that invokes RCU callbacks.
*/
static int __init rcu_spawn_kthreads(void)
{
struct sched_param sp;

rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
sp.sched_priority = RCU_BOOST_PRIO;
sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
return 0;
}
early_initcall(rcu_spawn_kthreads);

#else /* #ifdef CONFIG_RCU_BOOST */

/*
* Start up softirq processing of callbacks.
*/
void invoke_rcu_callbacks(void)
{
raise_softirq(RCU_SOFTIRQ);
}

void rcu_init(void)
{
open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
}

#endif /* #else #ifdef CONFIG_RCU_BOOST */

#ifdef CONFIG_DEBUG_LOCK_ALLOC
#include <linux/kernel_stat.h>

Expand All @@ -897,12 +959,6 @@ void __init rcu_scheduler_starting(void)

#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

#ifdef CONFIG_RCU_BOOST
#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
#else /* #ifdef CONFIG_RCU_BOOST */
#define RCU_BOOST_PRIO 1
#endif /* #else #ifdef CONFIG_RCU_BOOST */

#ifdef CONFIG_RCU_TRACE

#ifdef CONFIG_RCU_BOOST
Expand Down

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