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rcu: Make RCU_FAST_NO_HZ account for pauses out of idle 

Both Steven Rostedt's new idle-capable trace macros and the RCU_NONIDLE()
macro can cause RCU to momentarily pause out of idle without the rest
of the system being involved.  This can cause rcu_prepare_for_idle()
to run through its state machine too quickly, which can in turn result
in needless scheduling-clock interrupts.

This commit therefore adds code to enable rcu_prepare_for_idle() to
distinguish between an initial entry to idle on the one hand (which needs
to advance the rcu_prepare_for_idle() state machine) and an idle reentry
due to idle-capable trace macros and RCU_NONIDLE() on the other hand
(which should avoid advancing the rcu_prepare_for_idle() state machine).
Additional state is maintained to allow the timer to be correctly reposted
when returning after a momentary pause out of idle, and even more state
is maintained to detect when new non-lazy callbacks have been enqueued
(which may require re-evaluation of the approach to idleness).

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2012-02-28 11:02:21 -08:00 committed by Paul E. McKenney
parent 2ee3dc8066
commit c57afe80db
3 changed files with 56 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
kernel/rcutree.c
View File

@ -1829,6 +1829,8 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
rdp->qlen++;
if (lazy)
rdp->qlen_lazy++;
else
rcu_idle_count_callbacks_posted();
if (__is_kfree_rcu_offset((unsigned long)func))
trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,

1
kernel/rcutree.h
View File

@ -471,6 +471,7 @@ static void __cpuinit rcu_prepare_kthreads(int cpu);
static void rcu_prepare_for_idle_init(int cpu);
static void rcu_cleanup_after_idle(int cpu);
static void rcu_prepare_for_idle(int cpu);
static void rcu_idle_count_callbacks_posted(void);
static void print_cpu_stall_info_begin(void);
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
static void print_cpu_stall_info_end(void);

57
kernel/rcutree_plugin.h
View File

@ -1938,6 +1938,14 @@ static void rcu_prepare_for_idle(int cpu)
{
}
/*
* Don't bother keeping a running count of the number of RCU callbacks
* posted because CONFIG_RCU_FAST_NO_HZ=n.
*/
static void rcu_idle_count_callbacks_posted(void)
{
}
#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
/*
@ -1981,6 +1989,10 @@ static void rcu_prepare_for_idle(int cpu)
static DEFINE_PER_CPU(int, rcu_dyntick_drain);
static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
static DEFINE_PER_CPU(unsigned long, rcu_idle_gp_timer_expires);
static DEFINE_PER_CPU(bool, rcu_idle_first_pass);
static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted);
static DEFINE_PER_CPU(unsigned long, rcu_nonlazy_posted_snap);
/*
* Allow the CPU to enter dyntick-idle mode if either: (1) There are no
@ -1993,6 +2005,8 @@ static DEFINE_PER_CPU(struct timer_list, rcu_idle_gp_timer);
*/
int rcu_needs_cpu(int cpu)
{
/* Flag a new idle sojourn to the idle-entry state machine. */
per_cpu(rcu_idle_first_pass, cpu) = 1;
/* If no callbacks, RCU doesn't need the CPU. */
if (!rcu_cpu_has_callbacks(cpu))
return 0;
@ -2095,6 +2109,26 @@ static void rcu_cleanup_after_idle(int cpu)
*/
static void rcu_prepare_for_idle(int cpu)
{
/*
* If this is an idle re-entry, for example, due to use of
* RCU_NONIDLE() or the new idle-loop tracing API within the idle
* loop, then don't take any state-machine actions, unless the
* momentary exit from idle queued additional non-lazy callbacks.
* Instead, repost the rcu_idle_gp_timer if this CPU has callbacks
* pending.
*/
if (!per_cpu(rcu_idle_first_pass, cpu) &&
(per_cpu(rcu_nonlazy_posted, cpu) ==
per_cpu(rcu_nonlazy_posted_snap, cpu))) {
if (rcu_cpu_has_callbacks(cpu))
mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
per_cpu(rcu_idle_gp_timer_expires, cpu));
return;
}
per_cpu(rcu_idle_first_pass, cpu) = 0;
per_cpu(rcu_nonlazy_posted_snap, cpu) =
per_cpu(rcu_nonlazy_posted, cpu) - 1;
/*
* If there are no callbacks on this CPU, enter dyntick-idle mode.
* Also reset state to avoid prejudicing later attempts.
@ -2127,11 +2161,15 @@ static void rcu_prepare_for_idle(int cpu)
per_cpu(rcu_dyntick_drain, cpu) = 0;
per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
if (rcu_cpu_has_nonlazy_callbacks(cpu))
mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
jiffies + RCU_IDLE_GP_DELAY);
per_cpu(rcu_idle_gp_timer_expires, cpu) =
jiffies + RCU_IDLE_GP_DELAY;
else
mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
jiffies + RCU_IDLE_LAZY_GP_DELAY);
per_cpu(rcu_idle_gp_timer_expires, cpu) =
jiffies + RCU_IDLE_LAZY_GP_DELAY;
mod_timer(&per_cpu(rcu_idle_gp_timer, cpu),
per_cpu(rcu_idle_gp_timer_expires, cpu));
per_cpu(rcu_nonlazy_posted_snap, cpu) =
per_cpu(rcu_nonlazy_posted, cpu);
return; /* Nothing more to do immediately. */
} else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
/* We have hit the limit, so time to give up. */
@ -2171,6 +2209,17 @@ static void rcu_prepare_for_idle(int cpu)
trace_rcu_prep_idle("Callbacks drained");
}
/*
* Keep a running count of callbacks posted so that rcu_prepare_for_idle()
* can detect when something out of the idle loop posts a callback.
* Of course, it had better do so either from a trace event designed to
* be called from idle or from within RCU_NONIDLE().
*/
static void rcu_idle_count_callbacks_posted(void)
{
__this_cpu_add(rcu_nonlazy_posted, 1);
}
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
#ifdef CONFIG_RCU_CPU_STALL_INFO