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Date: Tue, 19 Oct 2010 10:06:45 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Lai Jiangshan <laijs@...fujitsu.com>
Cc: Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] rcu,cleanup: move synchronize_sched_expedited()
out of sched.c
On Tue, Oct 19, 2010 at 03:56:56PM +0800, Lai Jiangshan wrote:
>
> The first version of synchronize_sched_expedited() use the migration code
> of the scheduler code, so it have to be implemented in sched.c
>
> but now, the synchronize_sched_expedited() does not use such code,
> it is time to move it out of sched.c.
>
> Different rcu implementation' synchronize_sched_expedited() are also
> different. so we move synchronize_sched_expedited() to kernel/rcu{tree,tiny}_plugin.h
> instead of kerenl/rcupdate.c
Good stuff, Lai!!!
Please see below for one small but important comment.
Thanx, Paul
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> rcutiny_plugin.h | 6 ++++
> rcutree_plugin.h | 71 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> sched.c | 69 -----------------------------------------------------
> 3 files changed, 77 insertions(+), 69 deletions(-)
> diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
> index 6ceca4f..87e226c 100644
> --- a/kernel/rcutiny_plugin.h
> +++ b/kernel/rcutiny_plugin.h
> @@ -461,6 +461,12 @@ void synchronize_rcu(void)
> }
> EXPORT_SYMBOL_GPL(synchronize_rcu);
>
> +void synchronize_sched_expedited(void)
> +{
> + barrier();
> +}
> +EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
> +
It would be better to put this in include/linux/rcutiny.h under
#ifdef CONFIG_TINY_PREEMPT_RCU. The reason is that this would
allow the EXPORT_SYMBOL_GPL() to be dropped, saving significant
memory, at least from a TINY_RCU viewpoint. Could you please
resubmit with this change?
(The reason we don't do the same for synchronize_sched() is that
it needs cond_resched() in the CONFIG_TINY_RCU case.)
> static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
> static unsigned long sync_rcu_preempt_exp_count;
> static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 71a4147..9e62b01 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -25,6 +25,7 @@
> */
>
> #include <linux/delay.h>
> +#include <linux/stop_machine.h>
>
> /*
> * Check the RCU kernel configuration parameters and print informative
> @@ -581,6 +582,76 @@ void synchronize_rcu(void)
> }
> EXPORT_SYMBOL_GPL(synchronize_rcu);
>
> +#ifndef CONFIG_SMP
> +
> +void synchronize_sched_expedited(void)
> +{
> + barrier();
> +}
> +EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
> +
> +#else /* #ifndef CONFIG_SMP */
> +
> +static atomic_t synchronize_sched_expedited_count = ATOMIC_INIT(0);
> +
> +static int synchronize_sched_expedited_cpu_stop(void *data)
> +{
> + /*
> + * There must be a full memory barrier on each affected CPU
> + * between the time that try_stop_cpus() is called and the
> + * time that it returns.
> + *
> + * In the current initial implementation of cpu_stop, the
> + * above condition is already met when the control reaches
> + * this point and the following smp_mb() is not strictly
> + * necessary. Do smp_mb() anyway for documentation and
> + * robustness against future implementation changes.
> + */
> + smp_mb(); /* See above comment block. */
> + return 0;
> +}
> +
> +/*
> + * Wait for an rcu-sched grace period to elapse, but use "big hammer"
> + * approach to force grace period to end quickly. This consumes
> + * significant time on all CPUs, and is thus not recommended for
> + * any sort of common-case code.
> + *
> + * Note that it is illegal to call this function while holding any
> + * lock that is acquired by a CPU-hotplug notifier. Failing to
> + * observe this restriction will result in deadlock.
> + */
> +void synchronize_sched_expedited(void)
> +{
> + int snap, trycount = 0;
> +
> + smp_mb(); /* ensure prior mod happens before capturing snap. */
> + snap = atomic_read(&synchronize_sched_expedited_count) + 1;
> + get_online_cpus();
> + while (try_stop_cpus(cpu_online_mask,
> + synchronize_sched_expedited_cpu_stop,
> + NULL) == -EAGAIN) {
> + put_online_cpus();
> + if (trycount++ < 10)
> + udelay(trycount * num_online_cpus());
> + else {
> + synchronize_sched();
> + return;
> + }
> + if (atomic_read(&synchronize_sched_expedited_count) - snap > 0) {
> + smp_mb(); /* ensure test happens before caller kfree */
> + return;
> + }
> + get_online_cpus();
> + }
> + atomic_inc(&synchronize_sched_expedited_count);
> + smp_mb__after_atomic_inc(); /* ensure post-GP actions seen after GP. */
> + put_online_cpus();
> +}
> +EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
> +
> +#endif /* #else #ifndef CONFIG_SMP */
> +
> static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
> static long sync_rcu_preempt_exp_count;
> static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
> diff --git a/kernel/sched.c b/kernel/sched.c
> index abf8440..9dc7775 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -9332,72 +9332,3 @@ struct cgroup_subsys cpuacct_subsys = {
> };
> #endif /* CONFIG_CGROUP_CPUACCT */
>
> -#ifndef CONFIG_SMP
> -
> -void synchronize_sched_expedited(void)
> -{
> - barrier();
> -}
> -EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
> -
> -#else /* #ifndef CONFIG_SMP */
> -
> -static atomic_t synchronize_sched_expedited_count = ATOMIC_INIT(0);
> -
> -static int synchronize_sched_expedited_cpu_stop(void *data)
> -{
> - /*
> - * There must be a full memory barrier on each affected CPU
> - * between the time that try_stop_cpus() is called and the
> - * time that it returns.
> - *
> - * In the current initial implementation of cpu_stop, the
> - * above condition is already met when the control reaches
> - * this point and the following smp_mb() is not strictly
> - * necessary. Do smp_mb() anyway for documentation and
> - * robustness against future implementation changes.
> - */
> - smp_mb(); /* See above comment block. */
> - return 0;
> -}
> -
> -/*
> - * Wait for an rcu-sched grace period to elapse, but use "big hammer"
> - * approach to force grace period to end quickly. This consumes
> - * significant time on all CPUs, and is thus not recommended for
> - * any sort of common-case code.
> - *
> - * Note that it is illegal to call this function while holding any
> - * lock that is acquired by a CPU-hotplug notifier. Failing to
> - * observe this restriction will result in deadlock.
> - */
> -void synchronize_sched_expedited(void)
> -{
> - int snap, trycount = 0;
> -
> - smp_mb(); /* ensure prior mod happens before capturing snap. */
> - snap = atomic_read(&synchronize_sched_expedited_count) + 1;
> - get_online_cpus();
> - while (try_stop_cpus(cpu_online_mask,
> - synchronize_sched_expedited_cpu_stop,
> - NULL) == -EAGAIN) {
> - put_online_cpus();
> - if (trycount++ < 10)
> - udelay(trycount * num_online_cpus());
> - else {
> - synchronize_sched();
> - return;
> - }
> - if (atomic_read(&synchronize_sched_expedited_count) - snap > 0) {
> - smp_mb(); /* ensure test happens before caller kfree */
> - return;
> - }
> - get_online_cpus();
> - }
> - atomic_inc(&synchronize_sched_expedited_count);
> - smp_mb__after_atomic_inc(); /* ensure post-GP actions seen after GP. */
> - put_online_cpus();
> -}
> -EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
> -
> -#endif /* #else #ifndef CONFIG_SMP */
--
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