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Date: Tue, 9 Dec 2014 09:02:38 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Lai Jiangshan <laijs@...fujitsu.com>
Cc: linux-kernel@...r.kernel.org,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] tiny_rcu: directly force QS when call_rcu_[bh|sched]()
on idle_task
On Tue, Dec 09, 2014 at 05:53:34PM +0800, Lai Jiangshan wrote:
> For RCU in UP, context-switch = QS = GP, thus we can force a
> context-switch when any call_rcu_[bh|sched]() is happened on idle_task.
> After doing so, rcu_idle/irq_enter/exit() are useless, so we can simply
> make these functions empty.
>
> More important, this change does not change the functionality logically.
> Note: raise_softirq(RCU_SOFTIRQ)/rcu_sched_qs() in rcu_idle_enter() and
> outmost rcu_irq_exit() will have to wake up the ksoftirqd
> (due to in_interrupt() == 0).
>
> Before this patch After this patch:
> call_rcu_sched() in idle; call_rcu_sched() in idle
> set resched
> do other stuffs; do other stuffs
> outmost rcu_irq_exit() outmost rcu_irq_exit() (empty function)
> (or rcu_idle_enter()) (or rcu_idle_enter(), also empty function)
> start to resched. (see above)
> rcu_sched_qs() rcu_sched_qs()
> QS,and GP and advance cb QS,and GP and advance cb
> wake up the ksoftirqd wake up the ksoftirqd
> set resched
> resched to ksoftirqd (or other) resched to ksoftirqd (or other)
>
> These two code patches are almost the same.
>
> Size changed after patched:
>
> size kernel/rcu/tiny-old.o kernel/rcu/tiny-patched.o
> text data bss dec hex filename
> 3449 206 8 3663 e4f kernel/rcu/tiny-old.o
> 2406 144 8 2558 9fe kernel/rcu/tiny-patched.o
>
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
Queued for testing, thank you!
Thanx, Paul
> ---
> kernel/rcu/rcu.h | 6 +++
> kernel/rcu/tiny.c | 99 +++------------------------------------------
> kernel/rcu/tiny_plugin.h | 2 +-
> kernel/rcu/tree.c | 6 ---
> 4 files changed, 14 insertions(+), 99 deletions(-)
>
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 07bb02e..80adef7 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -137,4 +137,10 @@ int rcu_jiffies_till_stall_check(void);
>
> void rcu_early_boot_tests(void);
>
> +/*
> + * This function really isn't for public consumption, but RCU is special in
> + * that context switches can allow the state machine to make progress.
> + */
> +extern void resched_cpu(int cpu);
> +
> #endif /* __LINUX_RCU_H */
> diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> index 805b6d5..85287ec 100644
> --- a/kernel/rcu/tiny.c
> +++ b/kernel/rcu/tiny.c
> @@ -47,54 +47,14 @@ static void __call_rcu(struct rcu_head *head,
> void (*func)(struct rcu_head *rcu),
> struct rcu_ctrlblk *rcp);
>
> -static long long rcu_dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
> -
> #include "tiny_plugin.h"
>
> -/* Common code for rcu_idle_enter() and rcu_irq_exit(), see kernel/rcu/tree.c. */
> -static void rcu_idle_enter_common(long long newval)
> -{
> - if (newval) {
> - RCU_TRACE(trace_rcu_dyntick(TPS("--="),
> - rcu_dynticks_nesting, newval));
> - rcu_dynticks_nesting = newval;
> - return;
> - }
> - RCU_TRACE(trace_rcu_dyntick(TPS("Start"),
> - rcu_dynticks_nesting, newval));
> - if (IS_ENABLED(CONFIG_RCU_TRACE) && !is_idle_task(current)) {
> - struct task_struct *idle __maybe_unused = idle_task(smp_processor_id());
> -
> - RCU_TRACE(trace_rcu_dyntick(TPS("Entry error: not idle task"),
> - rcu_dynticks_nesting, newval));
> - ftrace_dump(DUMP_ALL);
> - WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
> - current->pid, current->comm,
> - idle->pid, idle->comm); /* must be idle task! */
> - }
> - rcu_sched_qs(); /* implies rcu_bh_inc() */
> - barrier();
> - rcu_dynticks_nesting = newval;
> -}
> -
> /*
> * Enter idle, which is an extended quiescent state if we have fully
> - * entered that mode (i.e., if the new value of dynticks_nesting is zero).
> + * entered that mode.
> */
> void rcu_idle_enter(void)
> {
> - unsigned long flags;
> - long long newval;
> -
> - local_irq_save(flags);
> - WARN_ON_ONCE((rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK) == 0);
> - if ((rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK) ==
> - DYNTICK_TASK_NEST_VALUE)
> - newval = 0;
> - else
> - newval = rcu_dynticks_nesting - DYNTICK_TASK_NEST_VALUE;
> - rcu_idle_enter_common(newval);
> - local_irq_restore(flags);
> }
> EXPORT_SYMBOL_GPL(rcu_idle_enter);
>
> @@ -103,55 +63,14 @@ EXPORT_SYMBOL_GPL(rcu_idle_enter);
> */
> void rcu_irq_exit(void)
> {
> - unsigned long flags;
> - long long newval;
> -
> - local_irq_save(flags);
> - newval = rcu_dynticks_nesting - 1;
> - WARN_ON_ONCE(newval < 0);
> - rcu_idle_enter_common(newval);
> - local_irq_restore(flags);
> }
> EXPORT_SYMBOL_GPL(rcu_irq_exit);
>
> -/* Common code for rcu_idle_exit() and rcu_irq_enter(), see kernel/rcu/tree.c. */
> -static void rcu_idle_exit_common(long long oldval)
> -{
> - if (oldval) {
> - RCU_TRACE(trace_rcu_dyntick(TPS("++="),
> - oldval, rcu_dynticks_nesting));
> - return;
> - }
> - RCU_TRACE(trace_rcu_dyntick(TPS("End"), oldval, rcu_dynticks_nesting));
> - if (IS_ENABLED(CONFIG_RCU_TRACE) && !is_idle_task(current)) {
> - struct task_struct *idle __maybe_unused = idle_task(smp_processor_id());
> -
> - RCU_TRACE(trace_rcu_dyntick(TPS("Exit error: not idle task"),
> - oldval, rcu_dynticks_nesting));
> - ftrace_dump(DUMP_ALL);
> - WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
> - current->pid, current->comm,
> - idle->pid, idle->comm); /* must be idle task! */
> - }
> -}
> -
> /*
> * Exit idle, so that we are no longer in an extended quiescent state.
> */
> void rcu_idle_exit(void)
> {
> - unsigned long flags;
> - long long oldval;
> -
> - local_irq_save(flags);
> - oldval = rcu_dynticks_nesting;
> - WARN_ON_ONCE(rcu_dynticks_nesting < 0);
> - if (rcu_dynticks_nesting & DYNTICK_TASK_NEST_MASK)
> - rcu_dynticks_nesting += DYNTICK_TASK_NEST_VALUE;
> - else
> - rcu_dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
> - rcu_idle_exit_common(oldval);
> - local_irq_restore(flags);
> }
> EXPORT_SYMBOL_GPL(rcu_idle_exit);
>
> @@ -160,15 +79,6 @@ EXPORT_SYMBOL_GPL(rcu_idle_exit);
> */
> void rcu_irq_enter(void)
> {
> - unsigned long flags;
> - long long oldval;
> -
> - local_irq_save(flags);
> - oldval = rcu_dynticks_nesting;
> - rcu_dynticks_nesting++;
> - WARN_ON_ONCE(rcu_dynticks_nesting == 0);
> - rcu_idle_exit_common(oldval);
> - local_irq_restore(flags);
> }
> EXPORT_SYMBOL_GPL(rcu_irq_enter);
>
> @@ -179,7 +89,7 @@ EXPORT_SYMBOL_GPL(rcu_irq_enter);
> */
> bool notrace __rcu_is_watching(void)
> {
> - return rcu_dynticks_nesting;
> + return true;
> }
> EXPORT_SYMBOL(__rcu_is_watching);
>
> @@ -347,6 +257,11 @@ static void __call_rcu(struct rcu_head *head,
> rcp->curtail = &head->next;
> RCU_TRACE(rcp->qlen++);
> local_irq_restore(flags);
> +
> + if (unlikely(is_idle_task(current))) {
> + /* force scheduling for rcu_sched_qs() */
> + resched_cpu(0);
> + }
> }
>
> /*
> diff --git a/kernel/rcu/tiny_plugin.h b/kernel/rcu/tiny_plugin.h
> index 858c565..b5fc5e5 100644
> --- a/kernel/rcu/tiny_plugin.h
> +++ b/kernel/rcu/tiny_plugin.h
> @@ -147,7 +147,7 @@ static void check_cpu_stall(struct rcu_ctrlblk *rcp)
> js = ACCESS_ONCE(rcp->jiffies_stall);
> if (*rcp->curtail && ULONG_CMP_GE(j, js)) {
> pr_err("INFO: %s stall on CPU (%lu ticks this GP) idle=%llx (t=%lu jiffies q=%ld)\n",
> - rcp->name, rcp->ticks_this_gp, rcu_dynticks_nesting,
> + rcp->name, rcp->ticks_this_gp, DYNTICK_TASK_EXIT_IDLE,
> jiffies - rcp->gp_start, rcp->qlen);
> dump_stack();
> }
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 3f5dd16..b5ef579 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -940,12 +940,6 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
> }
>
> /*
> - * This function really isn't for public consumption, but RCU is special in
> - * that context switches can allow the state machine to make progress.
> - */
> -extern void resched_cpu(int cpu);
> -
> -/*
> * Return true if the specified CPU has passed through a quiescent
> * state by virtue of being in or having passed through an dynticks
> * idle state since the last call to dyntick_save_progress_counter()
> --
> 1.7.4.4
>
--
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