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Message-ID: <20101221192849.GP2143@linux.vnet.ibm.com>
Date: Tue, 21 Dec 2010 11:28:49 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...e.hu>,
Steven Rostedt <rostedt@...dmis.org>,
Lai Jiangshan <laijs@...fujitsu.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Anton Blanchard <anton@....ibm.com>,
Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 10/15] nohz_task: Enter in extended quiescent state
when in userspace
On Mon, Dec 20, 2010 at 04:24:17PM +0100, Frederic Weisbecker wrote:
> A nohz task can safely enter into extended quiescent state when
> it goes into userspace, this avoids a remote cpu to force the
> nohz task to be interrupted in order to notify quiescent states.
>
> We enter into an extended quiescent state when:
>
> - A nohz task resumes to userspace and is alone running on the
> CPU (we check if the local cpu is in nohz mode, which means
> no other task compete on that CPU). If the tick is still running
> then entering into extended QS will be done later from the second
> case:
>
> - When the tick stops and verify the current task is a nohz one,
> is alone running on the CPU and runs in userspace.
>
> We exit the extended quiescent state when:
>
> - A nohz task enters the kernel and is alone running on the CPU.
> Again we check if the local cpu is in nohz mode for that. If
> the tick is still running then it means we are not in an extended
> QS and we don't do anything.
>
> - The tick restarts because a new task is enqueued.
>
> Whether the nohz task is in userspace or not is tracked by the
> per cpu nohz_task_ext_qs variable.
>
> Architectures need to provide some backend to notify userspace
> exit/entry in order to support this mode.
> It needs to implement the TIF_NOHZ flag that switches to slow
> path syscall mode and to notify exceptions entry/exit.
>
> We don't need to handle irqs or nmis as those are already handled
> by RCU through rcu_enter_irq/nmi helpers.
One question below...
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Lai Jiangshan <laijs@...fujitsu.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Anton Blanchard <anton@....ibm.com>
> Cc: Tim Pepper <lnxninja@...ux.vnet.ibm.com>
> ---
> arch/Kconfig | 4 +++
> include/linux/tick.h | 16 ++++++++++-
> kernel/sched.c | 3 ++
> kernel/time/tick-sched.c | 61 +++++++++++++++++++++++++++++++++++++++++++++-
> 4 files changed, 81 insertions(+), 3 deletions(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index e631791..d1ebea3 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -177,5 +177,9 @@ config HAVE_ARCH_JUMP_LABEL
>
> config HAVE_NO_HZ_TASK
> bool
> + help
> + Features necessary hooks for a task wanting to enter nohz
> + while running alone on a CPU: thread flag for syscall hooks
> + and exceptions entry/exit hooks.
>
> source "kernel/gcov/Kconfig"
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 7465a47..a704bb7 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -8,6 +8,7 @@
>
> #include <linux/clockchips.h>
> #include <linux/percpu-defs.h>
> +#include <asm/ptrace.h>
>
> #ifdef CONFIG_GENERIC_CLOCKEVENTS
>
> @@ -130,10 +131,21 @@ extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
>
> #ifdef CONFIG_NO_HZ_TASK
> DECLARE_PER_CPU(int, task_nohz_mode);
> +DECLARE_PER_CPU(int, nohz_task_ext_qs);
> +
> +extern void tick_nohz_task_enter_kernel(void);
> +extern void tick_nohz_task_exit_kernel(void);
> +extern void tick_nohz_task_enter_exception(struct pt_regs *regs);
> +extern void tick_nohz_task_exit_exception(struct pt_regs *regs);
> extern int tick_nohz_task_mode(void);
> -#else
> +
> +#else /* !NO_HZ_TASK */
> +static inline void tick_nohz_task_enter_kernel(void) { }
> +static inline void tick_nohz_task_exit_kernel(void) { }
> +static inline void tick_nohz_task_enter_exception(struct pt_regs *regs) { }
> +static inline void tick_nohz_task_exit_exception(struct pt_regs *regs) { }
> static inline int tick_nohz_task_mode(void) { return 0; }
> -#endif
> +#endif /* !NO_HZ_TASK */
>
> # else /* !NO_HZ */
> static inline void tick_nohz_stop_sched_tick(int inidle) { }
> diff --git a/kernel/sched.c b/kernel/sched.c
> index b99f192..4412493 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2464,6 +2464,9 @@ static void nohz_task_cpu_update(void *unused)
> if (rq->nr_running > 1 || rcu_pending(cpu) || rcu_needs_cpu(cpu)) {
If the task enters a system call in nohz mode, and then that system call
enqueues an RCU callback, this code path will pull that CPU out of nohz
mode, right?
Thanx, Paul
> __get_cpu_var(task_nohz_mode) = 0;
> tick_nohz_restart_sched_tick();
> +
> + if (__get_cpu_var(nohz_task_ext_qs))
> + rcu_exit_nohz();
> }
> }
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 88011b9..9a4aa39 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -720,6 +720,62 @@ void tick_check_idle(int cpu)
> }
>
> #ifdef CONFIG_NO_HZ_TASK
> +DEFINE_PER_CPU(int, nohz_task_ext_qs);
> +
> +void tick_nohz_task_exit_kernel(void)
> +{
> + unsigned long flags;
> +
> + if (!test_thread_flag(TIF_NOHZ))
> + return;
> +
> + local_irq_save(flags);
> +
> + __get_cpu_var(nohz_task_ext_qs) = 1;
> + /*
> + * Only enter extended QS if the tick is not running.
> + * Otherwise the tick will handle that later when it
> + * will decide to stop.
> + */
> + if (__get_cpu_var(task_nohz_mode))
> + rcu_enter_nohz();
> +
> + local_irq_restore(flags);
> +}
> +
> +void tick_nohz_task_enter_kernel(void)
> +{
> + unsigned long flags;
> +
> + if (!test_thread_flag(TIF_NOHZ))
> + return;
> +
> + local_irq_save(flags);
> +
> + __get_cpu_var(nohz_task_ext_qs) = 0;
> + /*
> + * If the tick was running, then we weren't in
> + * rcu extended period. Only exit extended QS
> + * if we were in such state.
> + */
> + if (__get_cpu_var(task_nohz_mode))
> + rcu_exit_nohz();
> +
> + local_irq_restore(flags);
> +}
> +
> +void tick_nohz_task_enter_exception(struct pt_regs *regs)
> +{
> + if (user_mode(regs))
> + tick_nohz_task_enter_kernel();
> +}
> +
> +void tick_nohz_task_exit_exception(struct pt_regs *regs)
> +{
> + if (user_mode(regs))
> + tick_nohz_task_exit_kernel();
> +}
> +
> int tick_nohz_task_mode(void)
> {
> return __get_cpu_var(task_nohz_mode);
> @@ -730,8 +786,11 @@ static void tick_nohz_task_stop_tick(void)
> if (!test_thread_flag(TIF_NOHZ) || __get_cpu_var(task_nohz_mode))
> return;
>
> - if (nohz_task_can_stop_tick())
> + if (nohz_task_can_stop_tick()) {
> __get_cpu_var(task_nohz_mode) = 1;
> + if (__get_cpu_var(nohz_task_ext_qs))
> + rcu_enter_nohz();
> + }
> }
> #else
> static void tick_nohz_task_stop_tick(void) { }
> --
> 1.7.3.2
>
--
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