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Message-ID: <20140708152358.GF6571@localhost.localdomain>
Date: Tue, 8 Jul 2014 17:24:00 +0200
From: Frederic Weisbecker <fweisbec@...il.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org,
laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
edumazet@...gle.com, dvhart@...ux.intel.com, oleg@...hat.com,
sbw@....edu
Subject: Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to
non-NO_HZ_FULL CPUs
On Mon, Jul 07, 2014 at 03:38:15PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>
> Binding the grace-period kthreads to the timekeeping CPU resulted in
> significant performance decreases for some workloads. For more detail,
> see:
>
> https://lkml.org/lkml/2014/6/3/395 for benchmark numbers
>
> https://lkml.org/lkml/2014/6/4/218 for CPU statistics
>
> It turns out that it is necessary to bind the grace-period kthreads
> to the timekeeping CPU only when all but CPU 0 is a nohz_full CPU
> on the one hand or if CONFIG_NO_HZ_FULL_SYSIDLE=y on the other.
> In other cases, it suffices to bind the grace-period kthreads to the
> set of non-nohz_full CPUs.
>
> This commit therefore creates a tick_nohz_not_full_mask that is the
> complement of tick_nohz_full_mask, and then binds the grace-period
> kthread to the set of CPUs indicated by this new mask, which covers
> the CONFIG_NO_HZ_FULL_SYSIDLE=n case. The CONFIG_NO_HZ_FULL_SYSIDLE=y
> case still binds the grace-period kthreads to the timekeeping CPU.
> This commit also includes the tick_nohz_full_enabled() check suggested
> by Frederic Weisbecker.
>
> Reported-by: Jet Chen <jet.chen@...el.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> [ paulmck: Created housekeeping_affine() per fweisbec feedback. ]
> ---
> include/linux/tick.h | 19 +++++++++++++++++++
> kernel/rcu/tree_plugin.h | 14 +++++++++-----
> kernel/time/tick-sched.c | 10 ++++++++++
> 3 files changed, 38 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index b84773cb9f4c..c39af3261351 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -12,6 +12,7 @@
> #include <linux/hrtimer.h>
> #include <linux/context_tracking_state.h>
> #include <linux/cpumask.h>
> +#include <linux/sched.h>
>
> #ifdef CONFIG_GENERIC_CLOCKEVENTS
>
> @@ -162,6 +163,7 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
> #ifdef CONFIG_NO_HZ_FULL
> extern bool tick_nohz_full_running;
> extern cpumask_var_t tick_nohz_full_mask;
> +extern cpumask_var_t tick_nohz_not_full_mask;
So I'm still puzzled by this mask.
How about creating a temporary cpumask on top of tick_nohz_full_mask
from housekeeping_affine().
If you wonder about performance, this can be called once for good from
rcu_spawn_gp_kthread() (that would be much better than checking that all
the time from the kthread itself anyway).
In case you're out of time, I can handle that. Just checking your opinion.
Thanks.
>
> static inline bool tick_nohz_full_enabled(void)
> {
> @@ -194,6 +196,23 @@ static inline void tick_nohz_full_kick_all(void) { }
> static inline void __tick_nohz_task_switch(struct task_struct *tsk) { }
> #endif
>
> +static inline bool is_housekeeping_cpu(int cpu)
> +{
> +#ifdef CONFIG_NO_HZ_FULL
> + if (tick_nohz_full_enabled())
> + return cpumask_test_cpu(cpu, tick_nohz_not_full_mask);
> +#endif
> + return true;
> +}
> +
> +static inline void housekeeping_affine(struct task_struct *t)
> +{
> +#ifdef CONFIG_NO_HZ_FULL
> + if (tick_nohz_full_enabled())
> + set_cpus_allowed_ptr(t, tick_nohz_not_full_mask);
> +#endif
> +}
> +
> static inline void tick_nohz_full_check(void)
> {
> if (tick_nohz_full_enabled())
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 719587af7b10..b39ba7239bd6 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2846,12 +2846,16 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
> */
> static void rcu_bind_gp_kthread(void)
> {
> -#ifdef CONFIG_NO_HZ_FULL
> - int cpu = tick_do_timer_cpu;
> + int __maybe_unused cpu;
>
> - if (cpu < 0 || cpu >= nr_cpu_ids)
> + if (!tick_nohz_full_enabled())
> return;
> - if (raw_smp_processor_id() != cpu)
> +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> + cpu = tick_do_timer_cpu;
> + if (cpu >= 0 && cpu < nr_cpu_ids && raw_smp_processor_id() != cpu)
> set_cpus_allowed_ptr(current, cpumask_of(cpu));
> -#endif /* #ifdef CONFIG_NO_HZ_FULL */
> +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> + if (!is_housekeeping_cpu(raw_smp_processor_id()))
> + housekeeping_affine(current);
> +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> }
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 6558b7ac112d..e023134d63a1 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -154,6 +154,7 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
>
> #ifdef CONFIG_NO_HZ_FULL
> cpumask_var_t tick_nohz_full_mask;
> +cpumask_var_t tick_nohz_not_full_mask;
> bool tick_nohz_full_running;
>
> static bool can_stop_full_tick(void)
> @@ -281,6 +282,7 @@ static int __init tick_nohz_full_setup(char *str)
> int cpu;
>
> alloc_bootmem_cpumask_var(&tick_nohz_full_mask);
> + alloc_bootmem_cpumask_var(&tick_nohz_not_full_mask);
> if (cpulist_parse(str, tick_nohz_full_mask) < 0) {
> pr_warning("NOHZ: Incorrect nohz_full cpumask\n");
> return 1;
> @@ -291,6 +293,8 @@ static int __init tick_nohz_full_setup(char *str)
> pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
> cpumask_clear_cpu(cpu, tick_nohz_full_mask);
> }
> + cpumask_andnot(tick_nohz_not_full_mask,
> + cpu_possible_mask, tick_nohz_full_mask);
> tick_nohz_full_running = true;
>
> return 1;
> @@ -332,9 +336,15 @@ static int tick_nohz_init_all(void)
> pr_err("NO_HZ: Can't allocate full dynticks cpumask\n");
> return err;
> }
> + if (!alloc_cpumask_var(&tick_nohz_not_full_mask, GFP_KERNEL)) {
> + pr_err("NO_HZ: Can't allocate not-full dynticks cpumask\n");
> + return err;
> + }
> err = 0;
> cpumask_setall(tick_nohz_full_mask);
> cpumask_clear_cpu(smp_processor_id(), tick_nohz_full_mask);
> + cpumask_clear(tick_nohz_not_full_mask);
> + cpumask_set_cpu(smp_processor_id(), tick_nohz_not_full_mask);
> tick_nohz_full_running = true;
> #endif
> return err;
> --
> 1.8.1.5
>
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