Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used

Paul E. McKenney
2014-06-13T01:24:38Z

On Fri, Jun 13, 2014 at 02:16:59AM +0200, Frederic Weisbecker wrote:
> CONFIG_NO_HZ_FULL may be enabled widely on distros nowadays but actual
> users should be a tiny minority, if actually any.
>
> Also there is a risk that affining the GP kthread to a single CPU could
> end up noticeably reducing RCU performances and increasing energy
> consumption.
>
> So lets affine the GP kthread only when nohz full is actually used
> (ie: when the nohz_full= parameter is filled or CONFIG_NO_HZ_FULL_ALL=y)
>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> ---
> kernel/rcu/tree_plugin.h | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index cbc2c45..726f52c 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -2843,12 +2843,16 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
> */
> static void rcu_bind_gp_kthread(void)
> {
> -#ifdef CONFIG_NO_HZ_FULL
> - int cpu = ACCESS_ONCE(tick_do_timer_cpu);
> + int cpu;
> +
> + if (!tick_nohz_full_enabled())
> + return;
> +
> + cpu = ACCESS_ONCE(tick_do_timer_cpu);
>
> if (cpu < 0 || cpu >= nr_cpu_ids)
> return;
> +
> if (raw_smp_processor_id() != cpu)
> set_cpus_allowed_ptr(current, cpumask_of(cpu));
> -#endif /* #ifdef CONFIG_NO_HZ_FULL */
> }

Hello, Frederic,

I have the following queued. Shall I port yours on top of mine, or is
there an issue with mine?

Thanx, Paul

------------------------------------------------------------------------

rcu: Bind grace-period kthreads to non-NO_HZ_FULL CPUs

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.

Reported-by: Jet Chen <jet.chen@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/tick.h b/include/linux/tick.h
index b84773cb9f4c..1fe0c05eee39 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -162,6 +162,7 @@ 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;

static inline bool tick_nohz_full_enabled(void)
{
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 7ce734040a5e..ec7627becaf0 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2863,7 +2863,12 @@ static void rcu_bind_gp_kthread(void)

if (cpu < 0 || cpu >= nr_cpu_ids)
return;
+#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
if (raw_smp_processor_id() != cpu)
set_cpus_allowed_ptr(current, cpumask_of(cpu));
+#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
+ if (!cpumask_test_cpu(raw_smp_processor_id(), tick_nohz_not_full_mask))
+ set_cpus_allowed_ptr(current, tick_nohz_not_full_mask);
+#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
#endif /* #ifdef CONFIG_NO_HZ_FULL */
}
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 9f8af69c67ec..02209e957e76 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -151,6 +151,7 @@ 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)
@@ -278,6 +279,7 @@ 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;
@@ -288,6 +290,8 @@ 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;
@@ -332,6 +336,8 @@ static int tick_nohz_init_all(void)
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;