From: Wanpeng Li <[email protected]>
On a machine, cpu 0 is used for housekeeping, other 39 cpus are in
nohz_full mode. We can observe huge time burn in the loop for seaching
nearest busy housekeeper cpu by ftrace.
2) | get_nohz_timer_target() {
2) 0.240 us | housekeeping_test_cpu();
2) 0.458 us | housekeeping_test_cpu();
...
2) 0.292 us | housekeeping_test_cpu();
2) 0.240 us | housekeeping_test_cpu();
2) 0.227 us | housekeeping_any_cpu();
2) + 43.460 us | }
This patch optimizes the searching logic by finding a nearest housekeeper
cpu in the housekeeping cpumask, it can minimize the worst searching time
from ~44us to < 10us in my testing.
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Signed-off-by: Wanpeng Li <[email protected]>
---
kernel/sched/core.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 83bd6bb..db550cf 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -548,11 +548,12 @@ int get_nohz_timer_target(void)
rcu_read_lock();
for_each_domain(cpu, sd) {
- for_each_cpu(i, sched_domain_span(sd)) {
+ for_each_cpu_and(i, sched_domain_span(sd),
+ housekeeping_cpumask(HK_FLAG_TIMER)) {
if (cpu == i)
continue;
- if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
+ if (!idle_cpu(i)) {
cpu = i;
goto unlock;
}
--
2.7.4
Cc Frederic,
On Tue, 18 Jun 2019 at 11:13, Wanpeng Li <[email protected]> wrote:
>
> From: Wanpeng Li <[email protected]>
>
> On a machine, cpu 0 is used for housekeeping, other 39 cpus are in
> nohz_full mode. We can observe huge time burn in the loop for seaching
> nearest busy housekeeper cpu by ftrace.
>
> 2) | get_nohz_timer_target() {
> 2) 0.240 us | housekeeping_test_cpu();
> 2) 0.458 us | housekeeping_test_cpu();
>
> ...
>
> 2) 0.292 us | housekeeping_test_cpu();
> 2) 0.240 us | housekeeping_test_cpu();
> 2) 0.227 us | housekeeping_any_cpu();
> 2) + 43.460 us | }
>
> This patch optimizes the searching logic by finding a nearest housekeeper
> cpu in the housekeeping cpumask, it can minimize the worst searching time
> from ~44us to < 10us in my testing.
>
> Cc: Ingo Molnar <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> kernel/sched/core.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 83bd6bb..db550cf 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -548,11 +548,12 @@ int get_nohz_timer_target(void)
>
> rcu_read_lock();
> for_each_domain(cpu, sd) {
> - for_each_cpu(i, sched_domain_span(sd)) {
> + for_each_cpu_and(i, sched_domain_span(sd),
> + housekeeping_cpumask(HK_FLAG_TIMER)) {
> if (cpu == i)
> continue;
>
> - if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
> + if (!idle_cpu(i)) {
> cpu = i;
> goto unlock;
> }
> --
> 2.7.4
>
On Wed, Jun 19, 2019 at 08:42:19AM +0800, Wanpeng Li wrote:
> Cc Frederic,
> On Tue, 18 Jun 2019 at 11:13, Wanpeng Li <[email protected]> wrote:
> >
> > From: Wanpeng Li <[email protected]>
> >
> > On a machine, cpu 0 is used for housekeeping, other 39 cpus are in
> > nohz_full mode. We can observe huge time burn in the loop for seaching
> > nearest busy housekeeper cpu by ftrace.
> >
> > 2) | get_nohz_timer_target() {
> > 2) 0.240 us | housekeeping_test_cpu();
> > 2) 0.458 us | housekeeping_test_cpu();
> >
> > ...
> >
> > 2) 0.292 us | housekeeping_test_cpu();
> > 2) 0.240 us | housekeeping_test_cpu();
> > 2) 0.227 us | housekeeping_any_cpu();
> > 2) + 43.460 us | }
> >
> > This patch optimizes the searching logic by finding a nearest housekeeper
> > cpu in the housekeeping cpumask, it can minimize the worst searching time
> > from ~44us to < 10us in my testing.
> >
> > Cc: Ingo Molnar <[email protected]>
> > Cc: Peter Zijlstra <[email protected]>
> > Signed-off-by: Wanpeng Li <[email protected]>
> > ---
> > kernel/sched/core.c | 5 +++--
> > 1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 83bd6bb..db550cf 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -548,11 +548,12 @@ int get_nohz_timer_target(void)
> >
> > rcu_read_lock();
> > for_each_domain(cpu, sd) {
> > - for_each_cpu(i, sched_domain_span(sd)) {
> > + for_each_cpu_and(i, sched_domain_span(sd),
> > + housekeeping_cpumask(HK_FLAG_TIMER)) {
> > if (cpu == i)
> > continue;
> >
> > - if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
> > + if (!idle_cpu(i)) {
> > cpu = i;
> > goto unlock;
> > }
Nice, but you also need to handle the default case that doesn't make much sense anymore.
It hasn't ever been clear anyway. The last iterated buzy housekeeper can become
a random candidate while current CPU is a better fallback if it is a housekeeper. Also
you're enhancing housekeeping_any_cpu() in another patch so give it a better chance:
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4778c48a7fda..c5229d71540a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -539,27 +539,32 @@ void resched_cpu(int cpu)
*/
int get_nohz_timer_target(void)
{
- int i, cpu = smp_processor_id();
+ int i, cpu = smp_processor_id(), default_cpu = -1;
struct sched_domain *sd;
- if (!idle_cpu(cpu) && housekeeping_cpu(cpu, HK_FLAG_TIMER))
- return cpu;
+ if (housekeeping_cpu(cpu, HK_FLAG_TIMER)) {
+ if (!idle_cpu(cpu))
+ return cpu;
+ default_cpu = cpu;
+ }
rcu_read_lock();
for_each_domain(cpu, sd) {
- for_each_cpu(i, sched_domain_span(sd)) {
+ for_each_cpu_and(i, sched_domain_span(sd),
+ housekeeping_cpumask(HK_FLAG_TIMER)) {
if (cpu == i)
continue;
- if (!idle_cpu(i) && housekeeping_cpu(i, HK_FLAG_TIMER)) {
+ if (!idle_cpu(i)) {
cpu = i;
goto unlock;
}
}
}
- if (!housekeeping_cpu(cpu, HK_FLAG_TIMER))
- cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
+ if (default_cpu == -1)
+ default_cpu = housekeeping_any_cpu(HK_FLAG_TIMER);
+ cpu = default_cpu;
unlock:
rcu_read_unlock();
return cpu;