Re: [PATCH v3 4/7] sched/fair: Introduce a CPU capacity comparison helper
Hi Valentin,
The reduced margin is helping our platforms, Please feel free to add my
tested-by tag:
Tested-by: Lingutla Chandrasekhar
On 3/11/2021 5:35 PM, Valentin Schneider wrote:
During load-balance, groups classified as group_misfit_task are filtered
out if they do not pass
group_smaller_max_cpu_capacity(, );
which itself employs fits_capacity() to compare the sgc->max_capacity of
both groups.
Due to the underlying margin, fits_capacity(X, 1024) will return false for
any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
{261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
CPUs, misfit migration will never intentionally upmigrate it to a CPU of
higher capacity due to the aforementioned margin.
One may argue the 20% margin of fits_capacity() is excessive in the advent
of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
is that fits_capacity() is meant to compare a utilization value to a
capacity value, whereas here it is being used to compare two capacity
values. As CPU capacity and task utilization have different dynamics, a
sensible approach here would be to add a new helper dedicated to comparing
CPU capacities.
Reviewed-by: Qais Yousef
Signed-off-by: Valentin Schneider
---
kernel/sched/fair.c | 7 +++
1 file changed, 7 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index db892f6e222f..ddb2ab3edf6d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
*/
#define fits_capacity(cap, max) ((cap) * 1280 < (max) * 1024)
+/*
+ * The margin used when comparing CPU capacities.
+ * is 'cap1' noticeably greater than 'cap2'
+ *
+ * (default: ~5%)
+ */
+#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
#endif
#ifdef CONFIG_CFS_BANDWIDTH
Re: [PATCH v3 4/7] sched/fair: Introduce a CPU capacity comparison helper
On 15/03/21 15:24, Vincent Guittot wrote: >> @@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu) >> */ >> #define fits_capacity(cap, max)((cap) * 1280 < (max) * 1024) >> >> +/* >> + * The margin used when comparing CPU capacities. >> + * is 'cap1' noticeably greater than 'cap2' >> + * >> + * (default: ~5%) >> + */ >> +#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078) > > defined but not used. > > Should be merged with next patch which start to use it > Will do.
Re: [PATCH v3 4/7] sched/fair: Introduce a CPU capacity comparison helper
On Thu, 11 Mar 2021 at 13:05, Valentin Schneider
wrote:
>
> During load-balance, groups classified as group_misfit_task are filtered
> out if they do not pass
>
> group_smaller_max_cpu_capacity(, );
>
> which itself employs fits_capacity() to compare the sgc->max_capacity of
> both groups.
>
> Due to the underlying margin, fits_capacity(X, 1024) will return false for
> any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
> {261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
> CPUs, misfit migration will never intentionally upmigrate it to a CPU of
> higher capacity due to the aforementioned margin.
>
> One may argue the 20% margin of fits_capacity() is excessive in the advent
> of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
> is that fits_capacity() is meant to compare a utilization value to a
> capacity value, whereas here it is being used to compare two capacity
> values. As CPU capacity and task utilization have different dynamics, a
> sensible approach here would be to add a new helper dedicated to comparing
> CPU capacities.
>
> Reviewed-by: Qais Yousef
> Signed-off-by: Valentin Schneider
> ---
> kernel/sched/fair.c | 7 +++
> 1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index db892f6e222f..ddb2ab3edf6d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
> */
> #define fits_capacity(cap, max)((cap) * 1280 < (max) * 1024)
>
> +/*
> + * The margin used when comparing CPU capacities.
> + * is 'cap1' noticeably greater than 'cap2'
> + *
> + * (default: ~5%)
> + */
> +#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
defined but not used.
Should be merged with next patch which start to use it
> #endif
>
> #ifdef CONFIG_CFS_BANDWIDTH
> --
> 2.25.1
>
[PATCH v3 4/7] sched/fair: Introduce a CPU capacity comparison helper
During load-balance, groups classified as group_misfit_task are filtered
out if they do not pass
group_smaller_max_cpu_capacity(, );
which itself employs fits_capacity() to compare the sgc->max_capacity of
both groups.
Due to the underlying margin, fits_capacity(X, 1024) will return false for
any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
{261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
CPUs, misfit migration will never intentionally upmigrate it to a CPU of
higher capacity due to the aforementioned margin.
One may argue the 20% margin of fits_capacity() is excessive in the advent
of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
is that fits_capacity() is meant to compare a utilization value to a
capacity value, whereas here it is being used to compare two capacity
values. As CPU capacity and task utilization have different dynamics, a
sensible approach here would be to add a new helper dedicated to comparing
CPU capacities.
Reviewed-by: Qais Yousef
Signed-off-by: Valentin Schneider
---
kernel/sched/fair.c | 7 +++
1 file changed, 7 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index db892f6e222f..ddb2ab3edf6d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
*/
#define fits_capacity(cap, max)((cap) * 1280 < (max) * 1024)
+/*
+ * The margin used when comparing CPU capacities.
+ * is 'cap1' noticeably greater than 'cap2'
+ *
+ * (default: ~5%)
+ */
+#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
#endif
#ifdef CONFIG_CFS_BANDWIDTH
--
2.25.1
