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[PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag

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  [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag Morten Rasmussen <morten.rasmussen@arm.com> - 2016-05-23 13:10 +0200
    Re: [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag Yuyang Du <yuyang.du@intel.com> - 2016-05-25 10:00 +0200
      Re: [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag Morten Rasmussen <morten.rasmussen@arm.com> - 2016-05-25 11:30 +0200

#1405257 — [PATCH 07/16] sched: Make SD_BALANCE_WAKE a topology flag

For systems with the SD_ASYM_CPUCAPACITY flag set on higher level in the
sched_domain hierarchy we need a way to enable wake-up balancing for the
lower levels as well as we may want to balance tasks that don't fit the
capacity of the previous cpu.

We have the option of introducing a new topology flag to express this
requirement, or let the existing SD_BALANCE_WAKE flag be set by the
architecture as a topology flag. The former means introducing yet
another flag, the latter breaks the current meaning of topology flags.
None of the options are really desirable.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
---
 kernel/sched/core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 558ec4a..8014b4a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5658,6 +5658,7 @@ static int sd_degenerate(struct sched_domain *sd)
 			 SD_BALANCE_NEWIDLE |
 			 SD_BALANCE_FORK |
 			 SD_BALANCE_EXEC |
+			 SD_BALANCE_WAKE |
 			 SD_SHARE_CPUCAPACITY |
 			 SD_ASYM_CPUCAPACITY |
 			 SD_SHARE_PKG_RESOURCES |
@@ -5690,6 +5691,7 @@ sd_parent_degenerate(struct sched_domain *sd, struct sched_domain *parent)
 				SD_BALANCE_NEWIDLE |
 				SD_BALANCE_FORK |
 				SD_BALANCE_EXEC |
+				SD_BALANCE_WAKE |
 				SD_ASYM_CPUCAPACITY |
 				SD_SHARE_CPUCAPACITY |
 				SD_SHARE_PKG_RESOURCES |
@@ -6308,6 +6310,7 @@ static int sched_domains_curr_level;
  * Odd ones out:
  * SD_ASYM_PACKING        - describes SMT quirks
  * SD_ASYM_CPUCAPACITY    - describes mixed capacity topologies
+ * SD_BALANCE_WAKE	  - controls wake-up balancing (expensive)
  */
 #define TOPOLOGY_SD_FLAGS		\
 	(SD_SHARE_CPUCAPACITY |		\
@@ -6315,6 +6318,7 @@ static int sched_domains_curr_level;
 	 SD_NUMA |			\
 	 SD_ASYM_PACKING |		\
 	 SD_ASYM_CPUCAPACITY |		\
+	 SD_BALANCE_WAKE |		\
 	 SD_SHARE_POWERDOMAIN)
 
 static struct sched_domain *
-- 
1.9.1

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#1406710

On Mon, May 23, 2016 at 11:58:49AM +0100, Morten Rasmussen wrote:
> For systems with the SD_ASYM_CPUCAPACITY flag set on higher level in the
> sched_domain hierarchy we need a way to enable wake-up balancing for the
> lower levels as well as we may want to balance tasks that don't fit the
> capacity of the previous cpu.
> 
> We have the option of introducing a new topology flag to express this
> requirement, or let the existing SD_BALANCE_WAKE flag be set by the
> architecture as a topology flag. The former means introducing yet
> another flag, the latter breaks the current meaning of topology flags.
> None of the options are really desirable.
 
I'd propose to replace SD_WAKE_AFFINE with SD_BALANCE_WAKE. And the
SD_WAKE_AFFINE semantic is simply "waker allowed":

waker_allowed = cpumask_test_cpu(cpu, tsk_cpus_allowed(p));

This can be implemented without current functionality change.

From there, the choice between waker and wakee, and fast path
select_idle_sibling() and the rest slow path should be reworked, which
I am thinking about.

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#1406771

On Wed, May 25, 2016 at 07:52:49AM +0800, Yuyang Du wrote:
> On Mon, May 23, 2016 at 11:58:49AM +0100, Morten Rasmussen wrote:
> > For systems with the SD_ASYM_CPUCAPACITY flag set on higher level in the
> > sched_domain hierarchy we need a way to enable wake-up balancing for the
> > lower levels as well as we may want to balance tasks that don't fit the
> > capacity of the previous cpu.
> > 
> > We have the option of introducing a new topology flag to express this
> > requirement, or let the existing SD_BALANCE_WAKE flag be set by the
> > architecture as a topology flag. The former means introducing yet
> > another flag, the latter breaks the current meaning of topology flags.
> > None of the options are really desirable.
>  
> I'd propose to replace SD_WAKE_AFFINE with SD_BALANCE_WAKE. And the
> SD_WAKE_AFFINE semantic is simply "waker allowed":
> 
> waker_allowed = cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> 
> This can be implemented without current functionality change.
> 
> From there, the choice between waker and wakee, and fast path
> select_idle_sibling() and the rest slow path should be reworked, which
> I am thinking about.

I don't really understand how that would work. If you change the
semantics of the flags you don't preserve current behaviour. To me it
sounds like at total rewrite of everything.

SD_BALANCE_WAKE controls whether we go slow path or not in case
want_affine is false. SD_WAKE_AFFINE controls whether we should consider
waking up near the waker instead of always waking up near the previous
cpu.

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