[PATCH] sched/fair: Simplify continue_balancing for newidle

[PATCH] sched/fair: Simplify continue_balancing for newidle

[Shrikanth Hegde]

newidle(CPU_NEWLY_IDLE) balancing doesn't stop the load balancing if the
continue_balancing flag is reset. Other two balancing (IDLE, BUSY) do
that. newidle balance stops the load balancing if rq has a task or there
is wakeup pending. The same checks are present in should_we_balance for
newidle. Hence use the return value and simplify continue_balancing
mechanism for newidle. Update the comment surrounding it as well.

No change in functionality intended.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/fair.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f00cb66cc479..d80535df8f03 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12307,6 +12307,7 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 {
 	unsigned long next_balance = jiffies + HZ;
 	int this_cpu = this_rq->cpu;
+	int continue_balancing = 1;
 	u64 t0, t1, curr_cost = 0;
 	struct sched_domain *sd;
 	int pulled_task = 0;
@@ -12321,8 +12322,9 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 		return 0;

 	/*
-	 * We must set idle_stamp _before_ calling idle_balance(), such that we
-	 * measure the duration of idle_balance() as idle time.
+	 * We must set idle_stamp _before_ calling sched_balance_rq()
+	 * for CPU_NEWLY_IDLE, such that we measure the this duration
+	 * as idle time.
 	 */
 	this_rq->idle_stamp = rq_clock(this_rq);

@@ -12361,7 +12363,6 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)

 	rcu_read_lock();
 	for_each_domain(this_cpu, sd) {
-		int continue_balancing = 1;
 		u64 domain_cost;

 		update_next_balance(sd, &next_balance);
@@ -12387,8 +12388,7 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 		 * Stop searching for tasks to pull if there are
 		 * now runnable tasks on this rq.
 		 */
-		if (pulled_task || this_rq->nr_running > 0 ||
-		    this_rq->ttwu_pending)
+		if (pulled_task || !continue_balancing)
 			break;
 	}
 	rcu_read_unlock();
--
2.39.3

Re: [PATCH] sched/fair: Simplify continue_balancing for newidle

[Ingo Molnar]

* Shrikanth Hegde <sshegde@linux.ibm.com> wrote:

> newidle(CPU_NEWLY_IDLE) balancing doesn't stop the load balancing if the
> continue_balancing flag is reset. Other two balancing (IDLE, BUSY) do
> that. newidle balance stops the load balancing if rq has a task or there
> is wakeup pending. The same checks are present in should_we_balance for
> newidle. Hence use the return value and simplify continue_balancing
> mechanism for newidle. Update the comment surrounding it as well.

Assuming there are no side-effects to balancing behavior.

> No change in functionality intended.

Is this actually true? Any change to behavior invalidates such a sentence.

>  	/*
> +	 * We must set idle_stamp _before_ calling sched_balance_rq()
> +	 * for CPU_NEWLY_IDLE, such that we measure the this duration
> +	 * as idle time.
>  	 */

'the this' ...?

Thanks,

	Ingo

Re: [PATCH] sched/fair: Simplify continue_balancing for newidle

[Shrikanth Hegde]

On 3/26/24 1:37 PM, Ingo Molnar wrote:
> 
> * Shrikanth Hegde <sshegde@linux.ibm.com> wrote:
> 
>> newidle(CPU_NEWLY_IDLE) balancing doesn't stop the load balancing if the
>> continue_balancing flag is reset. Other two balancing (IDLE, BUSY) do
>> that. newidle balance stops the load balancing if rq has a task or there
>> is wakeup pending. The same checks are present in should_we_balance for
>> newidle. Hence use the return value and simplify continue_balancing
>> mechanism for newidle. Update the comment surrounding it as well.
> 
> Assuming there are no side-effects to balancing behavior.

I ran hackbench. More or less same results with patch. But thats very 
limited set of benchmarks. Let me do some more testing with it and send the 
results. 

> 
>> No change in functionality intended.
> 
> Is this actually true? Any change to behavior invalidates such a sentence.

From what i think, code path is same and I don't see any functionality changing. 
Correct me if i am wrong. 

Currently, sched_balance_newidle does the same check to bail out as the
should_we_balance check in case of newidle.  i.e  

should_we_balance
	if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending)
			return 0;

sched_balance_newidle
		if (pulled_task || this_rq->nr_running > 0 ||
			this_rq->ttwu_pending)
			break;
		}

> 
>>  	/*
>> +	 * We must set idle_stamp _before_ calling sched_balance_rq()
>> +	 * for CPU_NEWLY_IDLE, such that we measure the this duration
>> +	 * as idle time.
>>  	 */
> 
> 'the this' ...?

Sorry for the typo. it should be.
"such that we measure this duration as idle time"

> 
> Thanks,
> 
> 	Ingo

Re: [PATCH] sched/fair: Simplify continue_balancing for newidle

[Dietmar Eggemann]

On 26/03/2024 10:00, Shrikanth Hegde wrote:
> 
> On 3/26/24 1:37 PM, Ingo Molnar wrote:
>>
>> * Shrikanth Hegde <sshegde@linux.ibm.com> wrote:
>>

[...]

>> Is this actually true? Any change to behavior invalidates such a sentence.
> 
> From what i think, code path is same and I don't see any functionality changing. 
> Correct me if i am wrong. 
> 
> Currently, sched_balance_newidle does the same check to bail out as the
> should_we_balance check in case of newidle.  i.e  
> 
> should_we_balance
> 	if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending)
> 			return 0;
> 
> sched_balance_newidle
> 		if (pulled_task || this_rq->nr_running > 0 ||
> 			this_rq->ttwu_pending)
> 			break;
> 		}

LGTM. Commit 792b9f65a568 ("sched: Allow newidle balancing to bail out
of load_balance") (Jun 22) made sure that we leave sched_balance_rq()
(former load_balance()) for CPU_NEWLY_IDLE asap to reduce wakeup latency.

So IMHO, we can use 'continue_balancing' instead of 'this_rq->nr_running
> 0 || this_rq->ttwu_pending' in sched_balance_newidle() (former
newidle_balance()).

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Re: [PATCH] sched/fair: Simplify continue_balancing for newidle

[Ingo Molnar]

* Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:

> On 26/03/2024 10:00, Shrikanth Hegde wrote:
> > 
> > On 3/26/24 1:37 PM, Ingo Molnar wrote:
> >>
> >> * Shrikanth Hegde <sshegde@linux.ibm.com> wrote:
> >>
> 
> [...]
> 
> >> Is this actually true? Any change to behavior invalidates such a sentence.
> > 
> > From what i think, code path is same and I don't see any functionality changing. 
> > Correct me if i am wrong. 
> > 
> > Currently, sched_balance_newidle does the same check to bail out as the
> > should_we_balance check in case of newidle.  i.e  
> > 
> > should_we_balance
> > 	if (env->dst_rq->nr_running > 0 || env->dst_rq->ttwu_pending)
> > 			return 0;
> > 
> > sched_balance_newidle
> > 		if (pulled_task || this_rq->nr_running > 0 ||
> > 			this_rq->ttwu_pending)
> > 			break;
> > 		}
> 
> LGTM. Commit 792b9f65a568 ("sched: Allow newidle balancing to bail out
> of load_balance") (Jun 22) made sure that we leave sched_balance_rq()
> (former load_balance()) for CPU_NEWLY_IDLE asap to reduce wakeup latency.
> 
> So IMHO, we can use 'continue_balancing' instead of 'this_rq->nr_running
> > 0 || this_rq->ttwu_pending' in sched_balance_newidle() (former
> newidle_balance()).
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

Thanks for the clarification, applied!

	Ingo

[tip: sched/core] sched/fair: Simplify the continue_balancing logic in sched_balance_newidle()

[tip-bot2 for Shrikanth Hegde]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     c829d6818b60c591f70c060b2bb75d76cf0cec6d
Gitweb:        https://git.kernel.org/tip/c829d6818b60c591f70c060b2bb75d76cf0cec6d
Author:        Shrikanth Hegde <sshegde@linux.ibm.com>
AuthorDate:    Mon, 25 Mar 2024 21:09:26 +05:30
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 26 Mar 2024 20:16:20 +01:00

sched/fair: Simplify the continue_balancing logic in sched_balance_newidle()

newidle(CPU_NEWLY_IDLE) balancing doesn't stop the load-balancing if the
continue_balancing flag is reset, but the other two balancing (IDLE, BUSY)
cases do that.

newidle balance stops the load balancing if rq has a task or there
is wakeup pending. The same checks are present in should_we_balance for
newidle. Hence use the return value and simplify continue_balancing
mechanism for newidle. Update the comment surrounding it as well.

No change in functionality intended.

Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/20240325153926.274284-1-sshegde@linux.ibm.com
---
 kernel/sched/fair.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 24a7530..1856e58 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12358,6 +12358,7 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 {
 	unsigned long next_balance = jiffies + HZ;
 	int this_cpu = this_rq->cpu;
+	int continue_balancing = 1;
 	u64 t0, t1, curr_cost = 0;
 	struct sched_domain *sd;
 	int pulled_task = 0;
@@ -12372,8 +12373,9 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 		return 0;
 
 	/*
-	 * We must set idle_stamp _before_ calling idle_balance(), such that we
-	 * measure the duration of idle_balance() as idle time.
+	 * We must set idle_stamp _before_ calling sched_balance_rq()
+	 * for CPU_NEWLY_IDLE, such that we measure the this duration
+	 * as idle time.
 	 */
 	this_rq->idle_stamp = rq_clock(this_rq);
 
@@ -12412,7 +12414,6 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 
 	rcu_read_lock();
 	for_each_domain(this_cpu, sd) {
-		int continue_balancing = 1;
 		u64 domain_cost;
 
 		update_next_balance(sd, &next_balance);
@@ -12438,8 +12439,7 @@ static int sched_balance_newidle(struct rq *this_rq, struct rq_flags *rf)
 		 * Stop searching for tasks to pull if there are
 		 * now runnable tasks on this rq.
 		 */
-		if (pulled_task || this_rq->nr_running > 0 ||
-		    this_rq->ttwu_pending)
+		if (pulled_task || !continue_balancing)
 			break;
 	}
 	rcu_read_unlock();