linux-kernel - Re: [PATCH v2 2/4] sched/fair: Move active balance logic to its own function

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Date:   Tue, 1 Oct 2019 12:48:17 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        peterz@...radead.org, vincent.guittot@...aro.org,
        tglx@...utronix.de, qais.yousef@....com
Subject: Re: [PATCH v2 2/4] sched/fair: Move active balance logic to its own
 function

On 01/10/2019 12:36, Srikar Dronamraju wrote:
>> +unlock:
>> +	raw_spin_unlock_irqrestore(&busiest->lock, flags);
>> +
>> +	if (status == started)
>> +		stop_one_cpu_nowait(cpu_of(busiest),
>> +				    active_load_balance_cpu_stop, busiest,
>> +				    &busiest->active_balance_work);
>> +
>> +	/* We've kicked active balancing, force task migration. */
>> +	if (status != cancelled_affinity)
>> +		sd->nr_balance_failed = sd->cache_nice_tries + 1;
> 
> Should we really update nr_balance_failed if status is cancelled?
> I do understand this behaviour was present even before this change. But
> still dont understand why we need to update if the current operation didn't
> kick active_load_balance.
> 

Agreed, I kept it as is to keep this as pure a code movement as possible,
but I don't see why this wouldn't be valid wouldn't be valid
(PoV of the current code):

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1fac444a4831..59f9e3583482 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9023,10 +9023,10 @@ static int load_balance(int this_cpu, struct rq *this_rq,
 				stop_one_cpu_nowait(cpu_of(busiest),
 					active_load_balance_cpu_stop, busiest,
 					&busiest->active_balance_work);
-			}
 
-			/* We've kicked active balancing, force task migration. */
-			sd->nr_balance_failed = sd->cache_nice_tries+1;
+				/* We've kicked active balancing, force task migration. */
+				sd->nr_balance_failed = sd->cache_nice_tries+1;
+			}
 		}
 	} else
 		sd->nr_balance_failed = 0;
---

Or even better, fold it in active_load_balance_cpu_stop(). I could add that
after the move.