linux-kernel - Re: [PATCH] sched: fix exec_start/task

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Date:	Fri, 16 May 2014 09:57:34 -0700
From:	bsegall@...gle.com
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: fix exec_start/task_hot on migrated tasks

Peter Zijlstra <peterz@...radead.org> writes:

> On Thu, May 15, 2014 at 03:59:20PM -0700, Ben Segall wrote:
>> task_hot checks exec_start on any runnable task, but if it has been
>> migrated since the it last ran, then exec_start is a clock_task from
>> another cpu. If the old cpu's clock_task was sufficiently far ahead of
>> this cpu's then the task will not be considered for another migration
>> until it has run. Instead reset exec_start whenever a task is migrated,
>> since it is presumably no longer hot anyway.
>> 
>> Signed-off-by: Ben Segall <bsegall@...gle.com>
>> ---
>>  kernel/sched/fair.c |    3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 28ccf50..9f8dfeb 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -4544,6 +4544,9 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
>>  		atomic_long_add(se->avg.load_avg_contrib,
>>  						&cfs_rq->removed_load);
>>  	}
>> +
>> +	/* We have migrated, no longer consider this task hot */
>> +	se.exec_start = 0;
>
> se->exec_start compiles loads better
Welp. Indeed it does.


From: Ben Segall <bsegall@...gle.com>
Date: Thu, 15 May 2014 15:38:10 -0700
Subject: [PATCH] sched: fix exec_start/task_hot on migrated tasks

task_hot checks exec_start on any runnable task, but if it has been
migrated since the it last ran, then exec_start is a clock_task from
another cpu. If the old cpu's clock_task was sufficiently far ahead of
this cpu's then the task will not be considered for another migration
until it has run. Instead reset exec_start whenever a task is migrated,
since it is presumably no longer hot anyway.

Signed-off-by: Ben Segall <bsegall@...gle.com>
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 28ccf50..dd3fa14 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4544,6 +4544,9 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
                atomic_long_add(se->avg.load_avg_contrib,
                                                &cfs_rq->removed_load);
        }
+
+       /* We have migrated, no longer consider this task hot */
+       se->exec_start = 0;
 }
 #endif /* CONFIG_SMP */
 
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