linux-kernel - Re: [PATCH 17/33] sched: Update clock of nohz busiest rq before balancing

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Date:	Fri, 8 Mar 2013 00:51:19 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Li Zhong <zhong@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung.kim@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 17/33] sched: Update clock of nohz busiest rq before balancing

2013/1/8 Li Zhong <zhong@...ux.vnet.ibm.com>:
> On Tue, 2013-01-08 at 03:08 +0100, Frederic Weisbecker wrote:
>> move_tasks() and active_load_balance_cpu_stop() both need
>> to have the busiest rq clock uptodate because they may end
>> up calling can_migrate_task() that uses rq->clock_task
>> to determine if the task running in the busiest runqueue
>> is cache hot.
>>
>> Hence if the busiest runqueue is tickless, update its clock
>> before reading it.
>>
>> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
>> Cc: Alessio Igor Bogani <abogani@...nel.org>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Chris Metcalf <cmetcalf@...era.com>
>> Cc: Christoph Lameter <cl@...ux.com>
>> Cc: Geoff Levand <geoff@...radead.org>
>> Cc: Gilad Ben Yossef <gilad@...yossef.com>
>> Cc: Hakan Akkan <hakanakkan@...il.com>
>> Cc: Ingo Molnar <mingo@...nel.org>
>> Cc: Li Zhong <zhong@...ux.vnet.ibm.com>
>> Cc: Namhyung Kim <namhyung.kim@....com>
>> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> Cc: Paul Gortmaker <paul.gortmaker@...driver.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Cc: Steven Rostedt <rostedt@...dmis.org>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> [ Forward port conflicts ]
>> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
>> ---
>>  kernel/sched/fair.c |   17 +++++++++++++++++
>>  1 files changed, 17 insertions(+), 0 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 3d65ac7..e78d81104 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5002,6 +5002,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>  {
>>       int ld_moved, cur_ld_moved, active_balance = 0;
>>       int lb_iterations, max_lb_iterations;
>> +     int clock_updated;
>>       struct sched_group *group;
>>       struct rq *busiest;
>>       unsigned long flags;
>> @@ -5045,6 +5046,7 @@ redo:
>>
>>       ld_moved = 0;
>>       lb_iterations = 1;
>> +     clock_updated = 0;
>>       if (busiest->nr_running > 1) {
>>               /*
>>                * Attempt to move tasks. If find_busiest_group has found
>> @@ -5068,6 +5070,14 @@ more_balance:
>>                */
>>               cur_ld_moved = move_tasks(&env);
>>               ld_moved += cur_ld_moved;
>> +
>> +             /*
>> +              * Move tasks may end up calling can_migrate_task() which
>> +              * requires an uptodate value of the rq clock.
>> +              */
>> +             update_nohz_rq_clock(busiest);
>> +             clock_updated = 1;
>
> According to the change log, it seems these lines should be added before
> move_tasks() above?

Yeah, but eventually it seems that can_migrate_task() doesn't make use
of rq clock anymore. I guess I'll just drop that patch.

Thanks.
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