linux-kernel - Re: [RFC][Patch 02/18] sched: increase SCHED_LOAD

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Date:	Thu, 28 Apr 2011 10:11:33 -0700
From:	Nikhil Rao <ncrao@...gle.com>
To:	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][Patch 02/18] sched: increase SCHED_LOAD_SCALE resolution

On Thu, Apr 28, 2011 at 2:54 AM, Nikunj A. Dadhania
<nikunj@...ux.vnet.ibm.com> wrote:
> On Wed, 20 Apr 2011 13:51:21 -0700, Nikhil Rao <ncrao@...gle.com> wrote:
>> Introduce SCHED_LOAD_RESOLUTION, which scales is added to SCHED_LOAD_SHIFT and
>> increases the resolution of SCHED_LOAD_SCALE. This patch sets the value of
>> SCHED_LOAD_RESOLUTION to 10, scaling up the weights for all sched entities by
>> a factor of 1024. With this extra resolution, we can handle deeper cgroup
>> hiearchies and the scheduler can do better shares distribution and load
>> load balancing on larger systems (especially for low weight task groups).
>>
>> This does not change the existing user interface, the scaled weights are only
>> used internally. We do not modify prio_to_weight values or inverses, but use
>> the original weights when calculating the inverse which is used to scale
>> execution time delta in calc_delta_mine(). This ensures we do not lose accuracy
>> when accounting time to the sched entities.
>>
>> Signed-off-by: Nikhil Rao <ncrao@...gle.com>
>> ---
>>  include/linux/sched.h |    3 ++-
>>  kernel/sched.c        |   18 ++++++++++--------
>>  2 files changed, 12 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 8d1ff2b..d2c3bab 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -794,7 +794,8 @@ enum cpu_idle_type {
>>  /*
>>   * Increase resolution of nice-level calculations:
>>   */
>> -#define SCHED_LOAD_SHIFT     10
>> +#define SCHED_LOAD_RESOLUTION        10
>> +#define SCHED_LOAD_SHIFT     (10 + SCHED_LOAD_RESOLUTION)
>>  #define SCHED_LOAD_SCALE     (1L << SCHED_LOAD_SHIFT)
>>
>>  /*
>> diff --git a/kernel/sched.c b/kernel/sched.c
>> index 50f97cc..bfee8ff 100644
>> --- a/kernel/sched.c
>> +++ b/kernel/sched.c
>> @@ -293,7 +293,7 @@ static DEFINE_SPINLOCK(task_group_lock);
>>   *  limitation from this.)
>>   */
>>  #define MIN_SHARES   2
>> -#define MAX_SHARES   (1UL << 18)
>> +#define MAX_SHARES   (1UL << 28)
> Shouldn't this change depend on SCHED_LOAD_RESOLUTION?
>
> #define MAX_SHARES      (1UL << (18 + SCHED_LOAD_RESOLUTION))
>

Right, and the other 18 bits come from SCHED_LOAD_SCALE + some slack,
which is also implicitly defined. I can make this more explicit in the
next rev as you suggest. I don't expect SCHED_LOAD_RESOLUTION to
change often though.

> Nikunj
>
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