Re: [PATCH 1/2] sched/nohz: add debugfs control over sched_tick_max_deferment

[Kevin Hilman <khilman@linaro.org>]

Frederic Weisbecker <fweisbec@gmail.com> writes:

> On Tue, Dec 17, 2013 at 01:23:07PM -0800, Kevin Hilman wrote:
>> Allow debugfs override of sched_tick_max_deferment in order to ease
>> finding/fixing the remaining issues with full nohz.
>> 
>> The value to be written is in jiffies, and -1 means the max deferment
>> is disabled (scheduler_tick_max_deferment() returns KTIME_MAX.)
>> 
>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>> Signed-off-by: Kevin Hilman <khilman@linaro.org>
>> ---
>>  kernel/sched/core.c | 16 +++++++++++++++-
>>  1 file changed, 15 insertions(+), 1 deletion(-)
>> 
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 5ac63c9a995a..4b1fe3e69fe4 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2175,6 +2175,8 @@ void scheduler_tick(void)
>>  }
>>  
>>  #ifdef CONFIG_NO_HZ_FULL
>> +static u32 sched_tick_max_deferment = HZ;
>> +
>>  /**
>>   * scheduler_tick_max_deferment
>>   *
>> @@ -2193,13 +2195,25 @@ u64 scheduler_tick_max_deferment(void)
>>  	struct rq *rq = this_rq();
>>  	unsigned long next, now = ACCESS_ONCE(jiffies);
>>  
>> -	next = rq->last_sched_tick + HZ;
>> +	if (sched_tick_max_deferment == -1)
>> +		return KTIME_MAX;
>> +
>> +	next = rq->last_sched_tick + sched_tick_max_deferment;
>>  
>>  	if (time_before_eq(next, now))
>>  		return 0;
>>  
>>  	return jiffies_to_usecs(next - now) * NSEC_PER_USEC;
>>  }
>> +
>> +static __init int sched_nohz_full_init_debug(void)
>> +{
>> +	debugfs_create_u32("sched_tick_max_deferment", 0644, NULL,
>> +			   &sched_tick_max_deferment);
>> +
>> +	return 0;
>> +}
>> +late_initcall(sched_nohz_full_init_debug);
>
> If the goal is mostly to turn off sched_tick_max_deferment (set to -1), we should
> perhaps make it a boolean sched feature (see kernel/sched/features.h) as it's a pretty
> well consolidated interface.

Well, I suspect folks may want to set it to various values, depending on
workload to experiment with the results.

Also, my first attempt was to add control over this via sysctl[1] (though
not sched_features) and you suggested[2] I use debugfs instead since this
should be a temporary hack until we can remove the 1Hz residual tick.

Kevin

[1] http://marc.info/?l=linux-kernel&m=137159992306877&w=2
[2] http://marc.info/?l=linux-kernel&m=137166737830821&w=2