Re: [PATCH] sched: fix exec_start/task_hot on migrated tasks

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 05/16/2014 07:47 PM, Peter Zijlstra wrote:
> On Fri, May 16, 2014 at 07:27:32PM +0530, Preeti Murthy wrote:
>>> 0 isn't strictly the right thing to do here, since the clock can wrap,
>>> but being wrong every ~585 years isn't too big an issue for this.
>>
>> I don't understand this. Will setting it to 0 not indicate beginning
>> of ticking? 
> 
> In modular spaces there is no beginnings nor endings.
> 
>> So when you find out for how long the task has run, the
>> difference would be larger than what would have been had you
>> let exec_start be at its previous value of the old cpu's clock_task right?
> 
> Nope, see the modular space thing, once every ~585 years we'll wrap the
> clock and 0 is within range of recently ran.
> 
>> Will not setting exec_start to the clock_task of the destination rq
>> during migration be better? This would be the closest we could
>> come to estimating the amount of time the task has run on this new
>> cpu while deciding task_hot or not no?
> 
> Setting it to the exact clock_task of the destination rq would make it
> hot on that rq, even though it hasn't yet ran there, so you'd have to do
> something like: rq_clock_task(dst_rq) - sysctl_sched_migration_cost.
> 
> But seeing as how that is far more work, and all this is heuristics
> anyhow and an extra fail term of 1/585 years is far below the current
> fail rate, all is well.
> 

Ok now I understand this.Thanks!

Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>