Re: [PATCH 07/10] sched/fair: Provide can_migrate_task_llc

[Valentin Schneider]

On 31/10/2018 19:14, Peter Zijlstra wrote:
> On Mon, Oct 29, 2018 at 07:34:50PM +0000, Valentin Schneider wrote:
>> On a sidenote, I find it a bit odd that the exec_start threshold depends on
>> sysctl_sched_migration_cost, which to me is more about idle_balance() cost
>> than "how long does it take for a previously run task to go cache cold".
> 
> A long long (think 2.6.20 long ago) there was code that did boot-time
> measurement of the various cache topology costs and that threshold was
> related to that.
> 
> That code got killed because of boot to boot variance and dubious
> benefits.
> 
> The migration cost is what it was replaced with as a single measure.
> 
> migration cost was then later abused in the newidle balance because it
> was over eager.


Thanks! I tried following the blame rabbit hole but got a bit lost along
the way.

> Ideally we'd get rid of it there, because we've now got
> that much more elaborate accounting, but Rohit tried and found some
> regression because of that.
> 
> Maybe we should remove it from newidle anyway.
> 

Well, if we ditch idle_balance() in favor of stealing, that would "solve"
it...