On 03/26/2013 11:55 PM, Vincent Guittot wrote:
So extrapolating that to a 4+4 big-little you'd get something like:
| little A9 || big A15 | | 0 | 1 | 2 | 3 || 4 | 5 | 6 | 7 |------+---+---+---+---++---+---+---+---+ buddy | 0 | 0 | 0 | 0 || 0 | 4 | 4 | 4 |
Right?
yes
So supposing the current ILB is 6, we'll only check 4, not 0-3, even though there might be a perfectly idle cpu in there.
We will check 4,5,7 at MC level in order to pack in the group of A15 (because they are not sharing the same power domain). If none of them are idle, we will look at CPU level and will check CPUs 0-3.
So you increase a fixed step here.
Also, your scheme fails to pack when cpus 0,4 are filled, even when there's idle cores around.
The primary target is to pack the tasks only when we are in a not busy system so you will have a power improvement without performance decrease. is_light_task function returns false and is_buddy_busy function true before the buddy is fully loaded and the scheduler will fall back into the default behavior which spreads tasks and races to idle.
We can extend the buddy CPU and the packing mechanism to fill one CPU before filling another buddy but it's not always the best choice for performance and/or power and thus it will imply to have a knob to select this full packing mode.
Just one buddy to pack tasks for whole level cpus definitely has scalability problem. That is not good for powersaving in most of scenarios.