On 26 March 2013 13:46, Peter Zijlstra peterz@infradead.org wrote:
On Fri, 2013-03-22 at 13:25 +0100, Vincent Guittot wrote:
During the creation of sched_domain, we define a pack buddy CPU for each CPU when one is available. We want to pack at all levels where a group of CPU can be power gated independently from others. On a system that can't power gate a group of CPUs independently, the flag is set at all sched_domain level and the buddy is set to -1. This is the default behavior. On a dual clusters / dual cores system which can power gate each core and cluster independently, the buddy configuration will be :
| Cluster 0 | Cluster 1 | | CPU0 | CPU1 | CPU2 | CPU3 |
buddy | CPU0 | CPU0 | CPU0 | CPU2 |
I suppose this is adequate for the 'small' systems you currently have; but given that Samsung is already bragging with its 'octo'-core Exynos 5 (4+4 big-little thing) does this solution scale?
The packing is only done at MC and CPU level to minimize the number of transition.
Isn't this basically related to picking the NO_HZ cpu; if the system isn't fully symmetric with its power gates you want the NO_HZ cpu to be the 'special' cpu. If it is symmetric we really don't care which core is left 'running' and we can even select a new pack cpu from the idle cores once the old one is fully utilized.
I agree that on a symmetric system, we don't really care about which core is selected but we want to use the same one whenever possible to prevent a ping pong between several cores or groups of cores, which is power consuming. By forcing a NOHZ cpu, your background activity will smoothly pack on this CPU and will not be spread on your system. When a CPU is fully loaded, we don't fall in a low CPU load use case and the periodic load balance can handle the situation to select a new target CPU which is close to the buddy CPU
Re-using (or integrating) with NO_HZ has the dual advantage that you'll make NO_HZ do the right thing for big-little (you typically want a little core to be the one staying 'awake' and once someone makes NO_HZ scale this all gets to scale along with it.
I think that you have answered to this question in your comment of patch 5, isn't it?
Vincent