On Tue, Jun 18, 2013 at 04:20:28PM +0100, Arjan van de Ven wrote:
On 6/14/2013 9:05 AM, Morten Rasmussen wrote:
Looking at the discussion it seems that people have slightly different views, but most agree that the goal is an integrated scheduling, frequency, and idle policy like you pointed out from the beginning.
... except that such a solution does not really work for Intel hardware.
The OS does not get to really pick the CPU "frequency" (never mind that frequency is not what gets controlled), the hardware picks the frequency. The OS can do some level of requests (best to think of this as a percentage more than frequency) but what you actually get is more often than not what you asked for.
You can look in hindsight what kind of performance you got (from some basic counters in MSRs), and the scheduler can use that to account backwards to what some process got. But to predict what you will get in the future...... that's near impossible on any realistic system nowadays (and even more so in the future).
The proposed power scheduler doesn't have to drive p-state selection if it doesn't make sense for the particular platform. The aim of the power scheduler is integration of power policies in general.
Treating "frequency" (well "performance) and idle separately is also a false thing to do (yes I know in 3.9/3.10 we still do that for Intel hw, but we're working on fixing that). They are by no means separate things. One guy's idle state is the other guys power budget (and thus performance)!.
I agree.
Based on our discussions so far, where it has become more clear where Intel is heading, and Ingo's reply I think we have three ways to ahead with the power-aware scheduling work. Each with their advantages and disadvantages:
1. We work on a generic power scheduler with appropriate abstractions that will work for all of us. Current and future Intel p-state policies will be implemented through the power scheduler.
Pros: We can arrive at fairly standard solution with standard tunables. There will be one interface to the scheduler.
Cons: Finding a suitable platform abstraction for the power scheduler.
2. Like 1, but we introduce a CONFIG_SCHED_POWER as suggested by Ingo, that makes it all go away.
Pros: Intel can keep intel_pstate.c others can use the power scheduler or their own driver.
Cons: Different platform specific drivers may need different interfaces to the scheduler. Harder to define cross-platform tunables.
3. We go for independent platform specific power policy driver that may or may not use existing frameworks, like intel_pstate.c.
Pros: No need to find common platform abstraction. Power policy is implemented in arch/* and won't affect others.
Cons: Same as 2. Everybody would have to implement their own frequency, idle and thermal solutions. Potential duplication of functionality.
In my opinion we should aim for 1., but start out with a CONFIG_SCHED_POWER and see where we get to. Feedback from everybody is essential to arrive at a generic solution.
Morten