On Fri, 7 Feb 2014, Preeti U Murthy wrote:
Hi Nicolas,
On 02/07/2014 06:47 AM, Nicolas Pitre wrote:
What about creating arch_cpu_idle_enter() and arch_cpu_idle_exit() in arch/powerpc/kernel/idle.c and calling ppc64_runlatch_off() and ppc64_runlatch_on() respectively from there instead? Would that work? That would make the idle consolidation much easier afterwards.
I would not suggest doing this. The ppc64_runlatch_*() routines need to be called when we are sure that the cpu is about to enter or has exit an idle state. Moving the ppc64_runlatch_on() routine to arch_cpu_idle_enter() for instance is not a good idea because there are places where the cpu can decide not to enter any idle state before the call to cpuidle_idle_call() itself. In that case communicating prematurely that we are in an idle state would not be a good idea.
So its best to add the ppc64_runlatch_* calls in the powernv cpuidle driver IMO. We could however create idle_loop_prologue/epilogue() variants inside it so that in addition to the runlatch routines we could potentially add more such similar routines that are powernv specific. If there are cases where there is work to be done prior to and post an entry into an idle state common to both pseries and powernv, we will probably put them in arch_cpu_idle_enter/exit(). But the runlatch routines are not suitable to be moved there as far as I can see.
OK.
However, one thing we need to do as much as possible is to remove those loops based on need_resched() from idle backend drivers. A somewhat common pattern is:
my_idle() { /* interrupts disabled on entry */ while (!need_resched()) { lowpower_wait_for_interrupts(); local_irq_enable(); /* IRQ serviced from here */ local_irq_disable(); } local_irq_enable(); /* interrupts enabled on exit */ }
To be able to keep statistics on the actual idleness of the CPU we'd need for all idle backends to always return to generic code on every interrupt similar to this:
my_idle() { /* interrupts disabled on entry */ lowpower_wait_for_interrupts(); local_irq_enable(); /* interrupts enabled on exit */ }
The generic code would be responsible for dealing with need_resched() and call back into the backend right away if necessary after updating some stats.
Do you see a problem with the runlatch calls happening around each interrrupt from such a simplified idle backend?
Nicolas