Hi Peter,
On 02/07/2014 06:11 PM, Peter Zijlstra wrote:
On Fri, Feb 07, 2014 at 05:11:26PM +0530, Preeti U Murthy wrote:
But observe the idle state "snooze" on powerpc. The power that this idle state saves is through the lowering of the thread priority of the CPU. After it lowers the thread priority, it is done. It cannot "wait_for_interrupts". It will exit my_idle(). It is now upto the generic idle loop to increase the thread priority if the need_resched flag is set. Only an interrupt routine can increase the thread priority. Else we will need to do it explicitly. And in such states which have a polling nature, the cpu will not receive a reschedule IPI.
That is why in the snooze_loop() we poll on need_resched. If it is set we up the priority of the thread using HMT_MEDIUM() and then exit the my_idle() loop. In case of interrupts, the priority gets automatically increased.
You can poll without setting TS_POLLING/TIF_POLLING_NRFLAGS just fine and get the IPI if that is what you want.
Depending on how horribly unprovisioned the thread gets at the lowest priority, that might actually be faster than polling and raising the prio whenever it does get ran.
So I am assuming you mean something like the below:
my_idle() { local_irq_enable(); /* Remove the setting of the polling flag */ HMT_low(); return index; }
And then exit into the generic idle loop. But the issue I see here is that the TS_POLLING/TIF_POLLING_NRFLAGS gets set immediately. So, if on testing need_resched() immediately after this returns that the TIF_NEED_RESCHED flag is set, the thread will exit at low priority right? We could raise the priority of the thread in arch_cpu_idle_exit() soon after setting the polling flag but that would mean for cases where the TIF_NEED_RESCHED flag is not set we unnecessarily raise the priority of the thread.
Thanks
Regards Preeti U Murthy