Re: [PATCH] Extend mwait idle to optimize away CAL and RES interruptsto an idle CPU -v1

[Venki Pallipadi]

On Wed, Feb 22, 2012 at 11:50 PM, Ingo Molnar <mingo@xxxxxxx> wrote:
>
> * Venkatesh Pallipadi <venki@xxxxxxxxxx> wrote:
>
>> * Do we need some accounting for these wakeups exported for powertop?
>
> If then tracepoints.
>
>> * We can also eliminate TS_POLLING flag in favor of this. But, that will have
>>   a lot more touchpoints and better done as a standlone change.
>
> Should most definitely be done for this series to be acceptble -
> as a preparatory patch in the series, with the feature at the
> end of the series.
>
>> +DECLARE_PER_CPU(atomic_t *, idle_task_ti_flags);
>
> That's ugly, we should access the idle task's ti flags directly.
>
> To have efficient percpu access to the idle threads another
> clean-up is needed: we should turn idle_thread_array into a
> full-structure PER_CPU area.
>
> For that we need a small variant of fork_idle(), which does not
> dup the init thread - pretty trivial.

OK. I looked a bit deeper into this and I understand what you suggested above.

>
> fork_idle() should also make sure it does not schedule the child
> thread: thus we'd also be able to further simplify smpboot.c and
> get rid of all that extremely ugly 'struct create_idle'
> gymnastics in smpboot.c.

But not this. I am not sure where fork_idle results in resched of the child.
As I saw it, fork_idle calls init_idle and that sets the affinity of
idle_task to target CPU. So, reschedule should not be a problem. What
am I missing here?

Also, I tried this silly test patch (Cut and paste... Sorry) and it
seemed to work fine both with and without CPU hotplug.

Thanks,
Venki

---
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 66d250c..36b80ef 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -686,7 +686,7 @@ static int __cpuinit do_boot_cpu(int apicid, int cpu)
                .done   = COMPLETION_INITIALIZER_ONSTACK(c_idle.done),
        };

-       INIT_WORK_ONSTACK(&c_idle.work, do_fork_idle);
+       // INIT_WORK_ONSTACK(&c_idle.work, do_fork_idle);

        alternatives_smp_switch(1);

@@ -703,12 +703,13 @@ static int __cpuinit do_boot_cpu(int apicid, int cpu)
                goto do_rest;
        }

-       schedule_work(&c_idle.work);
-       wait_for_completion(&c_idle.done);
+       // schedule_work(&c_idle.work);
+       // wait_for_completion(&c_idle.done);
+       c_idle.idle = fork_idle(cpu);

        if (IS_ERR(c_idle.idle)) {
                printk("failed fork for CPU %d\n", cpu);
-               destroy_work_on_stack(&c_idle.work);
+               // destroy_work_on_stack(&c_idle.work);
                return PTR_ERR(c_idle.idle);
        }

@@ -831,7 +832,7 @@ do_rest:
                smpboot_restore_warm_reset_vector();
        }

-       destroy_work_on_stack(&c_idle.work);
+       // destroy_work_on_stack(&c_idle.work);
        return boot_error;
 }

---

>
> Thanks,
>
>        Ingo
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