Re: [RFC] Extend mwait idle to optimize away IPIs when possible

[Suresh Siddha <suresh.b.siddha@intel.com>]

On Mon, 2012-02-06 at 12:42 -0800, Venkatesh Pallipadi wrote:
> * Lower overhead on Async IPI send path. Measurements on Westmere based
>   systems show savings on "no wait" smp_call_function_single with idle
>   target CPU (as measured on the sender side).
>   local socket smp_call_func cost goes from ~1600 to ~1200 cycles
>   remote socket smp_call_func cost goes from ~2000 to ~1800 cycles

Interesting that savings in the remote socket is less compared to the
local socket.

> +int smp_need_ipi(int cpu)
> +{
> +	int oldval;
> +
> +	if (!system_using_cpu_idle_sync || cpu == smp_processor_id())
> +		return 1;
> +
> +	oldval = atomic_cmpxchg(&per_cpu(cpu_idle_sync, cpu),
> +				CPU_STATE_IDLE, CPU_STATE_WAKING);

To avoid too many cache line bounces for the case when the cpu is in the
running state, we should do a read to check if the state is in idle
before going ahead with the locked operation?

> +
> +	if (oldval == CPU_STATE_RUNNING)
> +		return 1;
> +
> +	if (oldval == CPU_STATE_IDLE) {
> +		set_tsk_ipi_pending(idle_task(cpu));
> +		atomic_set(&per_cpu(cpu_idle_sync, cpu), CPU_STATE_WOKENUP);
> +	}
> +
> +	return 0;

We should probably disable interrupts around this, otherwise any delay
in transitioning to wokenup from waking will cause the idle cpu to be
stuck for similar amount of time.

thanks,
suresh