Re: [PATCH 0/2] sparc64: Use low latency path to resume idle cpu

[Vijay Kumar]

On 7/20/2017 2:57 PM, David Miller wrote:
> From: Vijay Kumar <vijay.ac.kumar@xxxxxxxxxx>
> Date: Sat,  8 Jul 2017 14:23:42 -0600
>
> > cpu_poke is a low latency path to resume the target cpu if suspended
> > using cpu_yield. Use cpu poke to resume cpu if supported by hypervisor.
> >
> > 	     hackbench results (lower is better):
> > Number of		
> > Process:		w/o fix		with fix
> > 1  			0.012		 0.010
> > 10			0.021		 0.019
> > 100			0.151		 0.148
> So this only works for a cpu which has yielded.
>
> The kernel sends reschedule events to both idle and non-idle cpus.
> That's why you have to have that fallback code to still send the
> mondo IPI right?
That is correct.

> For the case where POKE works, it seems like completely unnecessary
> overhead to set the PIL interrupt.  Just disable local cpu interrupts
> and call schedule_ipi() directly.
>
> I bet that improves your benchmark even more.

I had same thoughts initially but I had to go with this approach as 
scheduler_ipi is wrapped with irq_enter() and irq_exit(). Whereas POKE 
resumes the cpu in process context.

Comments in scheduler_ipi():

         * Not all reschedule IPI handlers call irq_enter/irq_exit, since
         * traditionally all their work was done from the interrupt return
         * path. Now that we actually do some work, we need to make sure
         * we do call them.
         *
         * Some archs already do call them, luckily irq_enter/exit nest
         * properly.
         *
         * Arguably we should visit all archs and update all handlers,
         * however a fair share of IPIs are still resched only so this 
would
         * somewhat pessimize the simple resched case.
         */
        irq_enter();


-Vijay