On 7/20/2017 1:11 AM, Thomas Gleixner wrote:
On Thu, 20 Jul 2017, Li, Aubrey wrote:
Don't get me wrong, even if a fast path is acceptable, we still need to
figure out if the coming idle is short and when to switch. I'm just worried
about if irq timings is not an ideal statistics, we have to skip it too.
There is no ideal solution ever.
Lets sit back and look at that from the big picture first before dismissing
a particular item upfront.
The current NOHZ implementation does:
predict = nohz_predict(timers, rcu, arch, irqwork);
if ((predict - now) > X)
stop_tick()
The C-State machinery does something like:
predict = cstate_predict(next_timer, scheduler);
cstate = cstate_select(predict);
That disconnect is part of the problem. What we really want is:
predict = idle_predict(timers, rcu, arch, irqwork, scheduler, irq timings);
two separate predictors is clearly a recipe for badness.
(likewise, C and P states try to estimate "performance sensitivity" and
sometimes estimate in opposite directions)
to be honest, performance sensitivity estimation is probably 10x more critical
for C state
selection than idle duration; a lot of modern hardware will do the energy
efficiency stuff
in a microcontroller when it coordinates between multiple cores in the system
on C and P states.
(both x86 and ARM have such microcontroller nowadays, at least for the higher
performance
designs)
