Sorry was away for short vacation.
On 28 January 2014 19:20, Frederic Weisbecker fweisbec@gmail.com wrote:
On Thu, Jan 23, 2014 at 07:50:40PM +0530, Viresh Kumar wrote:
Wait, I got the wrong code here. That's wasn't my initial intention. I actually wanted to write something like this:
wake_up_nohz_cpu(cpu);
if (!tbase_get_deferrable(timer->base) || idle_cpu(cpu))wake_up_nohz_cpu(cpu);Will that work?
Something is seriously wrong with me, again wrote rubbish code. Let me phrase what I wanted to write :)
"don't send IPI to a idle CPU for a deferrable timer."
Probably I code it correctly this time atleast.
- wake_up_nohz_cpu(cpu); + if (!(tbase_get_deferrable(timer->base) && idle_cpu(cpu))) + wake_up_nohz_cpu(cpu);
Well, this is going to wake up the target from its idle state, which is what we want to avoid if the timer is deferrable, right?
Yeah, sorry for doing it for second time :(
The simplest thing we want is:
if (!tbase_get_deferrable(timer->base) || tick_nohz_full_cpu(cpu)) wake_up_nohz_cpu(cpu);This spares the IPI for the common case where the timer is deferrable and we run in periodic or dynticks-idle mode (which should be 99.99% of the existing workloads).
I wasn't looking at this problem with NO_HZ_FULL in mind. As I thought its only about if the CPU is idle or not. And so the solution I was talking about was:
"don't send IPI to a idle CPU for a deferrable timer."
But I see that still failing with the code you wrote. For normal cases where we don't enable NO_HZ_FULL, we will still end up waking up idle CPUs which is what Lei Wen reported initially.
Also if a CPU is marked for NO_HZ_FULL and is not idle currently then we wouldn't send a IPI for a deferrable timer. But we actually need that, so that we can reevaluate the timers order again?
Then we can later optimize that and spare the IPI on full dynticks CPUs when they run idle, but that require some special care about subtle races which can't be dealt with a simple test on "idle_cpu(target)". And power consumption in full dynticks is already very suboptimized anyway.
So I suggest we start simple with the above test, and a big fat comment which explains what we are doing and what needs to be done in the future.