Sorry was away for short vacation. On 28 January 2014 19:20, Frederic Weisbecker <fweis...@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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
