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    SubjectRe: [PATCH 2/4] timer: relax tick stop in idle entry
    On Mon, Nov 16, 2015 at 05:41:03PM -0800, Josh Triplett wrote:
    > On Mon, Nov 16, 2015 at 03:26:40PM -0800, Paul E. McKenney wrote:
    > > On Mon, Nov 16, 2015 at 02:32:11PM -0800, Josh Triplett wrote:
    > > > On Mon, Nov 16, 2015 at 01:51:26PM -0800, Jacob Pan wrote:
    > > > > On Mon, 16 Nov 2015 16:06:57 +0100 (CET)
    > > > > Thomas Gleixner <tglx@linutronix.de> wrote:
    > > > >
    > > > > > > <idle>-0 [000] 30.093474: bprint:
    > > > > > > __tick_nohz_idle_enter: JPAN: tick_nohz_stop_sched_tick 609 delta
    > > > > > > 1000000 [JP] but sees delta is exactly 1 tick away. didn't stop
    > > > > > > tick.
    > > > > >
    > > > > > If the delta is 1 tick then it is not supposed to stop it. Did you
    > > > > > ever try to figure out WHY it is 1 tick?
    > > > > >
    > > > > > There are two code pathes which can set it to basemono + TICK_NSEC:
    > > > > >
    > > > > > if (rcu_needs_cpu(basemono, &next_rcu) ||
    > > > > > arch_needs_cpu() || irq_work_needs_cpu()) {
    > > > > > next_tick = basemono + TICK_NSEC;
    > > > > > } else {
    > > > > > next_tmr = get_next_timer_interrupt(basejiff,
    > > > > > basemono); ts->next_timer = next_tmr;
    > > > > > /* Take the next rcu event into account */
    > > > > > next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
    > > > > > }
    > > > > >
    > > > > > Can you please figure out WHY the tick is requested to continue
    > > > > > instead of blindly wreckaging the logic in that code?
    > > > >
    > > > > Looks like the it hits in both cases during forced idle.
    > > > > + Josh
    > > > > + Paul
    > > > >
    > > > > For the first case, it is always related to RCU. I found there are two
    > > > > CONFIG options to avoid this undesired tick in idle loop.
    > > > > 1. enable CONFIG_RCU_NOCB_CPU_ALL, offload to orcu kthreads
    > > > > 2. or enable CONFIG_RCU_FAST_NO_HZ (enter dytick idle w/ rcu callback)
    > > > >
    > > > > Either one works but my concern is that users may not realize the
    > > > > intricate CONFIG_ options and how they translate into energy savings.
    > > > > Consulted with Josh, it seems we could add a check here to recognize
    > > > > the forced idle state and relax rcu_needs_cpu() to return false even it
    > > > > has callbacks. Since we are blocking everybody for a short time (5 ticks
    > > > > default). It should not impact synchronize and kfree rcu.
    > > >
    > > > Right; as long as you're blocking *everybody*, and RCU priority boosting
    > > > doesn't come into play (meaning a real-time task is waiting on RCU
    > > > callbacks), then I don't see any harm in blocking RCU callbacks for a
    > > > while. You'd block completion of synchronize_rcu() and similar, as well
    > > > as memory reclamation, but since you've blocked *every* CPU systemwide
    > > > then that doesn't cause a problem.
    > >
    > > True enough. But how does RCU distinguish between this being a
    > > normal idle cycle that might last indefinitely on the one hand and the
    > > five-jiffy system-wide throttling on the other? OK, maybe there is a
    > > global variable that says that the just-now-starting idle period is
    > > system-wide throttling. But then what about the CPU that just went
    > > idle 10 microseconds ago, and therefore left its timer tick running?
    > > Fine and well, we could IPI it to wake it up and let it see that we
    > > are now doing thermal throttling. But then we presumably also have to
    > > IPI it at the end of the thermal-throttling interval in order for it to
    > > re-evaluate whether or not it should have the tick going. :-/
    > >
    > > On the one hand, I am sure that all of this can be made to work,
    > > but simply having systems using thermal throttling enable either
    > > CONFIG_RCU_NOCB_CPU_ALL or CONFIG_RCU_FAST_NO_HZ seems -way- simpler.
    > > CONFIG_RCU_FAST_NO_HZ is probably the better choice for generic workloads,
    > > but CONFIG_RCU_NOCB_CPU_ALL is the better choice for embedded workloads
    > > where it is less likely that RCU callbacks will be posted with continuous
    > > wild abandon.
    > >
    > > Or am I missing something subtle here?
    >
    > I agree that it seems preferable to make this require an existing RCU
    > solution rather than adding more complexity to the RCU idle path. One
    > possible thing that may affect the choice of solution: this needs to
    > idle *every* CPU, without leaving any CPU awake to handle callbacks or
    > similar.

    Fair point. When in the five-jiffy throttling state, what can wake up
    a CPU? In an earlier version of this proposal, the answer was "nothing",
    but maybe that has changed.

    Thanx, Paul



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