Messages in this thread | | | Date | Tue, 6 Apr 2010 00:11:23 +0200 | From | Dominik Brodowski <> | Subject | Re: A few questions and issues with dynticks, NOHZ and powertop |
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On Mon, Apr 05, 2010 at 02:38:52PM -0700, Paul E. McKenney wrote: > On Mon, Apr 05, 2010 at 11:03:40PM +0200, Dominik Brodowski wrote: > > Paul, > > > > I really appreaciate your reply -- thanks! I've done some more testing in > > the meantime: > > > > On Sun, Apr 04, 2010 at 01:47:25PM -0700, Paul E. McKenney wrote: > > > On Sun, Apr 04, 2010 at 06:39:24PM +0200, Dominik Brodowski wrote: > > > > On Sun, Apr 04, 2010 at 11:17:37AM -0400, Alan Stern wrote: > > > > > On Sun, 4 Apr 2010, Dominik Brodowski wrote: > > > > > > > > > > > Booting a SMP-capable kernel with "nosmp", or manually offlining one CPU > > > > > > (or -- though I haven't tested it -- booting a SMP-capable kernel on a > > > > > > system with merely one CPU) means that in up to about half of the calls to > > > > > > tick_nohz_stop_sched_tick() are aborted due to rcu_needs_cpu(). This is > > > > > > quite strange to me: AFAIK, RCU is an excellent tool for SMP, but not really > > > > > > needed for UP? > > > > > > > > > > I can't answer the real question here, not knowing enough about the RCU > > > > > implementation. However, your impression is wrong: RCU very definitely > > > > > _is_ useful and needed on UP systems. It coordinates among processes > > > > > (and interrupt handlers) as well as among processors. > > > > > > > > Okay, but still: can't this be sped up by much on UP (especially if > > > > CONFIG_RCU_FAST_NO_HZ is set), so that we can go to sleep right away? > > > > > > One situation that will prevent CONFIG_RCU_FAST_NO_HZ from putting the > > > machine to sleep right away is if there is an RCU callback posted that > > > spawns another RCU callback, and so on. CONFIG_RCU_FAST_NO_HZ will handle > > > one callback that spawns another, but it gives up if the second callback > > > spawns a third. > > > > Will the remaining callbacks be executed immediately afterwards (due to a > > need_resched() etc.), or only after the next tick? > > Only after the next tick. To see why, imagine an RCU callback that > re-registers itself -- which is a perfectly legal thing to do. The > only thing that will happen if we run through grace periods faster is > that we will have more invocations of that same callback to deal with. > > So we try for a bit, and if that doesn't get rid of all of the callbacks, > we hold off until the next jiffy. > > > > Might this be what is happening to you? > > > > > > If so, would you be willing to patch your kernel? RCU_NEEDS_CPU_FLUSHES > > > is currently set to 5, and might be set to (say) 8. This is defined > > > in kernel/rcutree_plugin.h, near line 990. > > > > Applied the patch by Lai Jiangshan, and tested 5 and 8: > > > > 5: Wakeups-from-idle: 33.4 (hrtimer_sched_timer: 78 %) > > 34% of calls to tick_nohz_stop_sched_tick fail due to > > rcu_needs_cpu() > > 8: Wakeups-from-idle: 36.5 (hrtimer_sched_timer: 83 %) > > 37% of calls to tick_nohz_stop_sched_tick fail due to > > rcu_needs_cpu() > > I don't recall your posting wakeups-from-idle for the original -- did > we get improvement? You did say "roughly 50%", but...
Actually, no. I'd say the 5-to-8 change has no significant effect at all; for the Patch by Lai Jiangshan, I'd need to re-run the test.
> OK, I see what is happening... > > What happens in the CONFIG_RCU_FAST_NO_HZ case is as follows: > > o Check to see if the holdoff period is in effect, and if so, > just check to see if RCU needs the CPU for later processing > without attempting to accelerate grace periods. > > o Check to see if there is some other non-dyntick-idle CPU. > If there is, reset holdoff state and just check to see if > RCU needs the CPU for later processing without attempting to > accelerate grace periods. > > o Check for initialization and hitting the RCU_NEEDS_CPU_FLUSHES > limit, again doing the "just check" thing if we hit the limit. > > o For each of RCU-sched and RCU-bh, note a quiescent state > and force the grace-period machinery, noting in each case > whether or not there are callbacks left to invoke. > > o If there are callbacks left to invoke, raise RCU_SOFTIRQ. > This softirq will process the callbacks. (Why not just invoke > the softirq function directly? Because lockdep yells at you > and I do not believe that this is a false positive.) > > o If there are callbacks left to invoke, tell the caller that > this CPU cannot yet enter dyntick-idle state. > > But if we told the caller that this CPU cannot yet enter dyntick-idle > state, then we also raised RCU_SOFTIRQ. Once the softirq returns, we > should once again try to enter dyntick-idle state. > > So a significant fraction of calls to rcu_needs_cpu() saying "no" does > not necessarily mean that we are taking significant time to get the > grace periods and callbacks out of the way. The funny loop involving > softirq is required due to locking-design issues. > > Or are you seeing significant delays between successive calls to > rcu_needs_cpu() on your setup?
Will check this, but all the data I'm seeing points to rcu_needs_cpu() not leading to additional wakeups. It might just be wrong reports by powertop, after all, for the UP case. Quoting my original mail:
> 5) powertop and hrtimer_start_range_ns (tick_sched_timer) on a SMP kernel > booted with "nosmp": > > Wakeups-from-idle per second : 9.9 interval: 15.0s > ... > 48.5% ( 9.4) <kernel core> : hrtimer_start (tick_sched_timer) > 26.1% ( 5.1) <kernel core> : cursor_timer_handler > (cursor_timer_handle > 20.6% ( 4.0) <kernel core> : usb_hcd_poll_rh_status (rh_timer_func) > 1.0% ( 0.2) <kernel core> : arm_supers_timer > (sync_supers_timer_fn) > 0.7% ( 0.1) <interrupt> : ata_piix > ... > > Accoding to http://www.linuxpowertop.org , the count in the brackets is > how > many wakeups per seconds were caused by one source. Adding all _except_ > 48.5% ( 9.4) <kernel core> : hrtimer_start (tick_sched_timer) > up leads to the 9.9.
Back to your mail:
> > tick_nohz_stop_sched_tick() doesn't fail in this case because of > > rcu_needs_cpu(). However, the improvements are hardly recognizable: > > > > TINY_RCU: Wakeups-from-idle: 33.9 (hrtimer_sched_timer: 53 %) > > TINY_RCU is set up to automatically do CONFIG_RCU_FAST_NO_HZ, and do > the same softirq dance, or that is the theory, anyway. Again, are you > seeing significant delays between successive calls to rcu_needs_cpu()?
Actually, rcu_needs_cpu() is statically defined to return 0 on TINY_RCU in include/linux/rcutiny.h .
Best, Dominik
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