On Fri, Feb 03, 2017 at 07:44:57AM -0800, Paul E. McKenney wrote: > On Fri, Feb 03, 2017 at 02:37:48PM +0100, Peter Zijlstra wrote: > > On Fri, Feb 03, 2017 at 01:59:34PM +0100, Mike Galbraith wrote: > > > On Fri, 2017-02-03 at 09:53 +0100, Peter Zijlstra wrote: > > > > On Fri, Feb 03, 2017 at 10:03:14AM +0530, Sachin Sant wrote: > > > > > > > > I ran few cycles of cpu hot(un)plug tests. In most cases it works except one > > > > > where I ran into rcu stall: > > > > > > > > > > [ 173.493453] INFO: rcu_sched detected stalls on CPUs/tasks: > > > > > [ 173.493473] > > > > 8-...: (2 GPs behind) idle=006/140000000000000/0 softirq=0/0 fqs=2996 > > > > > [ 173.493476] > > > > (detected by 0, t=6002 jiffies, g=885, c=884, q=6350) > > > > > > > > Right, I actually saw that too, but I don't think that would be related > > > > to my patch. I'll see if I can dig into this though, ought to get fixed > > > > regardless. > > > > > > FWIW, I'm not seeing stalls/hangs while beating hotplug up in tip. (so > > > next grew a wart?) > > > > I've seen it on tip. It looks like hot unplug goes really slow when > > there's running tasks on the CPU being taken down. > > > > What I did was something like: > > > > taskset -p $((1<<1)) $$ > > for ((i=0; i<20; i++)) do while :; do :; done & done > > > > taskset -p $((1<<0)) $$ > > echo 0 > /sys/devices/system/cpu/cpu1/online > > > > And with those 20 tasks stuck sucking cycles on CPU1, the unplug goes > > _really_ slow and the RCU stall triggers. What I suspect happens is that > > hotplug stops participating in the RCU state machine early, but only > > tells RCU about it really late, and in between it gets suspicious it > > takes too long. > > > > I've yet to dig through the RCU code to figure out the exact sequence of > > events, but found the above to be fairly reliable in triggering the > > issue. > If you send me the full splat from the dmesg and the RCU portions of > .config, I will take a look. Is this new behavior, or a new test? If new behavior, I would be most suspicious of these commits in -rcu which recently entered -tip: 19e4d983cda1 rcu: Place guard on rcu_all_qs() and rcu_note_context_switch() actions 913324b1364f rcu: Eliminate flavor scan in rcu_momentary_dyntick_idle() fcdcfefafa45 rcu: Pull rcu_qs_ctr into rcu_dynticks structure 0919a0b7e7a5 rcu: Pull rcu_sched_qs_mask into rcu_dynticks structure caa7c8e34293 rcu: Make rcu_note_context_switch() do deferred NOCB wakeups 41e4b159d516 rcu: Make rcu_all_qs() do deferred NOCB wakeups b457a3356a68 rcu: Make call_rcu() do deferred NOCB wakeups Does reverting any of these help? Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-next" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
