On Mon, Feb 06, 2017 at 07:10:48AM -0800, Paul E. McKenney wrote: > On Mon, Feb 06, 2017 at 11:53:10AM +0530, Sachin Sant wrote: > > > > >>> 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? > > > > > > > I have sent the required files to you via separate email. > > > > > 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? > > > > I tried reverting the above commits. That does not help. I can still recreate the issue. > > Thank you for testing, Sachin! > > Could you please try building and testing with CONFIG_RCU_BOOST=y? > You will need to enable CONFIG_RCU_EXPERT=y to see this Kconfig option. Ah, but looking ahead to your .config file, you have CONFIG_PREEMPT=n, which means boosting would not help and is not available in any case. So it looks like there is a very long loop within an RCU read-side critical section, and that this critical section needs to be broken up a bit -- 21 seconds in pretty much any kind of critical section is a bit excessive, after all. 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
