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. 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
