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? 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
