Re: [tip:sched/core] sched/core: Add debugging code to catch missing update_rq_clock() calls

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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.
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What I did was something like:
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 taskset -p $((1<<1)) $$
 for ((i=3D0; i<20; i++)) do while :; do :; done & done
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 taskset -p $((1<<0)) $$
 echo 0 > /sys/devices/system/cpu/cpu1/online
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And with those 20 tasks stuck sucking cycles on CPU1, the unplug =

goes

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_really_ slow and the RCU stall triggers. What I suspect happens is =

that

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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.
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I've yet to dig through the RCU code to figure out the exact =

sequence of

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events, but found the above to be fairly reliable in triggering the
issue.

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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?

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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:
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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
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Does reverting any of these help?

I tried reverting the above commits. That does not help. I can still =
recreate the issue.

Thanks
-Sachin=