False-positive RCU stall warnings on large systems...

[Daniel J Blueman]

Hi Paul,

On some of our larger servers with many hundreds of cores and when under 
high duress, we can see scheduler RCU stall warnings [1], so find we 
have to increase the hardcoded RCU_STALL_RAT_DELAY up from 2 and 
RCU_JIFFIES_TILL_FORCE_QS up from 3.

Is there a more sustainable way to account for this to avoid it being 
hard-coded, such as making it and dependent timeouts a fraction of 
CONFIG_RCU_CPU_STALL_TIMEOUT?

On the other hand, perhaps this is just caused by clock jitter (eg due 
to distance from a contended clock source)? So increasing these a bit 
may just be adequate in general...

Many thanks,
  Daniel

--- [1]

[ 3939.010085] INFO: rcu_sched detected stalls on CPUs/tasks: {} 
(detected by 1, t=29662 jiffies, g=3053, c=3052, q=598)
[ 3939.020008] INFO: Stall ended before state dump start
--
Daniel J Blueman
Principal Software Engineer, Numascale Asia
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