rcu_sched stall OR kernel panic on PowerEdge R640

[Sebastian Roth]

@Quazz said in rcu_sched stall OR kernel panic on PowerEdge R640:

as far as I know CONFIG_MAXSMP is in fact enabled by default on Kernel 4.4+ or so on all major distributions without issues

That’s valuable information! Any reference for this?

[Quazz]

@Sebastian-Roth Hmm, I may have been misremembering, though their CONFIG_NR_CPUS
is going to be much higher than 8 at the very least. (at least 512 afaik)

The only difference I can find is that CONFIG_MAXSMP enables CPUMASK_OFFSTACK
, which it requires to function correctly I believe (or any high CONFIG_NR_CPUS would at least)

[Sebastian Roth]

@Quazz @george1421 Ok, back from travels… what shall we do with this pending topic. I do understand that adding CONFIG_MAXSMP does fix the rcu_sched stall issue on PowerEdge R640. But do we know if this fixes rcu_sched stalls on other platforms as well? Would we get at least two more people to test this before we add it to the official kernel?

@george1421 Did you get to test this kernel on your fleet of Dell hardware to see if it might cause any other harm?

[george1421]

@Sebastian-Roth said in rcu_sched stall OR kernel panic on PowerEdge R640:

Did you get to test this kernel on your fleet of Dell hardware to see if it might cause any other harm?

TBH, no I did not test it. I haven’t found any other system that the max-cpu value fixed either. We had one dual core with the rcu_sched stall, but that was fixed with the current kernel and changing the acpi clock source that Quazz posted. I think the max-cpu will only impact CPUs with more than 8 cores.

[Sebastian Roth]

@george1421 Looking through a stack of other rcu_sched stall topics in the forums I can’t seem to find any thread where I’d think that people had CPUs with more than 8 cores. Sure sooner or later this will be state of the art but I don’t reckon we should step ahead of this. We know the current kernel works pretty good on most CPUs and I’d rather point people to this topic and provide compile instructions than setting CONFIG_MAXSMP as default. Hmm?

[george1421]

@Sebastian-Roth I’m still on the fence about this, I would say turn it on because the core count continues to rise on these processors. What can it hurt? And on the other side we really don’t know what the impact could be.

[Sebastian Roth]

@george1421 said in rcu_sched stall OR kernel panic on PowerEdge R640:

And on the other side we really don’t know what the impact could be.

Yes, because of that I don’t like switching it on.

What can it hurt?

I don’t know. Stalls on older CPUs?

[Quazz]

@Sebastian-Roth

There are kernel flags to disable SMP if necessary, so I think it’s pretty safe to compile with MAXSMP. Just my opinion of course; without a diverse test fleet it’s hard to say for sure since kernels can always have bugs or unforeseen interactions. But that would be true for any change we make.

I can’t find anything googling about stalls/problems with MAXSMP either. Only some people on embedded systems who want to reduce the size of their kernel, but that’s a targetted compile anyway.

There will be more and more systems entering the floor with more than 8 cores (our current NR_CPU value) given the recent CPU releases as well, so at the very least that number could use a bump.

[Sebastian Roth]

@Quazz said in rcu_sched stall OR kernel panic on PowerEdge R640:

I can’t find anything googling about stalls/problems with MAXSMP either. Only some people on embedded systems who want to reduce the size of their kernel, but that’s a targetted compile anyway.

Ok, you and George have convinced me this is most probably not going to cause us much trouble, so I will add the options as mentioned below.

[Sebastian Roth]

@george1421 @Quazz I just added the two kernel options as mentioned below to the x64 kernel config (not pushed the change yet).

While CONFIG_INTEL_IDLE is available in x86 (32 bit) config as well CONFIG_MAXSMP is not (depends on X86_64 [=y]). Should I leave CONFIG_NR_CPUS set to 8 (default I think) or increase it to 16, 32, 64?

For ARM kernel config we don’t have CONFIG_INTEL_IDLE nor CONFIG_MAXSMP but can adjust CONFIG_NR_CPUS too. 16, 32, 64?

[Quazz]

@Sebastian-Roth I think it’s fine to leave X86 to 8 since I don’t think they make huge multicore 32 bit CPUs. (wouldn’t really make sense to me anyway) Though I also think it wouldn’t necessarily hurt to change it, why bother if it’s not needed? I also believe 8 is default for X86 anyway

As for ARM: https://www.phoronix.com/scan.php?page=news_item&px=ARM64-256-Default-NR_CPUS

Default in Linux 5.1 ARM 64 is 256 now. (current default being 64)

[Sebastian Roth]

@Quazz Thanks, done: https://github.com/FOGProject/fos/issues/31