idle issues running sembench on 128 cpus

[Dave Kleikamp]

Thomas,
I've been looking at performance running sembench on a 128-cpu system 
and I'm running into some issues in the idle loop.

Initially, I was seeing a lot of contention on the clockevents_lock in 
clockevents_notify(). Assuming it is only protecting clockevents_chain, 
and not the handlers themselves, I changed this to an rwlock (with 
thoughts of using rcu if successful).

This didn't help, but exposed an underlying problem with high contention 
on tick_broadcast_lock in tick_broadcast_oneshot_control(). I think with 
this many cpus, tick_handle_oneshot_broadcast() is holding that lock a 
long time, causing the idle cpus to spin on the lock.

I am able to avoid this problem with either kernel parameter, 
"idle=mwait" or "processor.max_cstate=1". Similarly, defining 
CONFIG_INTEL_IDLE=y and using the kernel parameter 
intel_idle.max_cstate=1 exposes a different spinlock, pm_qos_lock, but I 
found this patch which fixes that contention:
https://lists.linux-foundation.org/pipermail/linux-pm/2011-February/030266.html
https://patchwork.kernel.org/patch/550721/

Of course, we'd like to find a way to reduce the spinlock contention and 
not resort to prohibiting the cpus from entering C3 state at all. I 
don't see a simple fix, and want to know if you've seen anything like 
this before and given it any thought.

I also don't know if it makes sense to be able to tune the cpuidle 
governors to add more resistance to enter the C3 state, or even being 
able to switch to a performance governor at runtime, similar to cpufreq.

I'd like to hear your thoughts before I dive any deeper into this.

Thanks,
Shaggy
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