Re: [PATCH RFC tip/core/rcu] Parallelize and economize NOCB kthread wakeups

[Peter Zijlstra]

On Fri, Jun 27, 2014 at 07:20:38AM -0700, Paul E. McKenney wrote:
> An 80-CPU system with a context-switch-heavy workload can require so
> many NOCB kthread wakeups that the RCU grace-period kthreads spend several
> tens of percent of a CPU just awakening things.  This clearly will not
> scale well: If you add enough CPUs, the RCU grace-period kthreads would
> get behind, increasing grace-period latency.
> 
> To avoid this problem, this commit divides the NOCB kthreads into leaders
> and followers, where the grace-period kthreads awaken the leaders each of
> whom in turn awakens its followers.  By default, the number of groups of
> kthreads is the square root of the number of CPUs, but this default may
> be overridden using the rcutree.rcu_nocb_leader_stride boot parameter.
> This reduces the number of wakeups done per grace period by the RCU
> grace-period kthread by the square root of the number of CPUs, but of
> course by shifting those wakeups to the leaders.  In addition, because
> the leaders do grace periods on behalf of their respective followers,
> the number of wakeups of the followers decreases by up to a factor of two.
> Instead of being awakened once when new callbacks arrive and again
> at the end of the grace period, the followers are awakened only at
> the end of the grace period.
> 
> For a numerical example, in a 4096-CPU system, the grace-period kthread
> would awaken 64 leaders, each of which would awaken its 63 followers
> at the end of the grace period.  This compares favorably with the 79
> wakeups for the grace-period kthread on an 80-CPU system.

Urgh, how about we kill the entire nocb nonsense and try again? This is
getting quite rediculous.

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