Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick

[Peter Zijlstra]

On Thu, Apr 13, 2017 at 10:31:00AM -0700, Paul E. McKenney wrote:
> On Thu, Apr 13, 2017 at 07:04:34PM +0200, Peter Zijlstra wrote:

> > And I have vague memories of it actually causing lock contention, but
> > I've forgotten how that worked.
> 
> That is a new one on me.  I can easily see how not skewing ticks could
> cause serious lock contention, but am missing how skewed ticks would
> do so.

It could've been something like cacheline bouncing. Where with a
synchronized tick, the (global) cacheline would get used by all CPUs on
a node before heading out to the next node etc.. Where with a skewed
tick, it would forever bounce around.