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Date: Thu, 13 Apr 2017 12:42:28 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Peter Zijlstra <peterz@...radead.org> Cc: linux-kernel@...r.kernel.org, mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com, akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com, josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com Subject: Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew_tick On Thu, Apr 13, 2017 at 08:29:39PM +0200, Peter Zijlstra wrote: > 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. In other words, motivating the order of the skewed ticks to be guided by hardware locality? Thanx, Paul
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