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Date: Wed, 24 Jun 2015 19:58:30 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>, tj@kernel.org, mingo@redhat.com,
        linux-kernel@vger.kernel.org, der.herr@hofr.at, dave@stgolabs.net,
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        torvalds@linux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock
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On Wed, Jun 24, 2015 at 10:10:17AM -0700, Paul E. McKenney wrote:
> > The thing is, once you start bailing on this condition your 'queue'
> > drains very fast and this is around the same time sync_rcu() would've
> > released the waiters too.
> 
> In my experience, this sort of thing simply melts down on large systems.
> I am reworking this with multiple locks so as to keep the large-system
> contention down to a dull roar.

So with the MCS queue we're got less global trashing than you had with
the start/done tickets. Only the queue head on enqueue.