linux-kernel - Re: [PATCH tip/core/rcu 04/13] rcu: Make RCU_FANOUT_LEAF help text more explicit about skew

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Message-ID: <20170419154040.knkdg2j6awrp74ua@hirez.programming.kicks-ass.net>
Date:   Wed, 19 Apr 2017 17:40:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 Wed, Apr 19, 2017 at 08:08:09AM -0700, Paul E. McKenney wrote:
> And even that would not be completely sufficient.  After all, the state
> in the leaf rcu_node structure will be out of date during grace-period
> initialization and cleanup.  So to -completely- synchronize state for
> the incoming CPU, I would have to acquire the root rcu_node structure's
> lock and look at the live state.  Needless to say, the performance and
> scalability implications of acquiring a global lock on each and every
> idle exit event is not going to be at all pretty.

Arguably you could use a seqlock to read the global state. Will still
ponder things a bit more, esp. those bugs you pointed me at from just
reading gpnum.