linux-kernel - Re: RCU branching for the v4.19 merge window

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Message-Id: <20180522160534.GT3803@linux.vnet.ibm.com>
Date:   Tue, 22 May 2018 09:05:34 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: RCU branching for the v4.19 merge window

On Tue, May 22, 2018 at 11:27:36AM -0400, Steven Rostedt wrote:
> On Thu, 17 May 2018 07:40:44 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > Hello, Steve!
> > 
> > Another year, another difficult-to-branch set of RCU commits.
> > 
> > In happy contrast to last year, I can make some branches (SRCU, some
> > of the torture commits, and a few miscellaneous commits), but I will
> > likely end up with several short branches and one huge one.  My thought
> > is to keep the long branch, but email the patches out in a few separate
> > serieses, with each depending on its predecessor.  For example, one series
> > from the big branch would be folding the ->gpnum and ->completed fields
> > into a single ->gp_seq, which helps the RCU-flavor consolidation task.
> > Another series suppresses some rare false-positive splats that have been
> > plaguing me for more than a year.  Yet another series within this huge
> > branch applies and optimizes funnel locking for grace-period startup.
> > 
> > The problem is that the conversion to ->gp_seq has a very large footprint,
> > which of course generates lots of conflicts.  I could of course collapse
> > these commits into a single commit, but if I did that I would also defer
> > to the merge window following v4.19 due to the resulting loss of bisection
> > within that change.
> > 
> > Any advice?
> > 
> > The commits are for-mingo..rcu/dev in my -rcu tree.
> 
> I don't see these branches (and I don't pull tags).

You don't see them yet because I don't create them until after -rc1 time,
which is a few weeks out.  If you go far enough down from rcu/dev you
will see branches (about 90 commits down from HEAD), but these branches
are already in -tip for v4.18.

> How bad are the conflicts? Or is it too late to respond to help (sorry,
> was on vacation :-)

The conflicts are already causing me substantial hassles when rebasing
bug fixes back to the buggy commits, so the conflicts are non-trivial.
Hence my reaching out to you, given your discomfort with last year's
long-chain RCU submission.

And you do have some time to respond.  ;-)

							Thanx, Paul