linux-kernel - Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU implementation

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Date:	Tue, 02 Sep 2008 15:41:54 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	cl@...ux-foundation.org, mingo@...e.hu, akpm@...ux-foundation.org,
	manfred@...orfullife.com, dipankar@...ibm.com,
	josht@...ux.vnet.ibm.com, schamp@....com, niv@...ibm.com,
	dvhltc@...ibm.com, ego@...ibm.com, laijs@...fujitsu.com,
	rostedt@...dmis.org
Subject: Re: [PATCH, RFC, tip/core/rcu] v3 scalable classic RCU
	implementation

On Tue, 2008-09-02 at 09:26 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (a.p.zijlstra@...llo.nl) wrote:
> > On Sat, 2008-08-30 at 07:10 -0700, Paul E. McKenney wrote:
> > > On Sat, Aug 30, 2008 at 11:33:00AM +0200, Peter Zijlstra wrote:
> > > > On Fri, 2008-08-29 at 17:49 -0700, Paul E. McKenney wrote:
> > > > 
> > > > > Some shortcomings:
> > > > > 
> > > > > o	Entering and leaving dynticks idle mode is a quiescent state,
> > > > > 	but the current patch doesn't take advantage of this (noted
> > > > > 	by Manfred).  It appears that it should be possible to make
> > > > > 	nmi_enter() and nmi_exit() provide an in_nmi(), which would make
> > > > > 	it possible for rcu_irq_enter() and rcu_irq_exit() to figure
> > > > > 	out whether it is safe to tell RCU about the quiescent state --
> > > > > 	and also greatly simplify the code.
> > > > 
> > > > Already done and available in the -tip tree, curtesy of Mathieu.
> > > 
> > > Very cool!!!  I see one of his patches at http://lkml.org/lkml/2008/4/17/342,
> > > but how do I find out which branch of -tip this is on?  (I am learning
> > > git, but it is a slow process...)
> > > 
> > > This would also simplify preemptable RCU's dyntick interface, removing
> > > the need for proofs.
> > 
> > Not sure - my git-foo isn't good enough either :-(
> > 
> > All I can offer is that its available in tip/master (the collective
> > merge of all of tip's branches) as commit:
> > 0d84b78a606f1562532cd576ee8733caf5a4aed3, which I found using
> > git-annotate include/linux/hardirq.h
> > 
> > How to find from which particular topic branch it came from, I too am
> > clueless.
> > 
> 
> If you're interested in knowing the topic it came from : it's required
> so a following patch can use a "popf; ret" instead of iret to return
> from trap handlers executed in NMI context. There is an architectural
> problem on x86 causing NMIs to be reactivated after the first iret
> encountered, which leads to NMI handler races if nmi handlers trap. This
> works around the problem by returning from the trap handlers without
> using the iret instruction.
> 
> It's useful to Immediate Values which put a temporary breakpoint in the
> instruction stream when proceeding to code modification and also useful
> to LTTng (available in the -lttng tree) which writes tracing data to
> vmap'd memory buffers (which can cause a minor page fault).
> 
> I'm glad to see NMI context detection is useful to others too !

While an interesting detail, its not the answer to the question.

Given a bunch of topic branches, and a branch that has all those topic
merged, how, for any particular commit from the merge branch, do you
find from which topic branch it originiated?

IOW, the answer to the above question would have been a series of git
commands that would have resulted in something like tip/tracing/nmisafe



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