linux-kernel - Re: Slow DOWN, please!!!

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Date:	Wed, 30 Apr 2008 15:08:26 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Noe <dpn@...merica.net>, davem@...emloft.net,
	linux-kernel@...r.kernel.org, jirislaby@...il.com,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: Slow DOWN, please!!!



On Wed, 30 Apr 2008, Rafael J. Wysocki wrote:
> 
> How bisectable is linux-next, BTW?

Each _individual_ release will be entirely bisectable, since it's all git 
trees, and at no point does anything collapse individual commits together 
like -mm does.

HOWEVER. 

Due to the way linux-next works, each individual release will be basically 
unrelated to the previous one, so it gets a bit more exciting indeed when 
you say "the last linux-next version worked for me, but the current one 
does not".

Git can actually do this - you can make the previous (good) linux-next 
version be one branch, and the not-directly-related next linux-next build 
be another, and then "git bisect" will _technically_ work, but:

 - it will not necessarily be as efficient (because the linux-next trees 
   will have re-done all the merges, so there will be new commits and 
   patterns in between them)

 - but much more distressingly, if the individual git trees that got 
   merged into linux-next were also using rebasing etc, now even all the 
   *base* commits will be different, and saying that the old release was 
   good tells you almost nothing about the new release!

   (The good news is that if only a couple of trees do that, the bisection 
   information from the other trees that don't do it will still be valid 
   and useful and help bisection)

 - also, while it's very easy for somebody who knows and understands git 
   branches, it's technically still quite a bit more challenging than just 
   following a single tree that never rebases (ie mine) and just bisecting 
   within that one.

So yes, git bisect will work in linux-next, and the fundamental nature of 
git-bisect will not change at all, but it's going to be a bit weaker 
"between different versions" of linux-next than it would be for the normal 
git tree that doesn't do the "merge different trees all over again" thing 
that linux-next does.

		Linus
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