Re: [GIT PULL] First batch of KVM changes for 4.1

From: Peter Zijlstra <peterz@infradead.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
	gleb@kernel.org, kvm@vger.kernel.org,
	Ralf Baechle <ralf@linux-mips.org>,
	mtosatti@redhat.com, luto@kernel.org
Subject: Re: [GIT PULL] First batch of KVM changes for 4.1
Date: Fri, 17 Apr 2015 12:36:54 +0200	[thread overview]
Message-ID: <20150417103654.GE5029@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <5530DBED.5080508@redhat.com>

On Fri, Apr 17, 2015 at 12:09:49PM +0200, Paolo Bonzini wrote:
> 
> 
> On 17/04/2015 11:17, Peter Zijlstra wrote:
> > On Fri, Apr 17, 2015 at 10:52:38AM +0200, Peter Zijlstra wrote:
> >> On Fri, Apr 10, 2015 at 05:01:29PM +0200, Paolo Bonzini wrote:
> >>>  include/linux/sched.h                              |    8 +
> >>>  kernel/sched/core.c                                |   15 +
> >>
> >> Can you please not puke over the scheduler without Acks from at least
> >> one maintainer?
> 
> Sorry, this was done while I was not handling the KVM tree.  At the very
> least the commit message should have included the original hashes of the
> commit and the revert.  This way one could have found the original Acks:
> 
>     commit 582b336ec2c0f0076f5650a029fcc9abd4a906f7
>     Author: Marcelo Tosatti <mtosatti@redhat.com>
>     Date:   Tue Nov 27 23:28:54 2012 -0200
> 
>     sched: add notifier for cross-cpu migrations
> 
>     Originally from Jeremy Fitzhardinge.
> 
>     Acked-by: Ingo Molnar <mingo@redhat.com>
>     Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Still not a good reason to sneak it back it now, after I got it taken
out. There was a reason it was removed, prior acks (esp. 2 year old
ones) do not count one whit _NOW_.

Also, Ingo later agreed that is was a mistake,

  http://marc.info/?l=linux-kernel&m=137346715521978&w=2

which is an effective retract of whatever ACK that was.

It was crap code then and its crap code now.

> >> I complained about this very thing two years ago:
> >>
> >>   http://marc.info/?l=linux-kernel&m=137345253916751
> >>
> >> And now it magically re-appears WTF!
> > 
> > And I really don't understand _why_ you need that extra callback in the
> > first place. You already have preempt notifiers, just track if you came
> > in on another cpu than you went out on and voila!
> 
> Then you pay for _all_ preemptions of _all_ processes in the guest,
> instead of the hopefully rare ones that do a CPU migration.

Now you make everybody pay for your crap, x86-64 paravirt or not. Keep
the cost by those who need it.

Please take it out, ASAP.