Date:   Thu, 16 Apr 2020 19:07:13 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "sashal@...nel.org" <sashal@...nel.org>,
        "leon@...nel.org" <leon@...nel.org>
CC:     "ecree@...arflare.com" <ecree@...arflare.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "gerlitz.or@...il.com" <gerlitz.or@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH AUTOSEL 4.9 09/26] net/mlx5e: Init ethtool steering for
 representors

On Thu, 2020-04-16 at 09:30 -0400, Sasha Levin wrote:
> On Thu, Apr 16, 2020 at 08:24:09AM +0300, Leon Romanovsky wrote:
> > On Thu, Apr 16, 2020 at 04:08:10AM +0000, Saeed Mahameed wrote:
> > > On Wed, 2020-04-15 at 20:00 -0400, Sasha Levin wrote:
> > > > On Wed, Apr 15, 2020 at 05:18:38PM +0100, Edward Cree wrote:
> > > > > Firstly, let me apologise: my previous email was too harsh
> > > > > and too
> > > > >  assertiveabout things that were really more uncertain and
> > > > > unclear.
> > > > > 
> > > > > On 14/04/2020 21:57, Sasha Levin wrote:
> > > > > > I've pointed out that almost 50% of commits tagged for
> > > > > > stable do
> > > > > > not
> > > > > > have a fixes tag, and yet they are fixes. You really deduce
> > > > > > things based
> > > > > > on coin flip probability?
> > > > > Yes, but far less than 50% of commits *not* tagged for stable
> > > > > have
> > > > > a fixes
> > > > >  tag.  It's not about hard-and-fast Aristotelian
> > > > > "deductions", like
> > > > > "this
> > > > >  doesn't have Fixes:, therefore it is not a stable
> > > > > candidate", it's
> > > > > about
> > > > >  probabilistic "induction".
> > > > > 
> > > > > > "it does increase the amount of countervailing evidence
> > > > > > needed to
> > > > > > conclude a commit is a fix" - Please explain this argument
> > > > > > given
> > > > > > the
> > > > > > above.
> > > > > Are you familiar with Bayesian statistics?  If not, I'd
> > > > > suggest
> > > > > reading
> > > > >  something like http://yudkowsky.net/rational/bayes/ which
> > > > > explains
> > > > > it.
> > > > > There's a big difference between a coin flip and a
> > > > > _correlated_
> > > > > coin flip.
> > > > 
> > > > I'd maybe point out that the selection process is based on a
> > > > neural
> > > > network which knows about the existence of a Fixes tag in a
> > > > commit.
> > > > 
> > > > It does exactly what you're describing, but also taking a bunch
> > > > more
> > > > factors into it's desicion process ("panic"? "oops"?
> > > > "overflow"?
> > > > etc).
> > > > 
> > > 
> > > I am not against AUTOSEL in general, as long as the decision to
> > > know
> > > how far back it is allowed to take a patch is made
> > > deterministically
> > > and not statistically based on some AI hunch.
> > > 
> > > Any auto selection for a patch without a Fixes tags can be
> > > catastrophic
> > > .. imagine a patch without a Fixes Tag with a single line that is
> > > fixing some "oops", such patch can be easily applied cleanly to
> > > stable-
> > > v.x and stable-v.y .. while it fixes the issue on v.x it might
> > > have
> > > catastrophic results on v.y ..
> > 
> > I tried to imagine such flow and failed to do so. Are you talking
> > about
> > anything specific or imaginary case?
> 
> It happens, rarely, but it does. However, all the cases I can think
> of
> happened with a stable tagged commit without a fixes where it's
> backport
> to an older tree caused unintended behavior (local denial of service
> in
> one case).
> 
> The scenario you have in mind is true for both stable and non-stable
> tagged patches, so it you want to restrict how we deal with commits
> that
> don't have a fixes tag shouldn't it be true for *all* commits?

All commits? even the ones without "oops" in them ? where does this
stop ? :) 
We _must_ have a hard and deterministic cut for how far back to take a
patch based on a human decision.. unless we are 100% positive
autoselection AI can never make a mistake.

Humans are allowed to make mistakes, AI is not.

If a Fixes tag is wrong, then a human will be blamed, and that is
perfectly fine, but if we have some statistical model that we know it
is going to be wrong 0.001% of the time.. and we still let it run..
then something needs to be done about this.

I know there are benefits to autosel, but overtime, if this is not
being audited, many pieces of the kernel will get broken unnoticed
until some poor distro decides to upgrade their kernel version.


> 
> > <...>
> > > > Let me put my Microsoft employee hat on here. We have
> > > > driver/net/hyperv/
> > > > which definitely wasn't getting all the fixes it should have
> > > > been
> > > > getting without AUTOSEL.
> > > > 
> > > 
> > > until some patch which shouldn't get backported slips through,
> > > believe
> > > me this will happen, just give it some time ..
> > 
> > Bugs are inevitable, I don't see many differences between bugs
> > introduced by manually cherry-picking or automatically one.
> 
> Oh bugs slip in, that's why I track how many bugs slipped via stable
> tagged commits vs non-stable tagged ones, and the statistic may
> surprise
> you.
> 

Statistics do not matter here, what really matters is that there is a
possibility of a non-human induced error, this should be a no no.
or at least make it an opt-in thing for those who want to take their
chances and keep a close eye on it..

> The solution here is to beef up your testing infrastructure rather
> than

So please let me opt-in until I beef up my testing infra.

> taking less patches; we still want to have *all* the fixes, right?
> 

if you can be sure 100% it is the right thing to do, then yes, please
don't hesitate to take that patch, even without asking anyone !!

Again, Humans are allowed to make mistakes.. AI is not.


> > Of course, it is true if this automatically cherry-picking works as
> > expected and evolving.
> > 
> > > > While net/ is doing great, drivers/net/ is not. If it's indeed
> > > > following
> > > > the same rules then we need to talk about how we get done
> > > > right.
> > > > 
> > > 
> > > both net and drivers/net are managed by the same maitainer and
> > > follow
> > > the same rules, can you elaborate on the difference ?
> > 
> > The main reason is a difference in a volume between net and
> > drivers/net.
> > While net/* patches are watched by many eyes and carefully selected
> > to be
> > ported to stable@, most of the drivers/net patches are not.
> > 
> > Except 3-5 the most active drivers, rest of the driver patches
> > almost never
> > asked to be backported.
> 
> Right, that's exactly my point: If you're not Mellanox, e1000*, etc
> you
> won't see it, but the smaller drivers aren't getting the same
> handling
> as the big ones.
> 
> I think that we all love the work DaveM does with net/ - it makes our
> lives a lot easier, and if the same thing would happen with
> drivers/net/
> I'll happily go away and never AUTOSEL a *net* commit, but looking at
> how our Hyper-V drivers look like it's clearly not there yet.
> 

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