Message-ID: <xbehqbtjp5wi4z2ppzrbmlj6vfazd2w5flz3tgjbo37tlisexa@caq633gciggt>
Date: Tue, 13 Feb 2024 18:12:38 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
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Subject: Re: [PATCH v3 00/35] Memory allocation profiling

On Wed, Feb 14, 2024 at 12:02:30AM +0100, David Hildenbrand wrote:
> On 13.02.24 23:59, Suren Baghdasaryan wrote:
> > On Tue, Feb 13, 2024 at 2:50 PM Kent Overstreet
> > <kent.overstreet@...ux.dev> wrote:
> > > 
> > > On Tue, Feb 13, 2024 at 11:48:41PM +0100, David Hildenbrand wrote:
> > > > On 13.02.24 23:30, Suren Baghdasaryan wrote:
> > > > > On Tue, Feb 13, 2024 at 2:17 PM David Hildenbrand <david@...hat.com> wrote:
> > > > > > 
> > > > > > On 13.02.24 23:09, Kent Overstreet wrote:
> > > > > > > On Tue, Feb 13, 2024 at 11:04:58PM +0100, David Hildenbrand wrote:
> > > > > > > > On 13.02.24 22:58, Suren Baghdasaryan wrote:
> > > > > > > > > On Tue, Feb 13, 2024 at 4:24 AM Michal Hocko <mhocko@...e.com> wrote:
> > > > > > > > > > 
> > > > > > > > > > On Mon 12-02-24 13:38:46, Suren Baghdasaryan wrote:
> > > > > > > > > > [...]
> > > > > > > > > > > We're aiming to get this in the next merge window, for 6.9. The feedback
> > > > > > > > > > > we've gotten has been that even out of tree this patchset has already
> > > > > > > > > > > been useful, and there's a significant amount of other work gated on the
> > > > > > > > > > > code tagging functionality included in this patchset [2].
> > > > > > > > > > 
> > > > > > > > > > I suspect it will not come as a surprise that I really dislike the
> > > > > > > > > > implementation proposed here. I will not repeat my arguments, I have
> > > > > > > > > > done so on several occasions already.
> > > > > > > > > > 
> > > > > > > > > > Anyway, I didn't go as far as to nak it even though I _strongly_ believe
> > > > > > > > > > this debugging feature will add a maintenance overhead for a very long
> > > > > > > > > > time. I can live with all the downsides of the proposed implementation
> > > > > > > > > > _as long as_ there is a wider agreement from the MM community as this is
> > > > > > > > > > where the maintenance cost will be payed. So far I have not seen (m)any
> > > > > > > > > > acks by MM developers so aiming into the next merge window is more than
> > > > > > > > > > little rushed.
> > > > > > > > > 
> > > > > > > > > We tried other previously proposed approaches and all have their
> > > > > > > > > downsides without making maintenance much easier. Your position is
> > > > > > > > > understandable and I think it's fair. Let's see if others see more
> > > > > > > > > benefit than cost here.
> > > > > > > > 
> > > > > > > > Would it make sense to discuss that at LSF/MM once again, especially
> > > > > > > > covering why proposed alternatives did not work out? LSF/MM is not "too far"
> > > > > > > > away (May).
> > > > > > > > 
> > > > > > > > I recall that the last LSF/MM session on this topic was a bit unfortunate
> > > > > > > > (IMHO not as productive as it could have been). Maybe we can finally reach a
> > > > > > > > consensus on this.
> > > > > > > 
> > > > > > > I'd rather not delay for more bikeshedding. Before agreeing to LSF I'd
> > > > > > > need to see a serious proposl - what we had at the last LSF was people
> > > > > > > jumping in with half baked alternative proposals that very much hadn't
> > > > > > > been thought through, and I see no need to repeat that.
> > > > > > > 
> > > > > > > Like I mentioned, there's other work gated on this patchset; if people
> > > > > > > want to hold this up for more discussion they better be putting forth
> > > > > > > something to discuss.
> > > > > > 
> > > > > > I'm thinking of ways on how to achieve Michal's request: "as long as
> > > > > > there is a wider agreement from the MM community". If we can achieve
> > > > > > that without LSF, great! (a bi-weekly MM meeting might also be an option)
> > > > > 
> > > > > There will be a maintenance burden even with the cleanest proposed
> > > > > approach.
> > > > 
> > > > Yes.
> > > > 
> > > > > We worked hard to make the patchset as clean as possible and
> > > > > if benefits still don't outweigh the maintenance cost then we should
> > > > > probably stop trying.
> > > > 
> > > > Indeed.
> > > > 
> > > > > At LSF/MM I would rather discuss functonal
> > > > > issues/requirements/improvements than alternative approaches to
> > > > > instrument allocators.
> > > > > I'm happy to arrange a separate meeting with MM folks if that would
> > > > > help to progress on the cost/benefit decision.
> > > > Note that I am only proposing ways forward.
> > > > 
> > > > If you think you can easily achieve what Michal requested without all that,
> > > > good.
> > > 
> > > He requested something?
> > 
> > Yes, a cleaner instrumentation. Unfortunately the cleanest one is not
> > possible until the compiler feature is developed and deployed. And it
> > still would require changes to the headers, so don't think it's worth
> > delaying the feature for years.
> > 
> 
> I was talking about this: "I can live with all the downsides of the proposed
> implementationas long as there is a wider agreement from the MM community as
> this is where the maintenance cost will be payed. So far I have not seen
> (m)any acks by MM developers".
> 
> I certainly cannot be motivated at this point to review and ack this,
> unfortunately too much negative energy around here.

David, this kind of reaction is exactly why I was telling Andrew I was
going to submit this as a direct pull request to Linus.

This is an important feature; if we can't stay focused ot the technical
and get it done that's what I'll do.

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