Date: Wed, 14 Feb 2024 00:22:08 +0100
From: David Hildenbrand <david@...hat.com>
To: Kent Overstreet <kent.overstreet@...ux.dev>
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 14.02.24 00:12, Kent Overstreet wrote:
> 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.

Kent, I started this with "Would it make sense" in an attempt to help 
Suren and you to finally make progress with this, one way or the other. 
I know that there were ways in the past to get the MM community to agree 
on such things.

I tried to be helpful, finding ways *not having to* bypass the MM 
community to get MM stuff merged.

The reply I got is mostly negative energy.

So you don't need my help here, understood.

But I will fight against any attempts to bypass the MM community.

-- 
Cheers,

David / dhildenb

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