linux-kernel - Re: [PATCH v1 4/6] perf threads: Move threads to its own files

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Date: Wed, 28 Feb 2024 15:43:59 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	Yang Jihong <yangjihong1@...wei.com>, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v1 4/6] perf threads: Move threads to its own files

On Tue, Feb 27, 2024 at 11:24 PM Ian Rogers <irogers@...gle.com> wrote:
>
> On Tue, Feb 27, 2024 at 10:40 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Tue, Feb 27, 2024 at 1:42 PM Ian Rogers <irogers@...gle.com> wrote:
> > >
> > > On Tue, Feb 27, 2024 at 11:17 AM Arnaldo Carvalho de Melo
> > > <acme@...nel.org> wrote:
> > > >
> > > > On Tue, Feb 27, 2024 at 09:31:33AM -0800, Namhyung Kim wrote:
> > > > > I can see some other differences like machine__findnew_thread()
> > > > > which I think is due to the locking change.  Maybe we can fix the
> > > > > problem before moving the code and let the code move simple.
> > > >
> > > > I was going to suggest that, agreed.
> > > >
> > > > We may start doing a refactoring, then find a bug, at that point we
> > > > first fix the problem them go back to refactoring.
> > >
> > > Sure I do this all the time. Your typical complaint on the v+1 patch
> > > set is to move the bug fixes to the front of the changes. On the v+2
> > > patch set the bug fixes get applied but not the rest of the patch
> > > series, etc.
> > >
> > > Here we are refactoring code for an rb-tree implementation of threads
> > > and worrying about its correctness. There's no indication it's not
> > > correct, it is largely copy and paste, there is also good evidence in
> > > the locking disciple it is more correct. The next patch deletes that
> > > implementation, replacing it with a hash table. Were I not trying to
> > > break things apart I could squash those 2 patches together, but I've
> > > tried to do the right thing. Now we're trying to micro correct, break
> > > apart, etc. a state that gets deleted. A reviewer could equally
> > > criticise this being 2 changes rather than 1, and the cognitive load
> > > of having to look at code that gets deleted. At some point it is a
> > > judgement call, and I think this patch is actually the right size. I
> > > think what is missing here is some motivation in the commit message to
> > > the findnew refactoring and so I'll add that.
> >
> > I'm not against your approach and actually appreciate your effort
> > to split rb-tree refactoring and hash table introduction.  What I'm
> > asking is just to separate out the code moving.  I think you can do
> > whatever you want in the current file.  Once you have the final code
> > you can move it to its own file exactly the same.  When I look at this
> > commit, say a few years later, I won't expect a commit that says
> > moving something to a new file has other changes.
>
> The problem is that the code in machine treats the threads lock as if
> it is a lock in machine. So there is __machine__findnew_thread which
> implies the thread lock is held. This change is making threads its own
> separate concept/collection and the lock belongs with that collection.
> Most of the implementation of threads__findnew matches
> __machine__findnew_thread, so we may be able to engineer a smaller
> line diff by moving "__machine__findnew_thread" code into threads.c,
> then renaming it to build the collection, etc. We could also build the
> threads collection inside of machine and then in a separate change
> move it to threads.[ch].  In the commit history this seems muddier
> than just splitting out threads as a collection. Also, some of the API
> design choices are motivated more by the hash table implementation of
> the next patch than trying to have a good rbtree abstracted collection
> of threads. Essentially it'd be engineering a collection of threads
> but only with a view to delete it in the next patch. I don't think it
> would be for the best and the commit history for deleted code is
> unlikely to be looked upon.

I think the conversation is repeating. :)  Why not do this?

1. refactor threads code in machine.c and fix the locking
2. move threads code to its own file
3. use hash table in threads

Thanks,
Namhyung