linux-kernel - Re: [PATCH v2 28/39] timekeeping: Fix a circular include dependency

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Date:   Fri, 27 Oct 2023 08:28:08 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>,
        Thomas Gleixner <tglx@...utronix.de>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH v2 28/39] timekeeping: Fix a circular include dependency

On Thu, Oct 26, 2023 at 11:35 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Fri, Oct 27, 2023, at 01:54, Kent Overstreet wrote:
> > On Fri, Oct 27, 2023 at 01:05:48AM +0200, Thomas Gleixner wrote:
> >> On Thu, Oct 26 2023 at 18:33, Suren Baghdasaryan wrote:
> >> > On Wed, Oct 25, 2023 at 5:33 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >> >> > This avoids a circular header dependency in an upcoming patch by only
> >> >> > making hrtimer.h depend on percpu-defs.h
> >> >>
> >> >> What's the actual dependency problem?
> >> >
> >> > Sorry for the delay.
> >> > When we instrument per-cpu allocations in [1] we need to include
> >> > sched.h in percpu.h to be able to use alloc_tag_save(). sched.h
> >>
> >> Including sched.h in percpu.h is fundamentally wrong as sched.h is the
> >> initial place of all header recursions.
> >>
> >> There is a reason why a lot of funtionalitiy has been split out of
> >> sched.h into seperate headers over time in order to avoid that.
> >
> > Yeah, it's definitely unfortunate. The issue here is that
> > alloc_tag_save() needs task_struct - we have to pull that in for
> > alloc_tag_save() to be inline, which we really want.
> >
> > What if we moved task_struct to its own dedicated header? That might be
> > good to do anyways...
>
> Yes, I agree that is the best way to handle it. I've prototyped
> a more thorough header cleanup with good results (much improved
> build speed) in the past, and most of the work to get there is
> to seperate out structures like task_struct, mm_struct, net_device,
> etc into headers that only depend on the embedded structure
> definitions without needing all the inline functions associated
> with them.

This is something I'll add to our automation todos which I plan to
talk about at plumbers; I feel like it should be possible to write a
script that given a header and identifier can split whatever
declaration out into a new header, update the old header, then add the
necessary includes for the newly created header to each dependent
(optional).
-- 
Thanks,
~Nick Desaulniers