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

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Date:   Thu, 26 Oct 2023 19:54:33 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
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Subject: Re: [PATCH v2 28/39] timekeeping: Fix a circular include dependency

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...