linux-kernel - Re: [PATCH 1/3] sched/core,x86: make struct thread

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Date:   Wed, 19 Oct 2016 16:19:46 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Mark Rutland <mark.rutland@....com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/3] sched/core,x86: make struct thread_info arch specific again

On Wed, Oct 19, 2016 at 11:28 AM, Mark Rutland <mark.rutland@....com> wrote:
> From: Heiko Carstens <heiko.carstens@...ibm.com>
>
> commit c65eacbe290b ("sched/core: Allow putting thread_info into
> task_struct") made struct thread_info a generic struct with only a
> single flags member if THREAD_INFO_IN_TASK_STRUCT is selected.
>
> This change however seems to be quite x86 centric, since at least the
> generic preemption code (asm-generic/preempt.h) assumes that struct
> thread_info also has a preempt_count member, which apparently was not
> true for x86.
>
> We could add a bit more ifdefs to solve this problem too, but it seems
> to be much simpler to make struct thread_info arch specific
> again. This also makes the conversion to THREAD_INFO_IN_TASK_STRUCT a
> bit easier for architectures that have a couple of arch specific stuff
> in their thread_info definition.
>
> The arch specific stuff _could_ be moved to thread_struct. However
> keeping them in thread_info makes it easier: accessing thread_info
> members is simple, since it is at the beginning of the task_struct,
> while the thread_struct is at the end. At least on s390 the offsets
> needed to access members of the thread_struct (with task_struct as
> base) are too large for various asm instructions.  This is not a
> problem when keeping these members within thread_info.

Acked-by: Andy Lutomirski <luto@...nel.org>

Ingo, there's a (somewhat weak) argument for sending this via
tip/urgent: it doesn't change generated code at all, and I think it
will avoid a silly depedency or possible conflict for the next merge
window, since both arm64 and s390 are going to need it.

--Andy