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

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Date:   Thu, 13 Oct 2016 16:51:04 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH 1/3] sched/core,x86: make struct thread_info arch specific again

On Thu, Oct 13, 2016 at 4:41 PM, Mark Rutland <mark.rutland@....com> wrote:
> Hi,
>
> On Thu, Oct 13, 2016 at 01:57:10PM +0200, Heiko Carstens wrote:
>> 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.
>
> The exact same applies for arm64 on all counts. This is also simpler than both
> attempts I had at this, so FWIW:
>
> Acked-by: Mark Rutland <mark.rutland@....com>
>
> To make merging less painful, I guess we'll need a stable branch with (just)
> this and whatever patch we end up with for fixing current_thread_info(), so we
> can independently merge the arch-specific parts.
>
> I guess it'd make sense for the tip tree to host that?
>

I wonder if this could even make 4.9.  It's pretty clearly a no-op.  Ingo?