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

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Message-ID: <20161020093353.GB10234@leverpostej>
Date:   Thu, 20 Oct 2016 10:33:53 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        "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 Thu, Oct 20, 2016 at 08:40:45AM +0200, Ingo Molnar wrote:
> 
> * Andy Lutomirski <luto@...capital.net> wrote:
> 
> > 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.
> 
> Can certainly do it if this is the final version of the patch. Mark?

Yes; this is the final version of this patch.

I can rebase the other two core patches atop, assuming this goes in for
a v4.9-rc* tag soon.

Thanks,
Mark.