linux-kernel - Re: [PATCH 0/3] nohz: Convert tick dependency mask to atomic

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Date:	Thu, 31 Mar 2016 08:54:19 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/3] nohz: Convert tick dependency mask to atomic_t


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> On Tue, Mar 29, 2016 at 03:05:14PM +0200, Ingo Molnar wrote:
> > 
> > * Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > > 
> > > * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > > 
> > > > On Tue, Mar 29, 2016 at 4:44 AM, Ingo Molnar <mingo@...nel.org> wrote:
> > > > >
> > > > > Harmonizing thread_info::flags does not look easy, given how much assembly code
> > > > > accesses this field.
> > > > 
> > > > It might not be too bad.
> > > > 
> > > > For 32-bit architectures (which is still most of them), it's just a
> > > > 
> > > >    unsigned int/long -> atomic_t
> > > > 
> > > > and for 64-bit architectures you end up with three choices:
> > > > 
> > > >  - it's already 32-bit (alpha, ia64, x86):
> > > > 
> > > >         unsigned int -> atomic_t
> > > > 
> > > >  - little-endian long:
> > > > 
> > > >         atomic_t flags
> > > >         unsigned int padding;
> > > > 
> > > >  - big-endian long (only powerpc? Maybe there's a big-endian MIPS still?)
> > > > 
> > > >         unsigned int padding;
> > > >         atomic_t flags;
> > > 
> > > Hm, that indeed sounds fairly nice and doable - I thought some architectures do 
> > > have a task flag above bit 31, but that does not appear to be so ...
> > > 
> > > Right now we seem to have 27 bits defined in include/linux/sched.h, with 5 more 
> > > bits left for the future. Here's their current usage histogram in the kernel 
> > > source:
> > > 
> > >   PF_KTHREAD                    : 68
> > >   PF_MEMALLOC                   : 65
> > 
> > Argh, my reading comprehension skills suck today.
> > 
> > That's a totally useless analysis of task_struct::flags, while we want to convert 
> > thread_info::flags...
> 
> Actually we want to convert that one too :-)
> In fact I planned to start there.

Sounds good to me! I also volunteer the x86 architecture to be the guinea pig to 
convert thread_info::flags to atomic_t ;-) [*]

Thanks,

	Ingo