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

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Date:	Fri, 25 Mar 2016 14:17:34 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.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

On Fri, Mar 25, 2016 at 09:48:47AM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@...il.com> wrote:
> 
> > As per Linus suggestion, lets convert the tick dependency mask to
> > atomic_t. Introduce atomic_fetch_or() and confine fetch_or() back to
> > scheduler guts.
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > 	timers/nohz
> > 
> > HEAD: 7b7e5da5733f58668181077ec394a718e08c392c
> > 
> > Thanks,
> > 	Frederic
> > ---
> > 
> > Frederic Weisbecker (3):
> >       atomic: Introduce atomic_fetch_or
> >       nohz: Convert tick dependency mask to atomic_t
> >       Revert "atomic: Export fetch_or()"
> > 
> > 
> >  include/linux/atomic.h   | 34 +++++++++++++--------------
> >  include/linux/sched.h    |  4 ++--
> >  kernel/sched/core.c      | 18 ++++++++++++++
> >  kernel/time/tick-sched.c | 61 ++++++++++++++++++++++++------------------------
> >  kernel/time/tick-sched.h |  2 +-
> >  5 files changed, 68 insertions(+), 51 deletions(-)
> 
> Could you please also convert the sched/core.c usage, so that we can get rid of 
> the private fetch_or() definition? Please also double check that it does not 
> result in worse code generation.

That involve converting thread_info::flags to atomic_t and given how much the type varies
across architectures:

       $ grep 'flags;' $(ls arch/*/include/asm/thread_info.h)
       arch/alpha/include/asm/thread_info.h:	unsigned int  flags;		/* low level flags */
       arch/arc/include/asm/thread_info.h:	unsigned long flags;			/* low level flags */
       arch/arm64/include/asm/thread_info.h:	unsigned long flags; 	     /* low level flags */
       arch/arm/include/asm/thread_info.h:	unsigned long flags; 	  /* low level flags */
       arch/avr32/include/asm/thread_info.h:	unsigned long flags;       /* low level flags */
       arch/blackfin/include/asm/thread_info.h:	 unsigned long flags;					  /* low level flags */
       arch/c6x/include/asm/thread_info.h:		 unsigned long flags;       /* low level flags */
       arch/cris/include/asm/thread_info.h:		 unsigned long flags; 	    /* low level flags */
       arch/frv/include/asm/thread_info.h:		 unsigned long flags; 	 /* low level flags */
       arch/h8300/include/asm/thread_info.h:		 unsigned long flags;	    /* low level flags */
       arch/hexagon/include/asm/thread_info.h:		 unsigned long flags;          /* low level flags */
       arch/ia64/include/asm/thread_info.h:		 __u32 flags;  		       /* thread_info flags (see TIF_*) */
       arch/m32r/include/asm/thread_info.h:		 unsigned long flags;      	    /* low level flags */
       arch/m68k/include/asm/thread_info.h:		 unsigned long flags;
       arch/metag/include/asm/thread_info.h:		 unsigned long flags;	    /* low level flags */
       arch/microblaze/include/asm/thread_info.h:	 unsigned long flags; /* low level flags */
       arch/mips/include/asm/thread_info.h:		 unsigned long flags; 	/* low level flags */
       arch/mn10300/include/asm/thread_info.h:		 unsigned long flags; 	     /* low level flags */
       arch/nios2/include/asm/thread_info.h:		 unsigned long flags; 	  /* low level flags */
       arch/openrisc/include/asm/thread_info.h:	 unsigned long flags;       /* low level flags */
       arch/parisc/include/asm/thread_info.h:		 unsigned long flags; 	       /* thread_info flags (see TIF_*) */
       arch/powerpc/include/asm/thread_info.h:		 unsigned long local_flags;    	  /* private flags for thread */
       arch/s390/include/asm/thread_info.h:		 unsigned long flags;     	   /* low level flags */
       arch/score/include/asm/thread_info.h:		 unsigned long flags; 	/* low level flags */
       arch/sh/include/asm/thread_info.h:		 unsigned long flags; 	     /* low level flags */
       arch/tile/include/asm/thread_info.h:		 unsigned long flags; 	  /* low level flags */
       arch/um/include/asm/thread_info.h:		 unsigned long flags;       /* low level flags */
       arch/unicore32/include/asm/thread_info.h:	 unsigned long flags; 	    /* low level flags */
       arch/x86/include/asm/thread_info.h:		 __u32	       flags; 	 /* low level flags */
       arch/xtensa/include/asm/thread_info.h:		 unsigned long flags;	      /* low level flags */

also given how much it is accessed (and that happens a lot in ASM as well). This conversion deserves quite a
whole project on its own.

It might be possible to do it incrementally though.