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Date: Tue, 15 Mar 2016 13:15:25 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        =?iso-8859-1?Q?Fr=E9d=E9ric?= Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] nohz: Change tick_dep_mask from 'unsigned long' to
 'unsigned int'
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> Linus noticed that the new tick_dep_mask types introduced in:
> 
>   d027d45d8a17 ("nohz: New tick dependency mask")
> 
> ... are sloppily defined as 'unsigned long' - which is wasteful
> to carry just 4 bits and which may also create suboptimal data
> types on 64-bit systems with word alignment padding holes in them.
> 
> Fix this by changing the type to the more natural 'unsigned int'.
> 
> (The xchg_or() API will work fine with 'unsigned int' as well.)
> 
> Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Chris Metcalf <cmetcalf@ezchip.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Chris Metcalf <cmetcalf@ezchip.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Luiz Capitulino <lcapitulino@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  include/linux/sched.h    |  4 ++--
>  kernel/time/tick-sched.c | 11 +++++------
>  2 files changed, 7 insertions(+), 8 deletions(-)

Hm, so the problem I did not consider is the following:

 triton:~/tip> git grep tick_dep_mask kernel | grep _bit
 kernel/time/tick-sched.c:       clear_bit(bit, &tick_dep_mask);
 kernel/time/tick-sched.c:       clear_bit(bit, &ts->tick_dep_mask);
 kernel/time/tick-sched.c:       clear_bit(bit, &tsk->tick_dep_mask);
 kernel/time/tick-sched.c:       clear_bit(bit, &sig->tick_dep_mask);

and the bitops natural type (and in fact the only supported bitops type) is 
'unsigned long'.

So it's not that easy to change a bitmask over to unsigned int.

Suggestions?

Thanks,

	Ingo