linux-kernel - Re: [RFC][PATCH] Dynamic Tick: Allow 32-bit machines to sleep for more than 2.15 seconds

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Message-ID: <20090421063523.GA8020@elte.hu>
Date:	Tue, 21 Apr 2009 08:35:23 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jon Hunter <jon-hunter@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] Dynamic Tick: Allow 32-bit machines to sleep for
	more than 2.15 seconds


* Jon Hunter <jon-hunter@...com> wrote:

> Hello,
>
> From reviewing the dynamic tick code, I noticed that the 
> "max_delta_ns" member of the "clock_event_device" structure, 
> represents the upper limit between timer events in nanoseconds. 
> The variable, "max_delta_ns", is defined as an unsigned long. An 
> unsigned long is a 32-bit integer for 32-bit machines and is a 
> 64-bit integer for 64-bit machines (if -m64 option is used for 
> gcc). Also see [1].
>
> The variable, "max_delta_ns", is configured by calling function 
> "clockevent_delta2ns()". The maximum value that "max_delta_ns" can 
> be set to by calling clockevent_delta2ns(), is LONG_MAX. For a 
> 32-bit machine LONG_MAX is equal to 0x7fffffff and in nanoseconds 
> this equates to ~2.15 seconds. Hence, the maximum sleep time for a 
> 32-bit machine is ~2.15 seconds, where as for a 64-bit machine it 
> will be many years.
>
> Therefore, I wanted to propose changing the type of max_delta_ns 
> to be "unsigned long long" instead of "unsigned long". My 
> understanding is that a variable of type "unsigned long long" is 
> 64-bits for both 32-bit and 64-bit machines and hence this would 
> allow a 32-bit machine to sleep for longer than ~2.15 seconds. 
> Please note that I also ended up making "min_delta_ns" of type 
> "unsigned long long" too and although this is probably very 
> unnecessary, it made the patch simpler. See below.
>
> Anyway, making this change has allowed my 32-bit machine to sleep 
> for longer than ~2.15 seconds and thought this could be of 
> interest to others. Your comments/feedback would be appreciated.
>
> [1] http://en.wikipedia.org/wiki/64-bit#64-bit_data_models
>
> Cheers
> Jon
>
>
> Signed-off-by: Jon Hunter <jon-hunter@...com>
> ---
>  include/linux/clockchips.h |    6 +++---
>  kernel/hrtimer.c           |    2 +-
>  kernel/time/clockevents.c  |   10 +++++-----
>  kernel/time/tick-oneshot.c |    2 +-
>  kernel/time/timer_list.c   |    4 ++--
>  5 files changed, 12 insertions(+), 12 deletions(-)

Looks like a good thing at first sight. Thomas, John, what do you 
think?

	Ingo
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