linux-kernel - Re: [PATCH 4/8] cpumask: convert kernel trace functions

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Message-ID: <alpine.DEB.1.10.0812192041090.29275@gandalf.stny.rr.com>
Date:	Fri, 19 Dec 2008 20:41:49 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Travis <travis@....com>
cc:	Ingo Molnar <mingo@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/8] cpumask: convert kernel trace functions


On Fri, 19 Dec 2008, Mike Travis wrote:

> Steven Rostedt wrote:
> > On Fri, 19 Dec 2008, Mike Travis wrote:
> > 
> >> Impact: Reduce memory usage, use new cpumask API.
> >>
> >> Convert kernel trace functions to use struct cpumask.
> >>
> > How does this save memory if we just allocate a cpumask var everytime
> > we allocate a ring buffer?  Is cpumask_var_t a mask of possible CPUS and 
> > not NR_CPUS?
> > 
> > Otherwise, I see this as just adding one extra pointer.
> > 
> > -- Steve
> 
> Hi Steve,
> 
> Yes, eventually, the cpumask_var_t will be allocated based on cpumask_size()
> which will become BITS_TO_LONG(nr_cpu_ids) instead of BITS_TO_LONGS(NR_CPUS)
> as soon as the kernel becomes "cpumask" clean.  (clean being that it ignores
> all bits >= nr_cpu_ids.)
> 
> Note that on small NR_CPUS count systems, cpumask_var_t is a static array
> and no allocation happens, so it only kicks in when CONFIG_CPUMASK_OFFSTACK=y.

For future memory savings...

Acked-by: Steven Rostedt <srostedt@...hat.com>

-- Steve

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