linux-kernel - Re: [PATCH] cpumask: alloc blank cpumask left over

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Date:	Fri, 05 Jun 2009 10:46:48 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Rusty Russell <rusty@...tcorp.com.au>, Avi Kivity <avi@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpumask: alloc blank cpumask left over

Linus Torvalds wrote:
> 
> On Fri, 5 Jun 2009, Rusty Russell wrote:
>> OK, here's what I've got in my tree.  Ingo, I think this should go in the
>> current -rc to avoid nasty bugs.
> 
> Why is the fix not to simply clear it in alloc?
> 
>> BTW, the original alloc_cpumask_var did zero; that was dropped after arguments
>> over efficiency and fitting with other interfaces, but I clearly had the old
>> semantics in my head for a while.
> 
> How could this ever be a efficiency issue? If you allocate cpumasks so 
> often that it's an efficiency problem, it would seem that you have bigger 
> issues than the memset. You'll have to initialize them some other way 
> anyway, so it's not like you can ever really avoid the dirty cachelines.
> 
> IOW, why isn't the fix just to clean up the horrible mess that is 
> alloc_cpumask_var_node() once and for all. Why not something like this? 
> The end result looks a lot simpler.
> 
> Or you could just add in that __GFP_ZERO there, and remove the memset. I 
> don't care. But the current code just looks messy and crazy, and that 
> FIXME is bogus. The end of the allocation needs to be cleared regardless.

some alloc_cpumask_var or alloc_cpumask_var_node calling are followed by 
cpumask_copy etc to assign the allocated cpuamsk_var_t.

about 66% calling are falling to that case.

YH
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