linux-kernel - Re: [PATCH 2.6.20-rc1 00/10] Kernel memory leak detector 0.13

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Date:	Wed, 27 Dec 2006 18:30:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Catalin Marinas <catalin.marinas@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.20-rc1 00/10] Kernel memory leak detector 0.13

* Ingo Molnar <mingo@...e.hu> wrote:

> > As I mentioned in a different e-mail, a way to remove the global 
> > hash table is to create per-cpu hashes. The only problem is that in 
> > these 8-10% of the cases, freeing would need to look up the other 
> > hashes. This would become a problem with a high number of CPUs but 
> > I'm not sure whether it would overtake the performance issues 
> > introduced by cacheline ping-ponging in the single-hash case.
> 
> i dont think it's worth doing that. So we should either do the current 
> global lock & hash (bad for scalability), or a pure per-CPU design. 
> The pure per-CPU design would have to embedd the CPU ID the object is 
> attached to into the allocated object. If that is not feasible then 
> only the global hash remains i think.

embedding the info shouldnt be /that/ hard in case of the SLAB: if the 
memleak info is at a negative offset from the allocated pointer. I.e. 
that if kmalloc() returns 'ptr', the memleak info could be at 
ptr-sizeof(memleak_info). That way you dont have to know the size of the 
object beforehand and there's absolutely no need for a global hash of 
any sort.

(it gets a bit more complex for page aligned allocations for the buddy 
and for vmalloc - but that could be solved by adding one extra pointer 
into struct page. That is a far more preferable cost than the 
locking/cache overhead of a global hash.)

	Ingo
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