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

From: Ingo Molnar
Date: Wed Dec 27 2006 - 12:34:02 EST

* Ingo Molnar <mingo@xxxxxxx> 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.)

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