Re: [PATCH UPDATED] percpu: use dynamic percpu allocator as the default percpu allocator

Christoph Lameter
2009-04-02T14:00:13Z

On Thu, 2 Apr 2009, Ingo Molnar wrote:

> to quote an earlier part of my mail:
>
> > > We encourage kfree(NULL) and it triggers commonly in the kernel
> > > today [on distro kernels we checked it can trigger once per
> > > syscall!] - so i think we should consider free_percpu(NULL) a
> > > possibly common pattern too. (even though today it's likely
> > > _not_ common at all.)
>
> I specifically mentioned that it is not at all common now.

What is this? Nonsense day? Consider it a common pattern although its
likely not common at all? April fools day?

> But there's no reason why an object shutdown fastpath with an
> optional percpu buffer (say for debug statistics, not enabled by
> default) couldnt look like this:
>
> percpu_free(NULL);
>
> We actually have such patterns of kfree(ptr) use, where the _common_
> case in a fastpath is kfree(NULL).

Speculation. A shutdown fastpath? The percpu allocation and free
operations are expensive and deal with teardown and setup of virtual
mappings. Those paths are *not* optimized for fastpath use. kfree is
different.