Re: [rfc][patch 1/2] mnt_want_write speedup 1

Dave Hansen
2008-12-29T23:00:41Z

On Mon, 2008-12-22 at 05:35 +0100, Nick Piggin wrote:
> > Is there a real good reason to allocate the percpu counters dynamically?
> > Might as well stick them in the vfsmount and let the one
> > kmem_cache_zalloc() in alloc_vfsmnt() do a bit larger of an allocation.
> > Did you think that was going to bloat it to a compound allocation or
> > something? I hate the #ifdefs. :)
>
> Distros want to ship big NR_CPUS kernels and have them run reasonably on
> small num_possible_cpus() systems. But also, it would help to avoid
> cacheline bouncing from false sharing (allocpercpu.c code can also mess
> this bug for small objects like these counters, but that's a problem
> with the allocpercpu code which should be fixed anyway).

I guess we could also play the old trick:

struct vfsmount
{
...
int mnt_writers[0];
};

And just

void __init mnt_init(void)
{
...
int size = sizeof(struct vfsmount) + num_possible_cpus() * sizeof(int)

- mnt_cache = kmem_cache_create("mnt_cache", sizeof(struct vfsmount),
+ mnt_cache = kmem_cache_create("mnt_cache", size,
0, SLAB_HWCACHE_ALIGN | SLAB_PANIC, NULL);

That should save us the dereference from the pointer and still let it be
pretty flexible.

-- Dave