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SubjectRe: [RFC] non-refcounted pages, application to slab?
Nick Piggin a écrit :
> If an allocator knows exactly the lifetime of its page, then there is no
> need to do refcounting or the final put_page_zestzero (atomic op + mem
> barriers).
>
> This is probably not worthwhile for most cases, but slab did strike me
> as a potential candidate (however the complication here is that some
> code I think uses the refcount of underlying pages of slab allocations
> eg nommu code). So it is not a complete patch, but I wonder if anyone
> thinks the savings might be worth the complexity?
>
> Is there any particular code that is really heavy on slab allocations?
> That isn't mostly handled by the slab's internal freelists?

Hi Nick

After reading your patch, I have some crazy idea.

The atomic op + mem barrier you want to avoid could be avoided more generally
just by changing atomic_dec_and_test(atomic_t *v).

If the current thread is the last referer (refcnt = 1), then it can safely set
the value to 0 because no other CPU can be touching the value (or else there
must be a bug somewhere, as the 'other cpu' could touch the value just after
us and we could free an object still in use by 'other cpu'

Something like :


--- include/asm-i386/atomic.h.orig 2006-01-25 12:11:46.000000000 +0100
+++ include/asm-i386/atomic.h 2006-01-25 12:13:07.000000000 +0100
@@ -130,6 +130,13 @@
printk("BUG: atomic counter underflow at:\n");
dump_stack();
}
+#ifdef CONFIG_SMP
+ /* avoid an atomic op if we are the last user of this atomic */
+ if (atomic_read(v) == 1) {
+ atomic_set(v, 0); /* not a real atomic op on most machines */
+ return 1;
+ }
+#endif
__asm__ __volatile__(
LOCK_PREFIX "decl %0; sete %1"
:"=m" (v->counter), "=qm" (c)

Thank you

Eric
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