On Thu, 15 Jan 2015 16:40:32 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote: > We had to insert a preempt enable/disable in the fastpath a while ago > in order to guarantee that tid and kmem_cache_cpu are retrieved on the > same cpu. It is the problem only for CONFIG_PREEMPT in which scheduler > can move the process to other cpu during retrieving data. > > Now, I reach the solution to remove preempt enable/disable in the fastpath. > If tid is matched with kmem_cache_cpu's tid after tid and kmem_cache_cpu > are retrieved by separate this_cpu operation, it means that they are > retrieved on the same cpu. If not matched, we just have to retry it. > > With this guarantee, preemption enable/disable isn't need at all even if > CONFIG_PREEMPT, so this patch removes it. > > I saw roughly 5% win in a fast-path loop over kmem_cache_alloc/free > in CONFIG_PREEMPT. (14.821 ns -> 14.049 ns) I'm surprised. preempt_disable/enable are pretty fast. I wonder why this makes a measurable difference. Perhaps preempt_enable()'s call to preempt_schedule() added pain? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>