+Cc linux-fsdevel
On Thu, Jun 01, 2017 at 11:07:19AM -0500, Christoph Lameter wrote:
> On Wed, 31 May 2017, akpm@xxxxxxxxxxxxxxxxxxxx wrote:
>
> > + struct buffer_head *evictee = bh;
> > + struct bh_lru *b;
> > + int i;
> > + b = this_cpu_ptr(&bh_lrus);
> > + for (i = 0; i < BH_LRU_SIZE; i++) {
> > + swap(evictee, b->bhs[i]);
>
> Could you try to use this_cpu_xchg here to see if it reduces latency
> further?
>
> for (i = 0; i < BH_LRU_SIZE; i++) {
> __this_cpu_xchg(bh_lrus->bhs[i], evictee)
>
> ...
>
I tried --- actually, 'evictee = __this_cpu_xchg(bh_lrus.bhs[i], evictee)'. But
it's much slower, nearly as slow as the original --- which perhaps is not
surprising since __this_cpu_xchg() is a cmpxchg rather than a simple load and
store. It may be even worse on non-x86 architectures. Also note that we still
have to disable IRQs because we need to stay on the same CPU throughout so that
only a single queue is operated on.
Eric
