On Fri, 8 Jan 2016 11:58:39 +0900
Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> On Thu, Jan 07, 2016 at 03:03:38PM +0100, Jesper Dangaard Brouer wrote:
> > This change is primarily an attempt to make it easier to realize the
> > optimizations the compiler performs in-case CONFIG_MEMCG_KMEM is not
> > enabled.
> >
> > Performance wise, even when CONFIG_MEMCG_KMEM is compiled in, the
> > overhead is zero. This is because, as long as no process have
> > enabled kmem cgroups accounting, the assignment is replaced by
> > asm-NOP operations. This is possible because memcg_kmem_enabled()
> > uses a static_key_false() construct.
> >
> > It also helps readability as it avoid accessing the p[] array like:
> > p[size - 1] which "expose" that the array is processed backwards
> > inside helper function build_detached_freelist().
>
> That part is cleande up but overall code doesn't looks readable to me.
True, I also don't like my "*s" indirection, even-though it gets removed
in the compiled code.
> How about below change?
Looks more C readable, but I have to verify that the compiler can still
realize the optimization, of just using "s" directly, when
CONFIG_MEMCG_KMEM is not activated.
> ---------------------->8------------------
> struct detached_freelist {
> + struct kmem_cache *s;
> struct page *page;
> void *tail;
> void *freelist;
> int cnt;
> }
I'll likely place "s" at another point, as 16 bytes alignment of this
struct can influence performance on Intel CPUs. (e.g. freelist+cnt
gets updated at almost the same time, and were 16 bytes aligned before)
> @@ -2852,8 +2853,11 @@ static int build_detached_freelist(struct kmem_cache *s, size_t size,
> if (!object)
> return 0;
>
> + /* Support for memcg */
> + df->s = cache_from_obj(s, object);
> +
> /* Start new detached freelist */
> - set_freepointer(s, object, NULL);
> + set_freepointer(df.s, object, NULL);
Not compile tested ;-) ... df->s
> df->page = virt_to_head_page(object);
> df->tail = object;
> df->freelist = object;
> @@ -2868,7 +2872,7 @@ static int build_detached_freelist(struct kmem_cache *s, size_t size,
> /* df->page is always set at this point */
> if (df->page == virt_to_head_page(object)) {
> /* Opportunity build freelist */
> - set_freepointer(s, object, df->freelist);
> + set_freepointer(df.s, object, df->freelist);
> df->freelist = object;
> df->cnt++;
> p[size] = NULL; /* mark object processed */
> @@ -2889,23 +2893,19 @@ static int build_detached_freelist(struct kmem_cache *s, size_t size,
>
>
> /* Note that interrupts must be enabled when calling this function. */
> -void kmem_cache_free_bulk(struct kmem_cache *orig_s, size_t size, void **p)
> +void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)
> {
> if (WARN_ON(!size))
> return;
>
> do {
> struct detached_freelist df;
> - struct kmem_cache *s;
> -
> - /* Support for memcg */
> - s = cache_from_obj(orig_s, p[size - 1]);
>
> size = build_detached_freelist(s, size, p, &df);
> if (unlikely(!df.page))
> continue;
>
> - slab_free(s, df.page, df.freelist, df.tail, df.cnt, _RET_IP_);
> + slab_free(df.s, df.page, df.freelist, df.tail, df.cnt, _RET_IP_);
Argh... line will be 81 chars wide...
> } while (likely(size));
> }
> EXPORT_SYMBOL(kmem_cache_free_bulk);
I'll try it out. Thanks for your suggestion.
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer
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