On Sun, Apr 18, 2021 at 08:00:31PM -0400, Waiman Long wrote:
> Currently, the object stock structure caches either reclaimable vmstat
> bytes or unreclaimable vmstat bytes in its object stock structure. The
> hit rate can be improved if both types of vmstat data can be cached
> especially for single-node system.
>
> This patch supports the cacheing of both type of vmstat data, though
> at the expense of a slightly increased complexity in the caching code.
> For large object (>= PAGE_SIZE), vmstat array is done directly without
> going through the stock caching step.
>
> On a 2-socket Cascade Lake server with instrumentation enabled, the
> miss rates are shown in the table below.
>
> Initial bootup:
>
> Kernel __mod_objcg_state mod_objcg_state %age
> ------ ----------------- --------------- ----
> Before patch 634400 3243830 19.6%
> After patch 419810 3182424 13.2%
>
> Parallel kernel build:
>
> Kernel __mod_objcg_state mod_objcg_state %age
> ------ ----------------- --------------- ----
> Before patch 24329265 142512465 17.1%
> After patch 24051721 142445825 16.9%
>
> There was a decrease of miss rate after initial system bootup. However,
> the miss rate for parallel kernel build remained about the same probably
> because most of the touched kmemcache objects were reclaimable inodes
> and dentries.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> mm/memcontrol.c | 79 +++++++++++++++++++++++++++++++------------------
> 1 file changed, 51 insertions(+), 28 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c13502eab282..a6dd18f6d8a8 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2212,8 +2212,8 @@ struct obj_stock {
> struct obj_cgroup *cached_objcg;
> struct pglist_data *cached_pgdat;
> unsigned int nr_bytes;
> - int vmstat_idx;
> - int vmstat_bytes;
> + int reclaimable_bytes; /* NR_SLAB_RECLAIMABLE_B */
> + int unreclaimable_bytes; /* NR_SLAB_UNRECLAIMABLE_B */
How about
int nr_slab_reclaimable_b;
int nr_slab_unreclaimable_b;
so you don't need the comments?
> #else
> int dummy[0];
> #endif
> @@ -3217,40 +3217,56 @@ void mod_objcg_state(struct obj_cgroup *objcg, struct pglist_data *pgdat,
> enum node_stat_item idx, int nr)
> {
> unsigned long flags;
> - struct obj_stock *stock = get_obj_stock(&flags);
> + struct obj_stock *stock;
> + int *bytes, *alt_bytes, alt_idx;
> +
> + /*
> + * Directly update vmstat array for big object.
> + */
> + if (unlikely(abs(nr) >= PAGE_SIZE))
> + goto update_vmstat;
This looks like an optimization independent of the vmstat item split?
> + stock = get_obj_stock(&flags);
> + if (idx == NR_SLAB_RECLAIMABLE_B) {
> + bytes = &stock->reclaimable_bytes;
> + alt_bytes = &stock->unreclaimable_bytes;
> + alt_idx = NR_SLAB_UNRECLAIMABLE_B;
> + } else {
> + bytes = &stock->unreclaimable_bytes;
> + alt_bytes = &stock->reclaimable_bytes;
> + alt_idx = NR_SLAB_RECLAIMABLE_B;
> + }
>
> /*
> - * Save vmstat data in stock and skip vmstat array update unless
> - * accumulating over a page of vmstat data or when pgdat or idx
> + * Try to save vmstat data in stock and skip vmstat array update
> + * unless accumulating over a page of vmstat data or when pgdat
> * changes.
> */
> if (stock->cached_objcg != objcg) {
> /* Output the current data as is */
> - } else if (!stock->vmstat_bytes) {
> - /* Save the current data */
> - stock->vmstat_bytes = nr;
> - stock->vmstat_idx = idx;
> - stock->cached_pgdat = pgdat;
> - nr = 0;
> - } else if ((stock->cached_pgdat != pgdat) ||
> - (stock->vmstat_idx != idx)) {
> - /* Output the cached data & save the current data */
> - swap(nr, stock->vmstat_bytes);
> - swap(idx, stock->vmstat_idx);
> + } else if (stock->cached_pgdat != pgdat) {
> + /* Save the current data and output cached data, if any */
> + swap(nr, *bytes);
> swap(pgdat, stock->cached_pgdat);
> + if (*alt_bytes) {
> + __mod_objcg_state(objcg, pgdat, alt_idx,
> + *alt_bytes);
> + *alt_bytes = 0;
> + }
As per the other email, I really don't think optimizing the pgdat
switch (in a percpu cache) is worth this level of complexity.
> } else {
> - stock->vmstat_bytes += nr;
> - if (abs(stock->vmstat_bytes) > PAGE_SIZE) {
> - nr = stock->vmstat_bytes;
> - stock->vmstat_bytes = 0;
> + *bytes += nr;
> + if (abs(*bytes) > PAGE_SIZE) {
> + nr = *bytes;
> + *bytes = 0;
> } else {
> nr = 0;
> }
> }
> - if (nr)
> - __mod_objcg_state(objcg, pgdat, idx, nr);
> -
> put_obj_stock(flags);
> + if (!nr)
> + return;
> +update_vmstat:
> + __mod_objcg_state(objcg, pgdat, idx, nr);
> }
>
> static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> @@ -3303,12 +3319,19 @@ static void drain_obj_stock(struct obj_stock *stock)
> /*
> * Flush the vmstat data in current stock
> */
> - if (stock->vmstat_bytes) {
> - __mod_objcg_state(old, stock->cached_pgdat, stock->vmstat_idx,
> - stock->vmstat_bytes);
> + if (stock->reclaimable_bytes || stock->unreclaimable_bytes) {
> + int bytes;
> +
> + if ((bytes = stock->reclaimable_bytes))
> + __mod_objcg_state(old, stock->cached_pgdat,
> + NR_SLAB_RECLAIMABLE_B, bytes);
> + if ((bytes = stock->unreclaimable_bytes))
> + __mod_objcg_state(old, stock->cached_pgdat,
> + NR_SLAB_UNRECLAIMABLE_B, bytes);
The int bytes indirection isn't necessary. It's easier to read even
with the extra lines required to repeat the long stock member names,
because that is quite a common pattern (if (stuff) frob(stuff)).
__mod_objcg_state() also each time does rcu_read_lock() toggling and a
memcg lookup that could be batched, which I think is further proof
that it should just be inlined here.
