On Tue, May 15, 2018 at 11:55:04AM +0300, Kirill Tkhai wrote:
> >> @@ -586,8 +586,23 @@ static unsigned long shrink_slab_memcg(gfp_t
> >> gfp_mask, int nid,
> >> continue;
> >>
> >> ret = do_shrink_slab(&sc, shrinker, priority);
> >> - if (ret == SHRINK_EMPTY)
> >> - ret = 0;
> >> + if (ret == SHRINK_EMPTY) {
> >> + clear_bit(i, map->map);
> >> + /*
> >> + * Pairs with mb in memcg_set_shrinker_bit():
> >> + *
> >> + * list_lru_add() shrink_slab_memcg()
> >> + * list_add_tail() clear_bit()
> >> + * <MB> <MB>
> >> + * set_bit() do_shrink_slab()
> >> + */
> >
> > Please improve the comment so that it isn't just a diagram.
>
> Please, say, which comment you want to see here.
I want the reader to understand why we need to invoke the shrinker twice
if it returns SHRINK_EMPTY. The diagram doesn't really help here IMO. So
I'd write Something like this:
ret = do_shrink_slab(&sc, shrinker, priority);
if (ret == SHRINK_EMPTY) {
clear_bit(i, map->map);
/*
* After the shrinker reported that it had no objects to free,
* but before we cleared the corresponding bit in the memcg
* shrinker map, a new object might have been added. To make
* sure, we have the bit set in this case, we invoke the
* shrinker one more time and re-set the bit if it reports that
* it is not empty anymore. The memory barrier here pairs with
* the barrier in memcg_set_shrinker_bit():
*
* list_lru_add() shrink_slab_memcg()
* list_add_tail() clear_bit()
* <MB> <MB>
* set_bit() do_shrink_slab()
*/
smp_mb__after_atomic();
ret = do_shrink_slab(&sc, shrinker, priority);
if (ret == SHRINK_EMPTY)
ret = 0;
else
memcg_set_shrinker_bit(memcg, nid, i);
