On Mon, Aug 07, 2023 at 07:09:33PM +0800, Qi Zheng wrote: > The shrinker_rwsem is a global read-write lock in shrinkers subsystem, > which protects most operations such as slab shrink, registration and > unregistration of shrinkers, etc. This can easily cause problems in the > following cases. .... > This commit uses the refcount+RCU method [5] proposed by Dave Chinner > to re-implement the lockless global slab shrink. The memcg slab shrink is > handled in the subsequent patch. .... > --- > include/linux/shrinker.h | 17 ++++++++++ > mm/shrinker.c | 70 +++++++++++++++++++++++++++++----------- > 2 files changed, 68 insertions(+), 19 deletions(-) There's no documentation in the code explaining how the lockless shrinker algorithm works. It's left to the reader to work out how this all goes together.... > diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h > index eb342994675a..f06225f18531 100644 > --- a/include/linux/shrinker.h > +++ b/include/linux/shrinker.h > @@ -4,6 +4,8 @@ > > #include <linux/atomic.h> > #include <linux/types.h> > +#include <linux/refcount.h> > +#include <linux/completion.h> > > #define SHRINKER_UNIT_BITS BITS_PER_LONG > > @@ -87,6 +89,10 @@ struct shrinker { > int seeks; /* seeks to recreate an obj */ > unsigned flags; > > + refcount_t refcount; > + struct completion done; > + struct rcu_head rcu; What does the refcount protect, why do we need the completion, etc? > + > void *private_data; > > /* These are for internal use */ > @@ -120,6 +126,17 @@ struct shrinker *shrinker_alloc(unsigned int flags, const char *fmt, ...); > void shrinker_register(struct shrinker *shrinker); > void shrinker_free(struct shrinker *shrinker); > > +static inline bool shrinker_try_get(struct shrinker *shrinker) > +{ > + return refcount_inc_not_zero(&shrinker->refcount); > +} > + > +static inline void shrinker_put(struct shrinker *shrinker) > +{ > + if (refcount_dec_and_test(&shrinker->refcount)) > + complete(&shrinker->done); > +} > + > #ifdef CONFIG_SHRINKER_DEBUG > extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker, > const char *fmt, ...); > diff --git a/mm/shrinker.c b/mm/shrinker.c > index 1911c06b8af5..d318f5621862 100644 > --- a/mm/shrinker.c > +++ b/mm/shrinker.c > @@ -2,6 +2,7 @@ > #include <linux/memcontrol.h> > #include <linux/rwsem.h> > #include <linux/shrinker.h> > +#include <linux/rculist.h> > #include <trace/events/vmscan.h> > > #include "internal.h" > @@ -577,33 +578,42 @@ unsigned long shrink_slab(gfp_t gfp_mask, int nid, struct mem_cgroup *memcg, > if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg)) > return shrink_slab_memcg(gfp_mask, nid, memcg, priority); > > - if (!down_read_trylock(&shrinker_rwsem)) > - goto out; > - > - list_for_each_entry(shrinker, &shrinker_list, list) { > + rcu_read_lock(); > + list_for_each_entry_rcu(shrinker, &shrinker_list, list) { > struct shrink_control sc = { > .gfp_mask = gfp_mask, > .nid = nid, > .memcg = memcg, > }; > > + if (!shrinker_try_get(shrinker)) > + continue; > + > + /* > + * We can safely unlock the RCU lock here since we already > + * hold the refcount of the shrinker. > + */ > + rcu_read_unlock(); > + > ret = do_shrink_slab(&sc, shrinker, priority); > if (ret == SHRINK_EMPTY) > ret = 0; > freed += ret; > + > /* > - * Bail out if someone want to register a new shrinker to > - * prevent the registration from being stalled for long periods > - * by parallel ongoing shrinking. > + * This shrinker may be deleted from shrinker_list and freed > + * after the shrinker_put() below, but this shrinker is still > + * used for the next traversal. So it is necessary to hold the > + * RCU lock first to prevent this shrinker from being freed, > + * which also ensures that the next shrinker that is traversed > + * will not be freed (even if it is deleted from shrinker_list > + * at the same time). > */ This comment really should be at the head of the function, describing the algorithm used within the function itself. i.e. how reference counts are used w.r.t. the rcu_read_lock() usage to guarantee existence of the shrinker and the validity of the list walk. I'm not going to remember all these little details when I look at this code in another 6 months time, and having to work it out from first principles every time I look at the code will waste of a lot of time... -Dave. -- Dave Chinner david@xxxxxxxxxxxxx