Re: [take13 1/3] kevent: Core files.

From: Eric Dumazet
Date: Wed Aug 23 2006 - 08:48:32 EST

Again Evgeniy I really begin to like kevent :)

On Wednesday 23 August 2006 13:24, Evgeniy Polyakov wrote:
+struct kevent
+       /* Used for kevent freeing.*/
+       struct rcu_head         rcu_head;
+       struct ukevent          event;
+       /* This lock protects ukevent manipulations, e.g. ret_flags changes.
+       spinlock_t              ulock;
+       /* Entry of user's queue. */
+       struct list_head        kevent_entry;
+       /* Entry of origin's queue. */
+       struct list_head        storage_entry;
+       /* Entry of user's ready. */
+       struct list_head        ready_entry;
+       u32                     flags;
+       /* User who requested this kevent. */
+       struct kevent_user      *user;
+       /* Kevent container. */
+       struct kevent_storage   *st;
+       struct kevent_callbacks callbacks;
+       /* Private data for different storages.
+        * poll()/select storage has a list of wait_queue_t containers
+        * for each ->poll() { poll_wait()' } here.
+        */
+       void                    *priv;

I wonder if you can reorder fields in this structure, so that 'read mostly'
fields are grouped together, maybe in a different cache line.
This should help reduce false sharing in SMP.
read mostly fields are (but you know better than me) : callbacks, rcu_head,
priv, user, event, ...

+#define KEVENT_MAX_EVENTS      4096

Could you please tell me (Forgive me if you already clarified this point) ,
what happens if the number of queued events reaches this value ?

+int kevent_init(struct kevent *k)
+       spin_lock_init(&k->ulock);
+       k->flags = 0;
+       if (unlikely(k->event.type >= KEVENT_MAX))
+               return kevent_break(k);

As long you are sure we cannot call kevent_enqueue()/kevent_dequeue() after a
failed kevent_init() it should be fine.

+int kevent_add_callbacks(const struct kevent_callbacks *cb, int pos)
+       struct kevent_callbacks *p;
+       if (pos >= KEVENT_MAX)
+               return -EINVAL;

if a negative pos is used here we might crash. KEVENT_MAX is a signed too, so
the compare is done on signed values.
If we consider callers always give a sane value, the test can be suppressed.
If we consider callers may be wrong, then we must do a correct test.
If you dont want to change function prototype, then change the test to :

if ((unsigned)pos >= KEVENT_MAX)
return -EINVAL;

Some people on lkml will prefer:
if (pos < 0 || pos >= KEVENT_MAX)
return -EINVAL;
#define KEVENT_MAX 6U /* unsigned constant */

+static kmem_cache_t *kevent_cache;

You probably want to add __read_mostly here to avoid false sharing.

+static kmem_cache_t *kevent_cache __read_mostly;

Same for other caches :
+static kmem_cache_t *kevent_poll_container_cache;
+static kmem_cache_t *kevent_poll_priv_cache;

About the hash table :

+struct kevent_user
+       struct list_head        kevent_list[KEVENT_HASH_MASK+1];
+       spinlock_t              kevent_lock;

epoll used to use a hash table too (its size was configurable at init time),
and was converted to a RB-tree for good reasons...(avoid a user to allocate a
big hash table in pinned memory and DOS)
Are you sure a process handling one million sockets will succeed using kevent
instead of epoll ?

Do you have a pointer to sample source code using mmap()/kevent interface ?
It's not clear to me how we can use it (and notice that a full wrap occured,
user app could miss x*KEVENT_MAX_EVENTS events ?). Do we still must use a
syscall to dequeue events ?

In particular you state sizeof(mukevent) is 40, while its 12:

+ * Note that kevents does not exactly fill the page (each mukevent is 40
+ * so we reuse 4 bytes at the begining of the first page to store index.
+ * Take that into account if you want to change size of struct ukevent.
+ */

+struct mukevent
+       struct kevent_id        id; /* size()=8 */
+       __u32                   ret_flags; /* size()=4 */

Thank you
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