linux-kernel - Re: [PATCH 1/2] lib: Resizable, Scalable, Concurrent Hash Table

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Date:	Tue, 29 Jul 2014 08:30:16 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kaber@...sh.net,
	paulmck@...ux.vnet.ibm.com, challa@...ronetworks.com,
	walpole@...pdx.edu, dev@...nvswitch.org
Subject: Re: [PATCH 1/2] lib: Resizable, Scalable, Concurrent Hash Table

On Tue, Jul 29, 2014 at 01:41:32PM +0200, Thomas Graf wrote:
> Generic implementation of a resizable, scalable, concurrent hash table
> based on [0]. The implementation supports both, fixed size keys specified
> via an offset and length, or arbitrary keys via own hash and compare
> functions.
> 
> Lookups are lockless and protected as RCU read side critical sections.
> Automatic growing/shrinking based on user configurable watermarks is
> available while allowing concurrent lookups to take place.
> 
> Objects to be hashed must include a struct rhash_head. The reason for not
> using the existing struct hlist_head is that the expansion and shrinking
> will have two buckets point to a single entry which would lead in obscure
> reverse chaining behaviour.
> 
> Code includes a boot selftest if CONFIG_TEST_RHASHTABLE is defined.
> 
> [0] https://www.usenix.org/legacy/event/atc11/tech/final_files/Triplett.pdf
> 
> Signed-off-by: Thomas Graf <tgraf@...g.ch>

Thanks for working on this!

Currently reading the algorithm implementation in more detail.  A couple
of minor issues below.

> --- /dev/null
> +++ b/include/linux/rhashtable.h
[...]
> +struct rhash_head {
> +	struct rhash_head		*next;
> +};

Arguably this could become a new singly-linked list type in list.h;
we don't currently have one.

> +/**
> + * rht_for_each_entry_safe - safely iterate over hash chain of given type
> + * @pos:	type * to use as a loop cursor.
> + * @n:		type * to use for temporary next object storage
> + * @head:	head of the hash chain (struct rhash_head *)
> + * @ht:		pointer to your struct rhashtable
> + * @member:	name of the rhash_head within the hashable struct.
> + *
> + * This hash chain list-traversal primitive allows for the looped code to
> + * remove the loop cursor from the list.
> + */
> +#define rht_for_each_entry_safe(pos, n, head, ht, member)		\
> +	for (pos = rht_entry_safe(rht_dereference(head, ht), \
> +				   typeof(*(pos)), member), \
> +	     n = rht_entry_safe(rht_dereference((pos)->member.next, ht), \
> +				 typeof(*(pos)), member); \
> +	     pos; \
> +	     pos = n, \
> +	     n = rht_entry_safe(rht_dereference((pos)->member.next, ht), \
> +				 typeof(*(pos)), member))

Given that removal requires special care, is this something actually
needed in the public interface, or only internally?  You don't
necessarily need to define a full set of list primitives.  (Unless of
course you make this a new list type in list.h.)

> --- /dev/null
> +++ b/lib/rhashtable.c
[...]
> +#define ASSERT_RHT_MUTEX(HT) BUG_ON(unlikely(!lockdep_rht_mutex_is_held(HT)))

BUG_ON and WARN_ON already include unlikely(); you don't need to add it
yourself.

> +/**
> + * rht_obj - cast hash head to outer object
> + * @ht:		hash table
> + * @he:		hashed node
> + */
> +void *rht_obj(const struct rhashtable *ht, const struct rhash_head *he)
> +{
> +	return (void *) he - ht->p.head_offset;
> +}
> +EXPORT_SYMBOL_GPL(rht_obj);

Should this be a static inline?  And, will the runtime indirection
involved in head_offset add unnecessary overhead for tables of a known
type?  (I'd expect a head_offset of 0 in common cases.)

- Josh Triplett
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