netdev - [PATCH net 1/2] rhashtable: unconditionally grow when max

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Date:	Wed, 25 Feb 2015 16:31:53 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	davem@...emloft.net
Cc:	tgraf@...g.ch, pablo@...filter.org, johunt@...mai.com,
	kaber@...sh.net, netdev@...r.kernel.org,
	Daniel Borkmann <daniel@...earbox.net>,
	Ying Xue <ying.xue@...driver.com>
Subject: [PATCH net 1/2] rhashtable: unconditionally grow when max_shift is not specified

While commit c0c09bfdc415 ("rhashtable: avoid unnecessary wakeup for
worker queue") rightfully moved part of the decision making of
whether we should expand or shrink from the expand/shrink functions
themselves into insert/delete functions in order to avoid unnecessary
worker wake-ups, it however introduced a regression by doing so.

Before that change, if no max_shift was specified (= 0) on rhashtable
initialization, rhashtable_expand() would just grow unconditionally
and lets the available memory be the limiting factor. After that
change, if no max_shift was specified, there would be _no_ expansion
step at all.

Given that netlink and tipc have a max_shift specified, it was not
visible there, but Josh Hunt reported that if nft that starts out
with a default element hint of 3 if not otherwise provided, would
slow i.e. inserts down trememdously as it cannot grow larger to
relax table occupancy.

Given that the test case verifies shrinks/expands manually, we also
must remove pointer to the helper functions to explicitly avoid
parallel resizing on insertions/deletions. test_bucket_stats() and
test_rht_lookup() could also be wrapped around rhashtable mutex to
explicitly synchronize a walk from resizing, but I think that defeats
the actual test case which intended to have explicit test steps,
i.e. 1) inserts, 2) expands, 3) shrinks, 4) deletions, with object
verification after each stage.

Reported-by: Josh Hunt <johunt@...mai.com>
Fixes: c0c09bfdc415 ("rhashtable: avoid unnecessary wakeup for worker queue")
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Cc: Ying Xue <ying.xue@...driver.com>
Cc: Josh Hunt <johunt@...mai.com>
---
 lib/rhashtable.c      | 2 +-
 lib/test_rhashtable.c | 2 --
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/lib/rhashtable.c b/lib/rhashtable.c
index e3a04e4..bcf119b 100644
--- a/lib/rhashtable.c
+++ b/lib/rhashtable.c
@@ -251,7 +251,7 @@ bool rht_grow_above_75(const struct rhashtable *ht, size_t new_size)
 {
 	/* Expand table when exceeding 75% load */
 	return atomic_read(&ht->nelems) > (new_size / 4 * 3) &&
-	       (ht->p.max_shift && atomic_read(&ht->shift) < ht->p.max_shift);
+	       (!ht->p.max_shift || atomic_read(&ht->shift) < ht->p.max_shift);
 }
 EXPORT_SYMBOL_GPL(rht_grow_above_75);

diff --git a/lib/test_rhashtable.c b/lib/test_rhashtable.c
index 58b9953..f9e9d73 100644
--- a/lib/test_rhashtable.c
+++ b/lib/test_rhashtable.c
@@ -202,8 +202,6 @@ static int __init test_rht_init(void)
 		.key_len = sizeof(int),
 		.hashfn = jhash,
 		.nulls_base = (3U << RHT_BASE_SHIFT),
-		.grow_decision = rht_grow_above_75,
-		.shrink_decision = rht_shrink_below_30,
 	};
 	int err;

-- 
1.9.3

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