Re: 2.6.14-rt22 (and mainline): netstat -anop triggers excessivelatencies
From: Steven Rostedt
Date: Wed Dec 21 2005 - 17:36:17 EST
On Wed, 2005-12-21 at 01:04 -0500, Lee Revell wrote:
> Here is a 13ms+ trace caused by "netstat -anop". I noticed that the
> preempt count is 0 so there are no locks to drop, would adding a
> cond_resched() be acceptable? Or should this be considered more
> evidence of the need for softirq preemption?
>
> preemption latency trace v1.1.5 on 2.6.14-rt22
> --------------------------------------------------------------------
> latency: 13824 us, #16533/16533, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0)
> -----------------
> | task: softirq-timer/0-3 (uid:0 nice:0 policy:1 rt_prio:1)
> -----------------
>
> _------=> CPU#
> / _-----=> irqs-off
> | / _----=> need-resched
> || / _---=> hardirq/softirq
> ||| / _--=> preempt-depth
> |||| /
> ||||| delay
> cmd pid ||||| time | caller
> \ / ||||| \ | /
> netstat-29157 0D.h2 0us : __trace_start_sched_wakeup (try_to_wake_up)
> netstat-29157 0D.h2 1us : __trace_start_sched_wakeup <<...>-3> (62 0)
> netstat-29157 0D.h. 2us : wake_up_process (wakeup_softirqd)
Hi Lee, I've seen the same thing here. I wrote up this patch as a case
study to see if it can work and it does. Now this sacrifices memory for
speed. I added a bitmask in the inet_hash to keep track of all the
buckets that are used, and use the find_next_bit to search the list in
established_get_first.
After running the netstat -anop I get the following:
preemption latency trace v1.1.5 on 2.6.14-rt22
--------------------------------------------------------------------
latency: 377 us, #225/225, CPU#0 | (M:preempt VP:0, KP:0, SP:0 HP:0)
-----------------
| task: softirq-timer/0-3 (uid:0 nice:0 policy:1 rt_prio:1)
-----------------
_------=> CPU#
/ _-----=> irqs-off
| / _----=> need-resched
|| / _---=> hardirq/softirq
||| / _--=> preempt-depth
|||| /
||||| delay
cmd pid ||||| time | caller
\ / ||||| \ | /
<idle>-0 0D.h4 0us : __trace_start_sched_wakeup (try_to_wake_up)
<idle>-0 0D.h4 0us : __trace_start_sched_wakeup <<...>-3> (62 0)
<idle>-0 0Dnh3 0us : try_to_wake_up <<...>-3> (62 8c)
<idle>-0 0Dnh2 1us : preempt_schedule (try_to_wake_up)
<idle>-0 0Dnh2 1us : wake_up_process (wakeup_softirqd)
[snip]
So it really does improve the latency here. Now is this worth the
overhead? This might be useful in other places to.
Ingo,
What do you think?
-- Steve
Index: linux-2.6.14-rt22/include/net/inet_hashtables.h
===================================================================
--- linux-2.6.14-rt22.orig/include/net/inet_hashtables.h 2005-10-27 20:02:08.000000000 -0400
+++ linux-2.6.14-rt22/include/net/inet_hashtables.h 2005-12-21 17:05:35.000000000 -0500
@@ -101,6 +101,7 @@
* is for TIME_WAIT sockets only.
*/
struct inet_ehash_bucket *ehash;
+ unsigned long *ebitmask;
/* Ok, let's try this, I give up, we do need a local binding
* TCP hash as well as the others for fast bind/connect.
@@ -157,6 +158,13 @@
return &hashinfo->ehash[hash & (hashinfo->ehash_size - 1)];
}
+static inline unsigned int inet_ehash_index(
+ struct inet_hashinfo *hashinfo,
+ unsigned int hash)
+{
+ return hash & (hashinfo->ehash_size - 1);
+}
+
extern struct inet_bind_bucket *
inet_bind_bucket_create(kmem_cache_t *cachep,
struct inet_bind_hashbucket *head,
@@ -229,11 +237,25 @@
wake_up(&hashinfo->lhash_wait);
}
+static inline void __inet_hash_setbit(unsigned long *bitmask, unsigned int index)
+{
+ if (bitmask)
+ set_bit(index, bitmask);
+}
+
+static inline void __inet_hash_clearbit(unsigned long *bitmask, unsigned int index)
+{
+ if (bitmask)
+ clear_bit(index, bitmask);
+}
+
static inline void __inet_hash(struct inet_hashinfo *hashinfo,
struct sock *sk, const int listen_possible)
{
struct hlist_head *list;
rwlock_t *lock;
+ unsigned long *bitmask = NULL;
+ unsigned int index = 0;
BUG_TRAP(sk_unhashed(sk));
if (listen_possible && sk->sk_state == TCP_LISTEN) {
@@ -243,12 +265,15 @@
} else {
struct inet_ehash_bucket *head;
sk->sk_hash = inet_sk_ehashfn(sk);
+ index = inet_ehash_index(hashinfo, sk->sk_hash);
head = inet_ehash_bucket(hashinfo, sk->sk_hash);
list = &head->chain;
lock = &head->lock;
+ bitmask = hashinfo->ebitmask;
write_lock(lock);
}
__sk_add_node(sk, list);
+ __inet_hash_setbit(bitmask, index);
sock_prot_inc_use(sk->sk_prot);
write_unlock(lock);
if (listen_possible && sk->sk_state == TCP_LISTEN)
@@ -267,6 +292,8 @@
static inline void inet_unhash(struct inet_hashinfo *hashinfo, struct sock *sk)
{
rwlock_t *lock;
+ unsigned long *bitmask = NULL;
+ unsigned int index = 0;
if (sk_unhashed(sk))
goto out;
@@ -276,12 +303,16 @@
inet_listen_wlock(hashinfo);
lock = &hashinfo->lhash_lock;
} else {
+ index = inet_ehash_index(hashinfo, sk->sk_hash);
lock = &inet_ehash_bucket(hashinfo, sk->sk_hash)->lock;
+ bitmask = hashinfo->ebitmask;
write_lock_bh(lock);
}
- if (__sk_del_node_init(sk))
+ if (__sk_del_node_init(sk)) {
+ __inet_hash_clearbit(bitmask, index);
sock_prot_dec_use(sk->sk_prot);
+ }
write_unlock_bh(lock);
out:
if (sk->sk_state == TCP_LISTEN)
Index: linux-2.6.14-rt22/mm/page_alloc.c
===================================================================
--- linux-2.6.14-rt22.orig/mm/page_alloc.c 2005-12-15 21:46:16.000000000 -0500
+++ linux-2.6.14-rt22/mm/page_alloc.c 2005-12-21 17:05:23.000000000 -0500
@@ -2601,3 +2601,30 @@
return table;
}
+
+void *__init alloc_large_system_bitmask(char *bitmaskname,
+ unsigned long bits, int flags)
+{
+ unsigned long words = bits / (sizeof(unsigned long)*8);
+ unsigned long size = words * sizeof(unsigned long);
+ unsigned long *bitmask = NULL;
+
+ if (flags & HASH_EARLY)
+ bitmask = alloc_bootmem(size);
+ else if (hashdist)
+ bitmask = __vmalloc(size, GFP_ATOMIC, PAGE_KERNEL);
+ else {
+ bitmask = kmalloc(size, GFP_ATOMIC);
+ if (!bitmask) {
+ unsigned long order;
+ for (order = 0; ((1UL << order) << PAGE_SHIFT) < size; order++)
+ ;
+ bitmask = (void*) __get_free_pages(GFP_ATOMIC, order);
+ }
+ }
+
+ if (!bitmask)
+ panic("Failed to allocate %s bitmask\n", bitmaskname);
+
+ return bitmask;
+}
Index: linux-2.6.14-rt22/net/ipv4/tcp.c
===================================================================
--- linux-2.6.14-rt22.orig/net/ipv4/tcp.c 2005-12-21 16:49:59.000000000 -0500
+++ linux-2.6.14-rt22/net/ipv4/tcp.c 2005-12-21 17:31:07.000000000 -0500
@@ -2038,6 +2038,8 @@
}
__setup("thash_entries=", set_thash_entries);
+void *__init alloc_large_system_bitmask(char *bitmaskname,
+ unsigned long bits, int flags);
void __init tcp_init(void)
{
struct sk_buff *skb = NULL;
@@ -2071,6 +2073,10 @@
NULL,
0);
tcp_hashinfo.ehash_size = (1 << tcp_hashinfo.ehash_size) >> 1;
+ tcp_hashinfo.ebitmask =
+ alloc_large_system_bitmask("TCP established",
+ tcp_hashinfo.ehash_size,
+ HASH_HIGHMEM);
for (i = 0; i < (tcp_hashinfo.ehash_size << 1); i++) {
rwlock_init(&tcp_hashinfo.ehash[i].lock);
Index: linux-2.6.14-rt22/net/ipv4/tcp_ipv4.c
===================================================================
--- linux-2.6.14-rt22.orig/net/ipv4/tcp_ipv4.c 2005-12-21 17:08:20.000000000 -0500
+++ linux-2.6.14-rt22/net/ipv4/tcp_ipv4.c 2005-12-21 17:26:18.000000000 -0500
@@ -1590,7 +1590,12 @@
struct tcp_iter_state* st = seq->private;
void *rc = NULL;
- for (st->bucket = 0; st->bucket < tcp_hashinfo.ehash_size; ++st->bucket) {
+ for (st->bucket = find_first_bit(tcp_hashinfo.ebitmask,
+ tcp_hashinfo.ehash_size);
+ st->bucket < tcp_hashinfo.ehash_size;
+ st->bucket = find_next_bit(tcp_hashinfo.ebitmask,
+ tcp_hashinfo.ehash_size,
+ st->bucket+1)) {
struct sock *sk;
struct hlist_node *node;
struct inet_timewait_sock *tw;
-
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