Re: route cache DoS testing and softirqs
From: Dipankar Sarma
Date: Wed Apr 07 2004 - 14:53:05 EST
On Wed, Apr 07, 2004 at 05:23:28PM +0200, Robert Olsson wrote:
> Dipankar Sarma writes:
>
> > Robert, you should try out rs-throttle-rcu.patch. The idea is that
> > we don't run too many callbacks in a single rcu. In my setup,
> > at 100kpps, I see as many as 30000 rcu callbacks in a single
> > tasklet handler. That is likely hurting even the softirq-only
> > RCU grace periods. Setting rcupdate.maxbatch=4 will do only 4 per
> > tasklet thus providing more quiescent points to the system.
>
> Hello!
>
> No bad things happens, lots of overflows and drop in performance
> and the userland app can stall for 32 sec. We seems to spin in
> softirq to much and still don't get things done.
Argh!! Andrea, this means that throttling rcu callbacks with
back-to-back rcu tasklets for better scheduling latency is bad
for this kind of DoS situation. I think we will have to address
the softirq limiting question.
That said, Robert, one last experiment - if you are running UP,
can you try the following patchset (should apply on top of vanilla
2.6.x) ? This implements direct invocation of callbacks instead
of waiting for rcu grace periods in UP kernel. This would be a
good data point to understand what happens.
Thanks
Dipankar
A new interface call_rcu_rt() allows the updates to happen immediately
in a UP kernel. This assumes that no RCU protection is needed
against interrupts.
include/linux/rcupdate.h | 14 ++++++++++++++
1 files changed, 14 insertions(+)
diff -puN include/linux/rcupdate.h~rcu-rt-latency include/linux/rcupdate.h
--- linux-2.6.0-test8-rcu/include/linux/rcupdate.h~rcu-rt-latency 2003-11-30 19:54:09.000000000 +0530
+++ linux-2.6.0-test8-rcu-dipankar/include/linux/rcupdate.h 2003-11-30 21:39:50.000000000 +0530
@@ -131,6 +131,20 @@ extern void rcu_check_callbacks(int cpu,
extern void FASTCALL(call_rcu(struct rcu_head *head,
void (*func)(void *arg), void *arg));
extern void synchronize_kernel(void);
+#if !defined(CONFIG_SMP) && defined(CONFIG_PREEMPT)
+static inline void call_rcu_rt(struct rcu_head *head,
+ void (*func)(void *arg), void *arg)
+{
+ func(arg);
+}
+#else
+static inline void call_rcu_rt(struct rcu_head *head,
+ void (*func)(void *arg), void *arg)
+{
+ call_rcu(head, func, arg);
+}
+#endif
+
#endif /* __KERNEL__ */
#endif /* __LINUX_RCUPDATE_H */
_
Use the new call_rcu_rt() primitive to immediately free dentries
to slab for UP/CONFIG_PREEMPT. This should improve scheduling
latencies by reducing rcu callback processing time.
fs/dcache.c | 2 +-
1 files changed, 1 insertion(+), 1 deletion(-)
diff -puN fs/dcache.c~dcache-rcu-latency fs/dcache.c
--- linux-2.6.0-test8-rcu/fs/dcache.c~dcache-rcu-latency 2003-11-30 21:22:02.000000000 +0530
+++ linux-2.6.0-test8-rcu-dipankar/fs/dcache.c 2003-11-30 21:22:16.000000000 +0530
@@ -78,7 +78,7 @@ static void d_free(struct dentry *dentry
{
if (dentry->d_op && dentry->d_op->d_release)
dentry->d_op->d_release(dentry);
- call_rcu(&dentry->d_rcu, d_callback, dentry);
+ call_rcu_rt(&dentry->d_rcu, d_callback, dentry);
}
/*
_
include/linux/rcupdate.h | 2 ++
net/ipv4/route.c | 26 +++++++++++++-------------
2 files changed, 15 insertions(+), 13 deletions(-)
diff -puN net/ipv4/route.c~rtcache-rcu-latency net/ipv4/route.c
--- linux-2.6.0-test8-rcu/net/ipv4/route.c~rtcache-rcu-latency 2003-12-15 01:42:09.000000000 +0530
+++ linux-2.6.0-test8-rcu-dipankar/net/ipv4/route.c 2003-12-15 02:15:13.000000000 +0530
@@ -223,11 +223,11 @@ static struct rtable *rt_cache_get_first
struct rt_cache_iter_state *st = seq->private;
for (st->bucket = rt_hash_mask; st->bucket >= 0; --st->bucket) {
- rcu_read_lock();
+ rcu_read_lock_bh();
r = rt_hash_table[st->bucket].chain;
if (r)
break;
- rcu_read_unlock();
+ rcu_read_unlock_bh();
}
return r;
}
@@ -239,10 +239,10 @@ static struct rtable *rt_cache_get_next(
smp_read_barrier_depends();
r = r->u.rt_next;
while (!r) {
- rcu_read_unlock();
+ rcu_read_unlock_bh();
if (--st->bucket < 0)
break;
- rcu_read_lock();
+ rcu_read_lock_bh();
r = rt_hash_table[st->bucket].chain;
}
return r;
@@ -278,7 +278,7 @@ static void *rt_cache_seq_next(struct se
static void rt_cache_seq_stop(struct seq_file *seq, void *v)
{
if (v && v != SEQ_START_TOKEN)
- rcu_read_unlock();
+ rcu_read_unlock_bh();
}
static int rt_cache_seq_show(struct seq_file *seq, void *v)
@@ -436,13 +436,13 @@ static struct file_operations rt_cpu_seq
static __inline__ void rt_free(struct rtable *rt)
{
- call_rcu(&rt->u.dst.rcu_head, (void (*)(void *))dst_free, &rt->u.dst);
+ call_rcu_rt(&rt->u.dst.rcu_head, (void (*)(void *))dst_free, &rt->u.dst);
}
static __inline__ void rt_drop(struct rtable *rt)
{
ip_rt_put(rt);
- call_rcu(&rt->u.dst.rcu_head, (void (*)(void *))dst_free, &rt->u.dst);
+ call_rcu_rt(&rt->u.dst.rcu_head, (void (*)(void *))dst_free, &rt->u.dst);
}
static __inline__ int rt_fast_clean(struct rtable *rth)
@@ -2216,7 +2216,7 @@ int __ip_route_output_key(struct rtable
hash = rt_hash_code(flp->fl4_dst, flp->fl4_src ^ (flp->oif << 5), flp->fl4_tos);
- rcu_read_lock();
+ rcu_read_lock_bh();
for (rth = rt_hash_table[hash].chain; rth; rth = rth->u.rt_next) {
smp_read_barrier_depends();
if (rth->fl.fl4_dst == flp->fl4_dst &&
@@ -2232,13 +2232,13 @@ int __ip_route_output_key(struct rtable
dst_hold(&rth->u.dst);
rth->u.dst.__use++;
RT_CACHE_STAT_INC(out_hit);
- rcu_read_unlock();
+ rcu_read_unlock_bh();
*rp = rth;
return 0;
}
RT_CACHE_STAT_INC(out_hlist_search);
}
- rcu_read_unlock();
+ rcu_read_unlock_bh();
return ip_route_output_slow(rp, flp);
}
@@ -2448,7 +2448,7 @@ int ip_rt_dump(struct sk_buff *skb, str
if (h < s_h) continue;
if (h > s_h)
s_idx = 0;
- rcu_read_lock();
+ rcu_read_lock_bh();
for (rt = rt_hash_table[h].chain, idx = 0; rt;
rt = rt->u.rt_next, idx++) {
smp_read_barrier_depends();
@@ -2459,12 +2459,12 @@ int ip_rt_dump(struct sk_buff *skb, str
cb->nlh->nlmsg_seq,
RTM_NEWROUTE, 1) <= 0) {
dst_release(xchg(&skb->dst, NULL));
- rcu_read_unlock();
+ rcu_read_unlock_bh();
goto done;
}
dst_release(xchg(&skb->dst, NULL));
}
- rcu_read_unlock();
+ rcu_read_unlock_bh();
}
done:
diff -puN include/linux/rcupdate.h~rtcache-rcu-latency include/linux/rcupdate.h
--- linux-2.6.0-test8-rcu/include/linux/rcupdate.h~rtcache-rcu-latency 2003-12-15 02:15:22.000000000 +0530
+++ linux-2.6.0-test8-rcu-dipankar/include/linux/rcupdate.h 2003-12-22 23:31:14.588119016 +0530
@@ -123,6 +123,8 @@ static inline int rcu_pending(int cpu)
#define rcu_read_lock() preempt_disable()
#define rcu_read_unlock() preempt_enable()
+#define rcu_read_lock_bh() local_bh_disable()
+#define rcu_read_unlock_bh() local_bh_enable()
extern void rcu_init(void);
extern void rcu_check_callbacks(int cpu, int user);
_