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SubjectRe: RCU latency regression in 2.6.16-rc1
On Tue, Jan 24, 2006 at 02:52:18AM -0500, Lee Revell wrote:
> I ported the latency tracer to 2.6.16 and got this 13ms latency within a
> few hours. This is a regression from 2.6.15.
>
> It appears that RCU can invoke ipv4_dst_destroy thousands of times in a
> single batch.
>
> <idle>-0 0d.s. 143us : tasklet_action (__do_softirq)
> <idle>-0 0d.s. 143us : rcu_process_callbacks (tasklet_action)
> <idle>-0 0d.s. 144us : __rcu_process_callbacks (rcu_process_callbacks)
> <idle>-0 0d.s. 145us : __rcu_process_callbacks (rcu_process_callbacks)
> <idle>-0 0d.s. 146us : dst_rcu_free (__rcu_process_callbacks)
> <idle>-0 0d.s. 147us : dst_destroy (dst_rcu_free)
> <idle>-0 0d.s. 148us : ipv4_dst_destroy (dst_destroy)
> <idle>-0 0d.s. 149us : kmem_cache_free (dst_destroy)
>
> [ etc - zillions of dst_rcu_free()s deleted ]

Are these predominantly coming from rt_run_flush() ? I had an old
patch that frees one hash chain per RCU callback. However the
cost of each RCU callback goes up here, so I am not sure whether
that helps. Atleast with shorter RCU callbacks, you get a chance
for preemption after the tasklet softirq has happened a few
times. Can you describe what you are running in your system ?
My old rt-flush-list patch is included below for reference.

Thanks
Dipankar



Reduce the number of RCU callbacks by flushing one hash chain
at a time. This is intended to reduce softirq overhead during
frequent flushing.


net/ipv4/route.c | 19 +++++++++++++------
1 files changed, 13 insertions(+), 6 deletions(-)
diff -puN net/ipv4/route.c~rcu-rt-flush-list net/ipv4/route.c
--- linux-2.6.0-test2-rcu/net/ipv4/route.c~rcu-rt-flush-list 2003-08-13 21:46:35.000000000 +0530
+++ linux-2.6.0-test2-rcu-dipankar/net/ipv4/route.c 2003-08-27 14:17:54.000000000 +0530
@@ -521,13 +521,22 @@ static void rt_check_expire(unsigned lon
mod_timer(&rt_periodic_timer, now + ip_rt_gc_interval);
}

+static void rt_list_free(struct rtable *rth)
+{
+ struct rtable *next;
+ for (; rth; rth = next) {
+ next = rth->u.rt_next;
+ dst_free(&rth->u.dst);
+ }
+}
+
/* This can run from both BH and non-BH contexts, the latter
* in the case of a forced flush event.
*/
static void rt_run_flush(unsigned long dummy)
{
int i;
- struct rtable *rth, *next;
+ struct rtable *rth;

rt_deadline = 0;

@@ -539,11 +548,9 @@ static void rt_run_flush(unsigned long d
if (rth)
rt_hash_table[i].chain = NULL;
spin_unlock_bh(&rt_hash_table[i].lock);
-
- for (; rth; rth = next) {
- next = rth->u.rt_next;
- rt_free(rth);
- }
+ if (rth)
+ call_rcu_bh(&rth->u.dst.rcu_head,
+ (void (*)(void *))rt_list_free, rth);
}
}

_
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