netdev - Re: [PATCH v4 net-next 1/1] net_sched: Introduce skbmod action

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Date:   Mon, 12 Sep 2016 16:10:37 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jamal Hadi Salim <jhs@...atatu.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org,
        xiyou.wangcong@...il.com, daniel@...earbox.net,
        john.r.fastabend@...el.com
Subject: Re: [PATCH v4 net-next 1/1] net_sched: Introduce skbmod action

On Mon, 2016-09-12 at 19:02 -0400, Jamal Hadi Salim wrote:
> On 16-09-12 06:26 PM, Eric Dumazet wrote:
> > On Mon, 2016-09-12 at 18:14 -0400, Jamal Hadi Salim wrote:
> >
> >> I noticed some very weird issues when I took that out.
> >> Running sufficiently large amount of traffic (ping -f is sufficient)
> >> I saw that when i did a dump it took anywhere between 6-15 seconds.
> >> With the read_lock in place response was immediate.
> >> I can go back and run things to verify - but it was very odd.
> >
> > This was on uni processor ?
> >
> 
> It was a VM.
> 
> > Looks like typical starvation caused by aggressive softirq.
> >
> 
> Well, then it is strange that in one case a tc dump of the rule
> was immediate and in the other case it was consistent for 5-15
> seconds.
> 

This needs investigation ;)

One possible loop under high stress would be possible in
__gnet_stats_copy_basic(), since we might restart the loop if we are
really really unlucky, but this would have nothing with your patches.



diff --git a/net/core/gen_stats.c b/net/core/gen_stats.c
index 508e051304fb62627e61b5065b2325edd1b84f2e..dc9dd8ae7d5405f76c775278dac7689655b21041 100644
--- a/net/core/gen_stats.c
+++ b/net/core/gen_stats.c
@@ -142,10 +142,14 @@ __gnet_stats_copy_basic(const seqcount_t *running,
 		return;
 	}
 	do {
-		if (running)
+		if (running) {
+			local_bh_disable();
 			seq = read_seqcount_begin(running);
+		}
 		bstats->bytes = b->bytes;
 		bstats->packets = b->packets;
+		if (running)
+			local_bh_enable();
 	} while (running && read_seqcount_retry(running, seq));
 }
 EXPORT_SYMBOL(__gnet_stats_copy_basic);