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

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Message-ID: <ea94bc64-a109-c033-7ecc-c51c19b214c8@mojatatu.com>
Date:   Mon, 12 Sep 2016 19:40:13 -0400
From:   Jamal Hadi Salim <jhs@...atatu.com>
To:     Eric Dumazet <eric.dumazet@...il.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 16-09-12 07:20 PM, Eric Dumazet wrote:
> On Mon, 2016-09-12 at 16:10 -0700, Eric Dumazet wrote:
>
>>
>> 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));
>>  }
>
> Ah well, forget this patch, re-enabling bh right before
> read_seqcount_retry() is not going to help.
>

I have to say I have seen some odd issues once in a while reading
generic action stats.
I had a program that opened a netlink socket into the kernel.
Every X seconds it does a dump of all the actions to read the
stats.
There is a very reproducible behavior that the stats
are not in sync with the kernel. Given generic stats is lockless
I thought maybe rcu or per-cpu stats was the issue. I havent had time
to look closely.
The solution is instead of keeping the socket open all the time;
I open, read stats, close (repeat every x seconds).

If there is something you want me to try - I could do sometimes
this week. Your patch above may be useful!

cheers,
jamal