netdev - Re: [Patch net 01/16] net_sched: introduce a workqueue for RCU callbacks of tc filter

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Date:   Thu, 26 Oct 2017 21:28:01 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Chris Mi <chrism@...lanox.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jiri Pirko <jiri@...nulli.us>,
        John Fastabend <john.fastabend@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [Patch net 01/16] net_sched: introduce a workqueue for RCU
 callbacks of tc filter

On Thu, Oct 26, 2017 at 9:05 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2017-10-26 at 18:24 -0700, Cong Wang wrote:
>> ...
>
>> On the other hand, this makes tcf_block_put() ugly and
>> harder to understand. Since David and Eric strongly dislike
>> adding synchronize_rcu(), this is probably the only
>> solution that could make everyone happy.
>
>
> ...
>
>> +static void tcf_block_put_deferred(struct work_struct *work)
>> +{
>> +     struct tcf_block *block = container_of(work, struct tcf_block, work);
>> +     struct tcf_chain *chain;
>>
>> +     rtnl_lock();
>>       /* Hold a refcnt for all chains, except 0, in case they are gone. */
>>       list_for_each_entry(chain, &block->chain_list, list)
>>               if (chain->index)
>> @@ -292,13 +308,27 @@ void tcf_block_put(struct tcf_block *block)
>>       list_for_each_entry(chain, &block->chain_list, list)
>>               tcf_chain_flush(chain);
>>
>> -     /* Wait for RCU callbacks to release the reference count. */
>> +     INIT_WORK(&block->work, tcf_block_put_final);
>> +     /* Wait for RCU callbacks to release the reference count and make
>> +      * sure their works have been queued before this.
>> +      */
>>       rcu_barrier();
>> +     tcf_queue_work(&block->work);
>> +     rtnl_unlock();
>> +}
>
>
> On a loaded server, rcu_barrier() typically takes 4 ms.
>
> Way better than synchronize_rcu() (about 90 ms) but still an issue when
> holding RTNL.
>
> We have thousands of filters, and management daemon restarts and rebuild
> TC hierarchy from scratch.
>
> Simply getting rid of 1000 old filters might block RTNL for a while, or
> maybe I misunderstood your patches.
>

Paul pointed out the same.

As I replied, this rcu_barrier() is NOT added by this patchset, it is already
there in current master branch.