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

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Message-ID: <1509079166.11887.33.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Thu, 26 Oct 2017 21:39:26 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...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, 2017-10-26 at 21:28 -0700, Cong Wang wrote:
> 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.

You added the rtnl_lock()  rtnl_unlock()...

I really do not care if hundreds of tasks (not owning rtnl) call
rcu_barrier()...

Also we are still using a 4.3 based kernel, and no rcu_barrier() is used
in filters dismantle ( unregister_tcf_proto_ops() is not used in our
workloads )

Somehow something went very wrong in net/sched in recent kernels.