On Thu, 2017-10-26 at 21:28 -0700, Cong Wang wrote:
> On Thu, Oct 26, 2017 at 9:05 PM, Eric Dumazet <eric.duma...@gmail.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.
