On Tue, 2015-10-27 at 12:02 -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Oct 27, 2015 at 07:18:01AM -0700, Eric Dumazet escreveu:
> > On Tue, 2015-10-27 at 07:31 -0500, Josh Cartwright wrote:
> >
> > > Okay, yes, I like the first suggestion better as well, I've included a
> > > patch below that does just that. I hope you don't mind me turning it
> > > into a Suggested-by :).
> > >
> > > Thanks for taking a look!
> > > Josh
> >
> >
> > > @@ -6969,7 +6969,7 @@ EXPORT_SYMBOL(free_netdev);
> > > void synchronize_net(void)
> > > {
> > > might_sleep();
> > > - if (rtnl_is_locked())
> > > + if (rtnl_is_locked() && !IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
> > > synchronize_rcu_expedited();
> > > else
> > > synchronize_rcu();
> >
> > No objection from me. Thanks.
> >
> > Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> The first suggestion, with it disabled by default seems to be the most
> flexible tho, i.e, Paul's original message plus the boot parameter line:
>
> Alternatively, a boot-time option could be used:
>
> int some_rt_boot_parameter = CONFIG_SYNC_NET_DEFAULT;
>
> if (rtnl_is_locked() && !some_rt_boot_parameter)
> synchronize_rcu_expedited();
> else
> synchronize_rcu();
>
> Then RT oriented kernel .config files would have CONFIG_SYNC_NET_DEFAULT
> set to 1, while upstream would have this default to 0.
>
> RT oriented kernel users could try using this in some scenarios where
> networking is not the critical path.
Well, if synchronize_rcu_expedited() is such a problem on RT, then maybe
a generic solution would make synchronize_rcu_expedited() to fallback
synchronize_rcu() after boot time on RT.
Not sure why networking use of synchronize_rcu_expedited() would be
problematic, and not the others.
scripts/checkpatch.pl has this comment about this :
# Check for expedited grace periods that interrupt non-idle non-nohz
# online CPUs. These expedited can therefore degrade real-time response
# if used carelessly, and should be avoided where not absolutely
# needed. It is always OK to use synchronize_rcu_expedited() and
# synchronize_sched_expedited() at boot time (before real-time applications
# start) and in error situations where real-time response is compromised in
# any case. Note that synchronize_srcu_expedited() does -not- interrupt
# other CPUs, so don't warn on uses of synchronize_srcu_expedited().
# Of course, nothing comes for free, and srcu_read_lock() and
# srcu_read_unlock() do contain full memory barriers in payment for
# synchronize_srcu_expedited() non-interruption properties.
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