On Sun, Oct 16, 2016 at 04:45:32AM +0300, Michael S. Tsirkin wrote: > On Fri, Oct 14, 2016 at 02:20:50AM -0700, Paul E. McKenney wrote: > > On Thu, Oct 13, 2016 at 07:25:56PM +0300, Michael S. Tsirkin wrote: > > > On Wed, Oct 12, 2016 at 01:32:23PM -0700, Paul E. McKenney wrote: > > > > On Wed, Oct 12, 2016 at 12:15:53PM -0500, Julia Cartwright wrote: > > > > > On Wed, Oct 12, 2016 at 12:49:56PM -0400, Luiz Capitulino wrote: > > > > > > On Wed, 12 Oct 2016 11:21:14 -0500 > > > > > > Julia Cartwright <julia@xxxxxx> wrote: > > > > > > > > > > > > > On Wed, Oct 12, 2016 at 11:12:51AM -0400, Luiz Capitulino wrote: > > > > > > > > Hi, > > > > > > > > > > > > > > > > We have the following patch applied to the RT tree: > > > > > > > > > > > > > > > > commit a9d3cc781a3306bfa332fa7cb6134b70696058d5 > > > > > > > > Author: Josh Cartwright <joshc@xxxxxx> > > > > > > > > Date: Tue Oct 27 07:31:53 2015 -0500 > > > > > > > > > > > > > > > > net: Make synchronize_rcu_expedited() conditional on !RT_FULL > > > > > > > > > > > > > > > > However, as noted by Michael, making rcu_normal=1 default in the > > > > > > > > RT kernel should have the same effect (ie. not calling > > > > > > > > synchronize_sched_expedited()). > > > > > > > > > > > > > > > > So, honest question, is there a reason not to: > > > > > > > > > > > > > > > > 1. Drop the patch above > > > > > > > > 2. Make rcu_normal=1 default? > > > > > > > > > > > > > > I think this is probably a cleaner way to fix the problems which > > > > > > > motivated this patch in the first place. In my defense, the above patch > > > > > > > predates rcu_normal :). > > > > > > > > > > > > No need to defend yourself! We debugged this very spike in one of > > > > > > our kernels that don't have rcu_normal. We decided to do exactly > > > > > > what you're doing before looking at upstream. Your patch helped > > > > > > us confirm that we were in the right track. > > > > > > > > > > Great! Glad I could help in some way! > > > > > > > > > > > > Something like this, perhaps? > > > > > > > > > > > > Looks good, but (honest question) what does it buy us using > > > > > > rcu_normal_after_boot vs rcu_normal? Is the boot process > > > > > > improved someway? > > > > > > > > > > That's the idea, although I don't have data to show much it actually > > > > > buys us. > > > > > > > > It means that grace periods can be expedited during boot. If you really > > > > care about boot speed, you can also set rcu_expedited=1 and also > > > > rcu_normal_after_boot=1, which will expedite all grace periods during > > > > the boot process, but stop doing so just before spawning init. > > > > After that point, any attempt to do an expedited grace period gets you > > > > a normal grace period instead. > > > > > > > > So you get fast boot and then clean realtime. > > > > > > > > > > As long as we're rcu_normal=1 before launching user-space, > > > > > > this should be fine. > > > > > > > > > > Agreed. > > > > > > > > Yes, you can also set them manually instead of at boot, if you wish. > > > > > > > > Thanx, Paul > > > > > > FWIW > > > > > > Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > > > > > > But I have a question - here's the commit that started > > > it all: > > > > > > > > > commit be3fc413da9eb17cce0991f214ab019d16c88c41 > > > Author: Eric Dumazet <eric.dumazet@xxxxxxxxx> > > > Date: Mon May 23 23:07:32 2011 +0000 > > > > > > net: use synchronize_rcu_expedited() > > > > > > synchronize_rcu() is very slow in various situations (HZ=100, > > > CONFIG_NO_HZ=y, CONFIG_PREEMPT=n) > > > > > > Extract from my (mostly idle) 8 core machine : > > > > > > synchronize_rcu() in 99985 us > > > synchronize_rcu() in 79982 us > > > synchronize_rcu() in 87612 us > > > synchronize_rcu() in 79827 us > > > synchronize_rcu() in 109860 us > > > synchronize_rcu() in 98039 us > > > synchronize_rcu() in 89841 us > > > synchronize_rcu() in 79842 us > > > synchronize_rcu() in 80151 us > > > synchronize_rcu() in 119833 us > > > synchronize_rcu() in 99858 us > > > synchronize_rcu() in 73999 us > > > synchronize_rcu() in 79855 us > > > synchronize_rcu() in 79853 us > > > > > > When we hold RTNL mutex, we would like to spend some cpu cycles but not > > > block too long other processes waiting for this mutex. > > > > > > We also want to setup/dismantle network features as fast as possible at > > > boot/shutdown time. > > > > > > > > > To make sure this does not regress for RT, > > > how about clearing this flag on shutdown as well? > > > > By that, you mean having some way to force all grace periods to be > > expedited during shutdown? Or am I missing your point? > > > > Thanx, Paul > > Exactly. And maybe kexec. If the relevant maintainers are OK with that, I am OK with it as long as it is non-default (at least to begin with) and does not introduce additional Kconfig questions. My guess is that a boot parameter would work best, but something to discuss. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
