On Wed, Apr 19, 2017 at 07:47:30AM -0700, Paul E. McKenney wrote: > On Wed, Apr 19, 2017 at 02:08:47PM +0200, Peter Zijlstra wrote: > > On Wed, Apr 19, 2017 at 01:48:08PM +0200, Christian Borntraeger wrote: > > > On 04/19/2017 01:28 PM, Peter Zijlstra wrote: > > > > > > > > So the thing Maz complained about is because KVM assumes > > > > synchronize_srcu() is 'free' when there is no srcu_read_lock() activity. > > > > This series 'breaks' that. > > > > > > Why is such a behaviour change not mentioned in the cover letter? > > > I could not find anything in the patch descriptions that would > > > indicate a slowdown. How much slower did it get? > > > > > > But indeed, there are several places at KVM startup which have been > > > reworked to srcu since normal rcu was too slow for several usecases. > > > (Mostly registering devices and related data structures at startup, > > > basically the qemu/kvm coldplug interaction) > > > > I suspect Paul is not considering this a 'normal' RCU feature, and > > therefore didn't think about changing this. > > > > I know I was fairly surprised by this requirement when I ran into it; > > and only accidentally remembered it now that maz complained. > > Indeed -- the natural thing to have done back when KVM's scalability was > first being worked on would have been to simply change synchronize_rcu() > to synchronize_rcu_expedited(). However, at that time, these things > did try_stop_cpus() and the like, which was really bad for latency. > Moving to SRCU avoided this problem. Of course, now that KVM uses > SRCU, why change unless there is a problem? Besides, I vaguely recall > some KVM cases where srcu_read_lock() is used from CPUs that look to > be idle or offline from RCU's perspective, and that sort of thing only > works for SRCU. > > Which reminds me... > > The RCU expedited primitives have been completely rewritten since then, > and no longer use try_stop_cpus(), no longer disturb idle CPUs, and no > longer disturb nohz_full CPUs running in userspace. In addition, there > is the rcupdate.rcu_normal kernel boot paramter for those who want to > completely avoid RCU expedited primitives. > > So it seems to me to be time for the patch below. Thoughts? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 333d383fad42b4bdef3d27d91e940a6eafed3f91 > Author: Paul E. McKenney <paul...@linux.vnet.ibm.com> > Date: Wed Apr 19 07:37:45 2017 -0700 > > checkpatch: Remove checks for expedited grace periods > > There was a time when the expedited grace-period primitives > (synchronize_rcu_expedited(), synchronize_rcu_bh_expedited(), and > synchronize_sched_expedited()) used rather antisocial kernel > facilities like try_stop_cpus(). However, they have since been > housebroken to use only single-CPU IPIs, and typically cause less > disturbance than a scheduling-clock interrupt. Furthermore, this > disturbance can be eliminated entirely using NO_HZ_FULL on the > one hand or the rcupdate.rcu_normal boot parameter on the other. > > This commit therefore removes checkpatch's complaints about use > of the expedited RCU primitives. > > Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <j...@joshtriplett.org> Still something to hesitate a bit before using, but not something checkpatch should warn about.
