Merge branch 'nohz/guest' of git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks into timers/nohz

Pull full dynticks support for virt guests from Frederic Weisbecker:

 "Some measurements showed that disabling the tick on the host while the
  guest is running can be interesting on some workloads. Indeed the
  host tick is irrelevant while a vcpu runs, it consumes CPU time and cache
  footprint for no good reasons.

  Full dynticks already works in every context, but RCU prevents it to
  be effective outside userspace, because the CPU needs to take part of
  RCU grace period completion as long as RCU may be used on it, which is
  the case in kernel context.

  However guest is similar to userspace and idle in that we know RCU is
  unused on such context. Therefore a CPU in guest/userspace/idle context
  can let other CPUs report its own RCU quiescent state on its behalf
  and shut down the tick safely, provided it isn't needed for other
  reasons than RCU. This is called RCU extended quiescent state.

  This was already implemented for idle and userspace. This patchset now
  brings it for guest contexts through the following steps:

  - Generalize the context tracking APIs to also track guest state
  - Rename/sanitize a few CPP symbols accordingly
  - Report guest entry/exit to RCU and define this context area as an RCU
    extended quiescent state."

Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index d12b2104d19b..cc8c61c5459c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -766,7 +766,8 @@ static inline void kvm_guest_enter(void)
 	 * one time slice). Lets treat guest mode as quiescent state, just like
 	 * we do with user-mode execution.
 	 */
-	rcu_virt_note_context_switch(smp_processor_id());
+	if (!context_tracking_cpu_is_enabled())
+		rcu_virt_note_context_switch(smp_processor_id());
 }
 
 static inline void kvm_guest_exit(void)