srcu: Expedite srcu_schedule_cbs_snp() callback invocation

Although Tree SRCU does reduce delays when there is at least one
synchronize_srcu_expedited() invocation pending, srcu_schedule_cbs_snp()
still waits for SRCU_INTERVAL before invoking callbacks.  Since
synchronize_srcu_expedited() now posts a callback and waits for
that callback to do a wakeup, this destroys the expedited nature of
synchronize_srcu_expedited().  This destruction became apparent to
Marc Zyngier in the guise of a guest-OS bootup slowdown from five
seconds to no fewer than forty seconds.

This commit therefore invokes callbacks immediately at the end of the
grace period when there is at least one synchronize_srcu_expedited()
invocation pending.  This brought Marc's guest-OS bootup times back
into the realm of reason.

Reported-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 12feeca18f46b91c704c2cc542bf857c72d5e47b..9ecf0acc18eb66d8f03ed25302a1225ff7f6f59b 100644 (file)
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -442,7 +442,8 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp)
        int cpu;
 
        for (cpu = snp->grplo; cpu <= snp->grphi; cpu++)
-               srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu), SRCU_INTERVAL);
+               srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu),
+                                     atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
 }
 
 /*