Commit 3563a438 authored Jul 28, 2016 by Paul E. McKenney

rcu: Avoid redundant quiescent-state chasing

Currently, __note_gp_changes() checks to see if the CPU has slept through
multiple grace periods. If it has, it resynchronizes that CPU's view
of the grace-period state, which includes whether or not the current
grace period needs a quiescent state from this CPU. The fact of this
need (or lack thereof) needs to be in two places, rdp->cpu_no_qs.b.norm
and rdp->core_needs_qs. The former tells RCU's context-switch code to
go get a quiescent state and the latter says that it needs to be reported.
The current code unconditionally sets the former to true, but correctly
sets the latter.

This does not result in failures, but it does unnecessarily increase
the amount of work done on average at context-switch time. This commit
therefore correctly sets both fields.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

parent e77b7041

kernel/rcu/tree.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -1846,6 +1846,7 @@ static bool __note_gp_changes(struct rcu_state rsp, struct rcu_node rnp,
		struct rcu_data *rdp)
		{
		bool ret;
		bool need_gp;

		/* Handle the ends of any preceding grace periods first. */
		if (rdp->completed == rnp->completed &&
		@@ -1872,9 +1873,10 @@ static bool __note_gp_changes(struct rcu_state rsp, struct rcu_node rnp,
		*/
		rdp->gpnum = rnp->gpnum;
		trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpustart"));
		rdp->cpu_no_qs.b.norm = true;
		need_gp = !!(rnp->qsmask & rdp->grpmask);
		rdp->cpu_no_qs.b.norm = need_gp;
		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_qs_ctr);
		rdp->core_needs_qs = !!(rnp->qsmask & rdp->grpmask);
		rdp->core_needs_qs = need_gp;
		zero_cpu_stall_ticks(rdp);
		WRITE_ONCE(rdp->gpwrap, false);
		}

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