linux-kernel - Re: [PATCH RFC 8/8] rcu: Fix cpustart tracepoint gp

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Message-Id: <20180514203320.GK26088@linux.vnet.ibm.com>
Date:   Mon, 14 May 2018 13:33:20 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com
Subject: Re: [PATCH RFC 8/8] rcu: Fix cpustart tracepoint gp_seq number

On Sun, May 13, 2018 at 08:15:41PM -0700, Joel Fernandes (Google) wrote:
> cpustart shows a stale gp_seq. This is because rdp->gp_seq is updated
> only at the end of the __note_gp_changes function. For this reason, use
> rnp->gp_seq instead. I believe we can't update rdp->gp_seq too early so
> lets just use the gp_seq from rnp instead.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

I took this one with the usual commit-log update.  Please take a look!

							Thanx, Paul

------------------------------------------------------------------------

commit 3e14c6a5225d201776edd9a96d9c9b4435855446
Author: Joel Fernandes (Google) <joel@...lfernandes.org>
Date:   Sun May 13 20:15:41 2018 -0700

    rcu: Fix cpustart tracepoint gp_seq number
    
    The "cpustart" trace event shows a stale gp_seq. This is because it uses
    rdp->gp_seq, which is updated only at the end of the __note_gp_changes()
    function. This commit therefore instead uses rnp->gp_seq.
    
    An alternative fix would be to update rdp->gp_seq earlier, but this would
    break RCU's detection of the beginning of a new-to-this-CPU grace period.
    
    Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
    Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index de6447dd73de..23b855f5c5cb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1796,7 +1796,7 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
 		 * set up to detect a quiescent state, otherwise don't
 		 * go looking for one.
 		 */
-		trace_rcu_grace_period(rsp->name, rdp->gp_seq, TPS("cpustart"));
+		trace_rcu_grace_period(rsp->name, rnp->gp_seq, TPS("cpustart"));
 		need_gp = !!(rnp->qsmask & rdp->grpmask);
 		rdp->cpu_no_qs.b.norm = need_gp;
 		rdp->rcu_qs_ctr_snap = __this_cpu_read(rcu_dynticks.rcu_qs_ctr);