linux-kernel - Re: [PATCH RFC 7/8] rcu: trace CleanupMore condition only if needed

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Date:   Mon, 14 May 2018 12:20:28 -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 7/8] rcu: trace CleanupMore condition only if needed

On Sun, May 13, 2018 at 08:15:40PM -0700, Joel Fernandes (Google) wrote:
> Currently the tree RCU clean up code records a CleanupMore trace event
> even if the GP was already in progress. This makes CleanupMore show up
> twice for no reason. Avoid it.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Good catch, and I applied this patch.  I did rework the commit log
a bit, so please look it over to make sure I didn't mess it up.

							Thanx, Paul

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

commit 52c4e689efd975f5383895b1bc1b91bc90fdd372
Author: Joel Fernandes (Google) <joel@...lfernandes.org>
Date:   Sun May 13 20:15:40 2018 -0700

    rcu: Produce last "CleanupMore" trace only if late-breaking request
    
    Currently the tree RCU clean-up code records a "CleanupMore" trace
    event in response to late-arriving grace-period requests even if the
    grace period was already requested. This makes "CleanupMore" show up an
    extra time (in addition to once for each rcu_node structure that was
    previously marked with the request) for no good reason.  This commit
    therefore avoids emitting this trace message unless the only if the only
    request for this next grace period arrived during or after the cleanup
    scan of the rcu_node structures.
    
    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 8063a0478870..de6447dd73de 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2072,7 +2072,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 	rsp->gp_state = RCU_GP_IDLE;
 	/* Check for GP requests since above loop. */
 	rdp = this_cpu_ptr(rsp->rda);
-	if (ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
+	if (!needgp && ULONG_CMP_LT(rnp->gp_seq, rnp->gp_seq_needed)) {
 		trace_rcu_this_gp(rnp, rdp, rnp->gp_seq_needed,
 				  TPS("CleanupMore"));
 		needgp = true;