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SubjectRe: [PATCH RFC 6/8] rcu: Add back the Startedleaf tracepoint
On Sun, May 13, 2018 at 08:15:39PM -0700, Joel Fernandes (Google) wrote:
> In recent discussion [1], the check for whether a leaf believes RCU is
> not idle, is being added back to funnel locking code, to avoid more
> locking. In this we are marking the leaf node for a future grace-period
> and bailing out since a GP is currently in progress. However the
> tracepoint is missing. Lets add it back.
>
> Also add a small comment about why we do this check (basically the point
> is to avoid locking intermediate nodes unnecessarily) and clarify the
> comments in the trace event header now that we are doing traversal of
> one or more intermediate nodes.
>
> [1] http://lkml.kernel.org/r/20180513190906.GL26088@linux.vnet.ibm.com
>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Looks like a good idea, but it does not apply -- which is not a surprise,
given the change rate in this code. I hand-applied as a modification
to c1b3f9fce26f ("rcu: Don't funnel-lock above leaf node if GP in progress")
with attribution, but with the changes below. Please let me know if I
am missing something.

Ah, I see -- this commit depends on your earlier name-change commit.
I therefore made this patch use the old names.

> ---
> include/trace/events/rcu.h | 4 ++--
> kernel/rcu/tree.c | 11 ++++++++++-
> 2 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index 539900a9f8c7..dc0bd11739c7 100644
> --- a/include/trace/events/rcu.h
> +++ b/include/trace/events/rcu.h
> @@ -91,8 +91,8 @@ TRACE_EVENT(rcu_grace_period,
> *
> * "Startleaf": Request a grace period based on leaf-node data.
> * "Prestarted": Someone beat us to the request
> - * "Startedleaf": Leaf-node start proved sufficient.
> - * "Startedleafroot": Leaf-node start proved sufficient after checking root.
> + * "Startedleaf": Leaf and one or more non-root nodes marked for future start.

Actually, we only get to that trace if all we did was mark the leaf
node, right?

> + * "Startedleafroot": all non-root nodes from leaf to root marked for future start.

I got rid of the "non-root" part, given that we had to have marked
the root to break out of the loop.

Thanx, Paul

> * "Startedroot": Requested a nocb grace period based on root-node data.
> * "NoGPkthread": The RCU grace-period kthread has not yet started.
> * "StartWait": Start waiting for the requested grace period.
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 40670047d22c..8401a253e7de 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1593,8 +1593,17 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp,
> goto unlock_out;
> }
> rnp_node->gp_seq_needed = gp_seq_start;
> - if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))
> +
> + /*
> + * Check if leaf believes a GP is in progress, if yes we can
> + * bail and avoid more locking. We have already marked the leaf.
> + */
> + if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
> + trace_rcu_this_gp(rnp_node, rdp, gp_seq_start,
> + TPS("Startedleaf"));
> goto unlock_out;
> + }
> +
> if (rnp_node != rnp && rnp_node->parent != NULL)
> raw_spin_unlock_rcu_node(rnp_node);
> if (!rnp_node->parent) {
> --
> 2.17.0.441.gb46fe60e1d-goog
>

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