Messages in this thread Patch in this message | | | Date | Mon, 21 May 2018 16:13:57 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v3 3/4] rcu: Use better variable names in funnel locking loop |
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On Sun, May 20, 2018 at 09:42:19PM -0700, Joel Fernandes wrote: > The funnel locking loop in rcu_start_this_gp uses rcu_root as a > temporary variable while walking the combining tree. This causes a > tiresome exercise of a code reader reminding themselves that rcu_root > may not be root. Lets just call it rnp, and rename other variables as > well to be more appropriate. > > Original patch: https://patchwork.kernel.org/patch/10396577/ > > Signed-off-by: Joel Fernandes <joel@joelfernandes.org>
Nice!
Please see feedback interspersed below.
Thanx, Paul
> --- > kernel/rcu/tree.c | 48 ++++++++++++++++++++++++----------------------- > 1 file changed, 25 insertions(+), 23 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 0ffd41ba304f..879c67a31116 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1526,7 +1526,7 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, > > /* > * rcu_start_this_gp - Request the start of a particular grace period > - * @rnp: The leaf node of the CPU from which to start. > + * @rnp_start: The leaf node of the CPU from which to start. > * @rdp: The rcu_data corresponding to the CPU from which to start. > * @gp_seq_req: The gp_seq of the grace period to start. > * > @@ -1540,12 +1540,12 @@ static void trace_rcu_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, > * > * Returns true if the GP thread needs to be awakened else false. > */ > -static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, > +static bool rcu_start_this_gp(struct rcu_node *rnp_start, struct rcu_data *rdp, > unsigned long gp_seq_req) > { > bool ret = false; > struct rcu_state *rsp = rdp->rsp; > - struct rcu_node *rnp_root; > + struct rcu_node *rnp, *rnp_root = NULL;
Unless I am going blind, this patch really isn't using rnp_root. It could be removed.
> > /* > * Use funnel locking to either acquire the root rcu_node > @@ -1556,34 +1556,36 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, > * scan the leaf rcu_node structures. Note that rnp->lock must > * not be released. > */ > - raw_lockdep_assert_held_rcu_node(rnp); > - trace_rcu_this_gp(rnp, rdp, gp_seq_req, TPS("Startleaf")); > - for (rnp_root = rnp; 1; rnp_root = rnp_root->parent) { > - if (rnp_root != rnp) > - raw_spin_lock_rcu_node(rnp_root); > - if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req) || > - rcu_seq_started(&rnp_root->gp_seq, gp_seq_req) || > - (rnp != rnp_root && > - rcu_seq_state(rcu_seq_current(&rnp_root->gp_seq)))) { > - trace_rcu_this_gp(rnp_root, rdp, gp_seq_req, > + raw_lockdep_assert_held_rcu_node(rnp_start); > + trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, TPS("Startleaf")); > + for (rnp = rnp_start; 1; rnp = rnp->parent) { > + if (rnp != rnp_start) > + raw_spin_lock_rcu_node(rnp); > + if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req) || > + rcu_seq_started(&rnp->gp_seq, gp_seq_req) || > + (rnp != rnp_start && > + rcu_seq_state(rcu_seq_current(&rnp->gp_seq)))) { > + trace_rcu_this_gp(rnp, rdp, gp_seq_req, > TPS("Prestarted")); > goto unlock_out; > } > - rnp_root->gp_seq_needed = gp_seq_req; > - if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) { > + rnp->gp_seq_needed = gp_seq_req; > + if (rcu_seq_state(rcu_seq_current(&rnp_start->gp_seq))) {
The original had a performance bug, which is quite a bit more obvious given the new names, so thank you for that! The above statement should instead be as follows:
if (rcu_seq_state(rcu_seq_current(&rnp->gp_seq))) {
It does not make sense to keep checking the starting rcu_node because changes to ->gp_seq happen first at the top of the tree. So we might take an earlier exit by checking the current rnp instead of rechecking rnp_start over and over.
Please feel free to make this change, which is probably best as a separate patch. That way this rename patch can remain a straightforward rename patch.
> /* > * We just marked the leaf, and a grace period > * is in progress, which means that rcu_gp_cleanup() > * will see the marking. Bail to reduce contention. > */ > - trace_rcu_this_gp(rnp, rdp, gp_seq_req, > + trace_rcu_this_gp(rnp_start, rdp, gp_seq_req, > TPS("Startedleaf")); > goto unlock_out; > } > - if (rnp_root != rnp && rnp_root->parent != NULL) > - raw_spin_unlock_rcu_node(rnp_root); > - if (!rnp_root->parent) > + if (rnp != rnp_start && rnp->parent != NULL) > + raw_spin_unlock_rcu_node(rnp); > + if (!rnp->parent) { > + rnp_root = rnp;
Since rnp_root is otherwise unused in the new version, the above statement can be dropped along with the "if" statement's braces and the declaration.
> break; /* At root, and perhaps also leaf. */ > + } > } > > /* If GP already in progress, just leave, otherwise start one. */ > @@ -1601,11 +1603,11 @@ static bool rcu_start_this_gp(struct rcu_node *rnp, struct rcu_data *rdp, > trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gp_seq), TPS("newreq")); > ret = true; /* Caller must wake GP kthread. */ > unlock_out: > - if (rnp != rnp_root) > - raw_spin_unlock_rcu_node(rnp_root); > + if (rnp != rnp_start) > + raw_spin_unlock_rcu_node(rnp); > /* Push furthest requested GP to leaf node and rcu_data structure. */ > - if (ULONG_CMP_GE(rnp_root->gp_seq_needed, gp_seq_req)) { > - rnp->gp_seq_needed = gp_seq_req; > + if (ULONG_CMP_GE(rnp->gp_seq_needed, gp_seq_req)) { > + rnp_start->gp_seq_needed = gp_seq_req; > rdp->gp_seq_needed = gp_seq_req; > } > return ret; > -- > 2.17.0.441.gb46fe60e1d-goog >
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