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Message-ID: <20200825215553.GB16846@paulmck-ThinkPad-P72>
Date: Tue, 25 Aug 2020 14:55:53 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc: linux-kernel@...r.kernel.org, boqun.feng@...il.com,
dave@...olabs.net, Ingo Molnar <mingo@...hat.com>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
neeraj.iitr10@...il.com, rcu@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>,
"Uladzislau Rezki (Sony)" <urezki@...il.com>, vineethrp@...il.com
Subject: Re: [PATCH v4 -rcu 4/4] rcu/trace: Add tracing for how segcb list
changes
On Mon, Aug 24, 2020 at 10:48:42PM -0400, Joel Fernandes (Google) wrote:
> Track how the segcb list changes before/after acceleration, during
> queuing and during dequeuing.
>
> This has proved useful to discover an optimization to avoid unwanted GP
> requests when there are no callbacks accelerated. The overhead is minimal as
> each segment's length is now stored in the respective segment.
>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Speaking of tracing, diagnostics, and debugging, a few instances of
ASSERT_EXCLUSIVE_WRITER() in the previous patch might save a lot
of debugging. Help KCSAN help us.
> ---
> include/trace/events/rcu.h | 25 +++++++++++++++++++++++++
> kernel/rcu/rcu_segcblist.c | 27 +++++++++++++++++++++++++++
> kernel/rcu/rcu_segcblist.h | 7 +++++++
> kernel/rcu/tree.c | 23 +++++++++++++++++++++++
> 4 files changed, 82 insertions(+)
>
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index 155b5cb43cfd..7b84df3c95df 100644
> --- a/include/trace/events/rcu.h
> +++ b/include/trace/events/rcu.h
> @@ -505,6 +505,31 @@ TRACE_EVENT_RCU(rcu_callback,
> __entry->qlen)
> );
>
> +TRACE_EVENT_RCU(rcu_segcb,
> +
> + TP_PROTO(const char *ctx, int *cb_count, unsigned long *gp_seq),
> +
> + TP_ARGS(ctx, cb_count, gp_seq),
> +
> + TP_STRUCT__entry(
> + __field(const char *, ctx)
> + __array(int, cb_count, 4)
> + __array(unsigned long, gp_seq, 4)
> + ),
> +
> + TP_fast_assign(
> + __entry->ctx = ctx;
> + memcpy(__entry->cb_count, cb_count, 4 * sizeof(int));
> + memcpy(__entry->gp_seq, gp_seq, 4 * sizeof(unsigned long));
> + ),
> +
> + TP_printk("%s cb_count: (DONE=%d, WAIT=%d, NEXT_READY=%d, NEXT=%d) "
> + "gp_seq: (DONE=%lu, WAIT=%lu, NEXT_READY=%lu, NEXT=%lu)", __entry->ctx,
> + __entry->cb_count[0], __entry->cb_count[1], __entry->cb_count[2], __entry->cb_count[3],
> + __entry->gp_seq[0], __entry->gp_seq[1], __entry->gp_seq[2], __entry->gp_seq[3])
> +
> +);
> +
> /*
> * Tracepoint for the registration of a single RCU callback of the special
> * kvfree() form. The first argument is the RCU type, the second argument
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 73a103464ea4..6419dbbaecde 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -378,6 +378,33 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
> rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0);
> }
>
> +/*
> + * Return how many CBs each segment along with their gp_seq values.
> + *
> + * This function is O(N) where N is the number of callbacks. Only used from
> + * tracing code which is usually disabled in production.
> + */
> +#ifdef CONFIG_RCU_TRACE
> +void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
> + int cbcount[RCU_CBLIST_NSEGS],
> + unsigned long gpseq[RCU_CBLIST_NSEGS])
> +{
Why not declare the arrays here and invoke the new trace event from
here also? That would simplify the usage and save a few lines of code.
Thanx, Paul
> + struct rcu_head **cur_tail;
> + int i;
> +
> + for (i = 0; i < RCU_CBLIST_NSEGS; i++)
> + cbcount[i] = 0;
> +
> + cur_tail = &(rsclp->head);
> +
> + for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
> + cbcount[i] = rcu_segcblist_get_seglen(rsclp, i);
> + gpseq[i] = rsclp->gp_seq[i];
> + cur_tail = rsclp->tails[i];
> + }
> +}
> +#endif
> +
> /*
> * Extract only those callbacks still pending (not yet ready to be
> * invoked) from the specified rcu_segcblist structure and place them in
> diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
> index b90725f81d77..5f7cdfed0ba4 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -105,3 +105,10 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq);
> bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq);
> void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
> struct rcu_segcblist *src_rsclp);
> +#ifdef CONFIG_RCU_TRACE
> +void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
> + int cbcount[RCU_CBLIST_NSEGS],
> + unsigned long gpseq[RCU_CBLIST_NSEGS]);
> +#else
> +#define rcu_segcblist_countseq(...)
> +#endif
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 51348144a4ea..16ad99a9ebba 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1476,6 +1476,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
> {
> unsigned long gp_seq_req;
> bool ret = false;
> + int cbs[RCU_CBLIST_NSEGS];
> + unsigned long gps[RCU_CBLIST_NSEGS];
>
> rcu_lockdep_assert_cblist_protected(rdp);
> raw_lockdep_assert_held_rcu_node(rnp);
> @@ -1484,6 +1486,10 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
> if (!rcu_segcblist_pend_cbs(&rdp->cblist))
> return false;
>
> + /* Count CBs for tracing. */
> + rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
> + trace_rcu_segcb("SegCbPreAcc", cbs, gps);
> +
> /*
> * Callbacks are often registered with incomplete grace-period
> * information. Something about the fact that getting exact
> @@ -1504,6 +1510,10 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
> else
> trace_rcu_grace_period(rcu_state.name, gp_seq_req, TPS("AccReadyCB"));
>
> + /* Count CBs for tracing. */
> + rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
> + trace_rcu_segcb("SegCbPostAcc", cbs, gps);
> +
> return ret;
> }
>
> @@ -2421,6 +2431,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
> long bl, count = 0;
> long pending, tlimit = 0;
> + int cbs[RCU_CBLIST_NSEGS];
> + unsigned long gps[RCU_CBLIST_NSEGS];
>
> /* If no callbacks are ready, just return. */
> if (!rcu_segcblist_ready_cbs(&rdp->cblist)) {
> @@ -2461,6 +2473,11 @@ static void rcu_do_batch(struct rcu_data *rdp)
> /* Invoke callbacks. */
> tick_dep_set_task(current, TICK_DEP_BIT_RCU);
> rhp = rcu_cblist_dequeue(&rcl);
> +
> + /* Count CBs for tracing. */
> + rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
> + trace_rcu_segcb("SegCbDequeued", cbs, gps);
> +
> for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
> rcu_callback_t f;
>
> @@ -2929,6 +2946,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
> unsigned long flags;
> struct rcu_data *rdp;
> bool was_alldone;
> + int cbs[RCU_CBLIST_NSEGS];
> + unsigned long gps[RCU_CBLIST_NSEGS];
>
> /* Misaligned rcu_head! */
> WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
> @@ -2974,6 +2993,10 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
> trace_rcu_callback(rcu_state.name, head,
> rcu_segcblist_n_cbs(&rdp->cblist));
>
> + /* Count CBs for tracing. */
> + rcu_segcblist_countseq(&rdp->cblist, cbs, gps);
> + trace_rcu_segcb("SegCBQueued", cbs, gps);
> +
> /* Go handle any RCU core processing required. */
> if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
> --
> 2.28.0.297.g1956fa8f8d-goog
>
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