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Message-ID: <20201105033716.GJ3249@paulmck-ThinkPad-P72>
Date: Wed, 4 Nov 2020 19:37:16 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
linux-kernel@...r.kernel.org,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Marco Elver <elver@...gle.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
"Uladzislau Rezki (Sony)" <urezki@...il.com>, fweisbec@...il.com,
neeraj.iitr10@...il.com
Subject: Re: [PATCH v8 3/6] rcu/trace: Add tracing for how segcb list changes
On Tue, Nov 03, 2020 at 08:47:21AM -0500, Joel Fernandes wrote:
[ . . . ]
> ---8<-----------------------
>
> From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> Subject: [PATCH] rcu/trace: Add tracing for how segcb list changes
>
> 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>
> ---
> include/trace/events/rcu.h | 25 +++++++++++++++++++++++++
> kernel/rcu/rcu_segcblist.c | 34 ++++++++++++++++++++++++++++++++++
> kernel/rcu/rcu_segcblist.h | 5 +++++
> kernel/rcu/tree.c | 9 +++++++++
> 4 files changed, 73 insertions(+)
>
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index 155b5cb43cfd..5f8f2ee1a936 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_stats,
> +
> + 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, RCU_CBLIST_NSEGS)
> + __array(unsigned long, gp_seq, RCU_CBLIST_NSEGS)
> + ),
> +
> + TP_fast_assign(
> + __entry->ctx = ctx;
> + memcpy(__entry->cb_count, cb_count, RCU_CBLIST_NSEGS * sizeof(int));
> + memcpy(__entry->gp_seq, gp_seq, RCU_CBLIST_NSEGS * 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 357c19bbcb00..2a03949d0b82 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -14,6 +14,7 @@
> #include <linux/types.h>
>
> #include "rcu_segcblist.h"
> +#include "rcu.h"
>
> /* Initialize simple callback list. */
> void rcu_cblist_init(struct rcu_cblist *rclp)
> @@ -328,6 +329,39 @@ 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 segments. Only used from
> + * tracing code which is usually disabled in production.
> + */
> +#ifdef CONFIG_RCU_TRACE
> +static void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
> + int cbcount[RCU_CBLIST_NSEGS],
Given that negative numbers are not possible here, shouldn't
the above be unsigned int?
> + unsigned long gpseq[RCU_CBLIST_NSEGS])
> +{
> + int i;
> +
> + for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
> + cbcount[i] = rcu_segcblist_get_seglen(rsclp, i);
> + gpseq[i] = rsclp->gp_seq[i];
> + }
OK, I will bite...
Why can't this function be inlined into the TP_fast_assign portion of the
rcu_segcb_stats trace event? Or is the usual do-while(0) loop that one
might use to define "i" somehow illegal where TP_fast_assign() is used?
That would also allow the gp_seq array to be memcpy()ed directly, without
the intervening loop.
Or you could take the hit of the extra storage and directly memcpy()
both arrays.
> +}
> +
> +void __trace_rcu_segcb_stats(struct rcu_segcblist *rsclp, const char *context)
> +{
> + int cbs[RCU_CBLIST_NSEGS];
> + unsigned long gps[RCU_CBLIST_NSEGS];
> +
> + if (!trace_rcu_segcb_stats_enabled())
> + return;
> +
> + rcu_segcblist_countseq(rsclp, cbs, gps);
> +
> + trace_rcu_segcb_stats(context, cbs, gps);
> +}
> +#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 cd35c9faaf51..7750734fa116 100644
> --- a/kernel/rcu/rcu_segcblist.h
> +++ b/kernel/rcu/rcu_segcblist.h
> @@ -103,3 +103,8 @@ 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 __trace_rcu_segcb_stats(struct rcu_segcblist *rsclp, const char *context);
> +#else
> +#define __trace_rcu_segcb_stats(...)
> +#endif
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 24c00020ab83..f6c6653b3ec2 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1497,6 +1497,8 @@ static bool rcu_accelerate_cbs(struct rcu_node *rnp, struct rcu_data *rdp)
> if (!rcu_segcblist_pend_cbs(&rdp->cblist))
> return false;
>
> + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPreAcc"));
You remembered TPS(), good! ;-)
Thanx, Paul
> +
> /*
> * Callbacks are often registered with incomplete grace-period
> * information. Something about the fact that getting exact
> @@ -1517,6 +1519,8 @@ 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"));
>
> + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbPostAcc"));
> +
> return ret;
> }
>
> @@ -2466,11 +2470,14 @@ static void rcu_do_batch(struct rcu_data *rdp)
> rcu_segcblist_extract_done_cbs(&rdp->cblist, &rcl);
> if (offloaded)
> rdp->qlen_last_fqs_check = rcu_segcblist_n_cbs(&rdp->cblist);
> +
> + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCbDequeued"));
> rcu_nocb_unlock_irqrestore(rdp, flags);
>
> /* Invoke callbacks. */
> tick_dep_set_task(current, TICK_DEP_BIT_RCU);
> rhp = rcu_cblist_dequeue(&rcl);
> +
> for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
> rcu_callback_t f;
>
> @@ -2983,6 +2990,8 @@ __call_rcu(struct rcu_head *head, rcu_callback_t func)
> trace_rcu_callback(rcu_state.name, head,
> rcu_segcblist_n_cbs(&rdp->cblist));
>
> + __trace_rcu_segcb_stats(&rdp->cblist, TPS("SegCBQueued"));
> +
> /* Go handle any RCU core processing required. */
> if (unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
> __call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */
> --
> 2.29.1.341.ge80a0c044ae-goog
>
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