Date:   Wed, 14 Oct 2020 20:05:03 -0400
From:   joel@...lfernandes.org
To:     Neeraj Upadhyay <neeraju@...eaurora.org>
Cc:     linux-kernel@...r.kernel.org, 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, "Paul E. McKenney" <paulmck@...nel.org>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [PATCH v6 3/4] rcu/trace: Add tracing for how segcb list changes

On Wed, Oct 14, 2020 at 08:52:17PM +0530, Neeraj Upadhyay wrote:
> 
> 
> On 9/23/2020 8:52 PM, 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>
> > ---
> >   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..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)
> 
> Use RCU_CBLIST_NSEGS in place of 4 ?

Done.

> > +		),
> > +
> > +		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 0e6d19bd3de9..df0f31e30947 100644
> > --- a/kernel/rcu/rcu_segcblist.c
> > +++ b/kernel/rcu/rcu_segcblist.c
> > @@ -13,6 +13,7 @@
> >   #include <linux/rcupdate.h>
> >   #include "rcu_segcblist.h"
> > +#include "rcu.h"
> >   /* Initialize simple callback list. */
> >   void rcu_cblist_init(struct rcu_cblist *rclp)
> > @@ -343,6 +344,39 @@ 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 callbacks. Only used from
> 
> N is number of segments?

Yes, will fix.

> > + * 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],
> > +			 unsigned long gpseq[RCU_CBLIST_NSEGS])
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < RCU_CBLIST_NSEGS; i++)
> > +		cbcount[i] = 0;
> > +
> 
> What is the reason for initializing to 0?

You are right, not needed. I'll remove.

> > +	for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
> > +		cbcount[i] = rcu_segcblist_get_seglen(rsclp, i);
> > +		gpseq[i] = rsclp->gp_seq[i];
> > +	}
> > +}
> > +
> > +void trace_rcu_segcb_list(struct rcu_segcblist *rsclp, char *context)
> > +{
> > +	int cbs[RCU_CBLIST_NSEGS];
> > +	unsigned long gps[RCU_CBLIST_NSEGS];
> > +
> > +	rcu_segcblist_countseq(rsclp, cbs, gps);
> > +
> > +	trace_rcu_segcb(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 3e0eb1056ae9..15c10d30f88c 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_list(struct rcu_segcblist *rsclp, char *context);
> > +#else
> > +#define trace_rcu_segcb_list(...)
> > +#endif
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 50af465729f4..e3381ff67fc6 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1492,6 +1492,8 @@ 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_list(&rdp->cblist, "SegCbPreAcc");
> 
> Use TPS("SegCbPreAcc") ?

Fixed, thanks!

thanks,

 - Joel

> 
> 
> Thanks
> Neeraj
> 
> > +
> >   	/*
> >   	 * Callbacks are often registered with incomplete grace-period
> >   	 * information.  Something about the fact that getting exact
> > @@ -1512,6 +1514,8 @@ 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_list(&rdp->cblist, "SegCbPostAcc");
> > +
> >   	return ret;
> >   }
> > @@ -2469,6 +2473,9 @@ 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);
> > +
> > +	trace_rcu_segcb_list(&rdp->cblist, "SegCbDequeued");
> > +
> >   	for (; rhp; rhp = rcu_cblist_dequeue(&rcl)) {
> >   		rcu_callback_t f;
> > @@ -2982,6 +2989,8 @@ __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_list(&rdp->cblist, "SegCBQueued");
> > +
> >   	/* Go handle any RCU core processing required. */
> >   	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
> >   	    unlikely(rcu_segcblist_is_offloaded(&rdp->cblist))) {
> > 
> 
> -- 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
> the Code Aurora Forum, hosted by The Linux Foundation

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