Re: [PATCH v9 4/7] rcu/trace: Add tracing for how segcb list changes

[Neeraj Upadhyay <neeraju@codeaurora.org>]

On 11/3/2020 7:56 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.
> 
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.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),

I think we need to use long[] instead of int[] for cb_count everywhere 
in this patch?


Thanks
Neeraj

> +
> +		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],
> +			 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];
> +	}
> +}
> +
> +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"));
> +
>   	/*
>   	 * 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 */
> 

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