Re: [PATCH tip/core/rcu 10/15] rcu: Add tracing for _rcu_barrier()

[Josh Triplett <josh@joshtriplett.org>]

On Fri, Jun 15, 2012 at 02:06:05PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@linaro.org>
> 
> This commit adds event tracing for _rcu_barrier() execution.  This
> is defined only if RCU_TRACE=y.
> 
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  include/trace/events/rcu.h |   45 ++++++++++++++++++++++++++++++++++++++++++++
>  kernel/rcutree.c           |   29 +++++++++++++++++++++++++++-
>  2 files changed, 73 insertions(+), 1 deletions(-)
> 
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index 1480900..cd63f79 100644
> --- a/include/trace/events/rcu.h
> +++ b/include/trace/events/rcu.h
> @@ -540,6 +540,50 @@ TRACE_EVENT(rcu_torture_read,
>  		  __entry->rcutorturename, __entry->rhp)
>  );
>  
> +/*
> + * Tracepoint for _rcu_barrier() execution.  The string "s" describes
> + * the _rcu_barrier phase:
> + *	"Begin": rcu_barrier_callback() started.
> + *	"Check": rcu_barrier_callback() checking for piggybacking.
> + *	"EarlyExit": rcu_barrier_callback() piggybacked, thus early exit.
> + *	"Inc1": rcu_barrier_callback() piggyback check counter incremented.
> + *	"Offline": rcu_barrier_callback() found offline CPU
> + *	"OnlineQ": rcu_barrier_callback() found online CPU with callbacks.
> + *	"OnlineNQ": rcu_barrier_callback() found online CPU, no callbacks.
> + *	"IRQ": An rcu_barrier_callback() callback posted on remote CPU.
> + *	"CB": An rcu_barrier_callback() invoked a callback, not the last.
> + *	"LastCB": An rcu_barrier_callback() invoked the last callback.
> + *	"Inc2": rcu_barrier_callback() piggyback check counter incremented.
> + * The "cpu" argument is the CPU or -1 if meaningless, the "cnt" argument
> + * is the count of remaining callbacks, and "done" is the piggybacking count.
> + */
> +TRACE_EVENT(rcu_barrier,
> +
> +	TP_PROTO(char *rcuname, char *s, int cpu, int cnt, unsigned long done),
> +
> +	TP_ARGS(rcuname, s, cpu, cnt, done),
> +
> +	TP_STRUCT__entry(
> +		__field(char *, rcuname)
> +		__field(char *, s)
> +		__field(int, cpu)
> +		__field(int, cnt)
> +		__field(unsigned long, done)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->rcuname = rcuname;
> +		__entry->s = s;
> +		__entry->cpu = cpu;
> +		__entry->cnt = cnt;
> +		__entry->done = done;
> +	),
> +
> +	TP_printk("%s %s cpu %d remaining %d # %lu",
> +		  __entry->rcuname, __entry->s, __entry->cpu, __entry->cnt,
> +		  __entry->done)
> +);
> +
>  #else /* #ifdef CONFIG_RCU_TRACE */
>  
>  #define trace_rcu_grace_period(rcuname, gpnum, gpevent) do { } while (0)
> @@ -563,6 +607,7 @@ TRACE_EVENT(rcu_torture_read,
>  #define trace_rcu_batch_end(rcuname, callbacks_invoked, cb, nr, iit, risk) \
>  	do { } while (0)
>  #define trace_rcu_torture_read(rcutorturename, rhp) do { } while (0)
> +#define trace_rcu_barrier(name, s, cpu, cnt, done) do { } while (0)
>  
>  #endif /* #else #ifdef CONFIG_RCU_TRACE */
>  
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 7c299d3..ebd5223 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -2257,6 +2257,17 @@ static int rcu_cpu_has_callbacks(int cpu)
>  }
>  
>  /*
> + * Helper function for _rcu_barrier() tracing.  If tracing is disabled,
> + * the compiler is expected to optimize this away.
> + */
> +static void _rcu_barrier_trace(struct rcu_state *rsp, char *s,
> +			       int cpu, unsigned long done)
> +{
> +	trace_rcu_barrier(rsp->name, s, cpu,
> +			  atomic_read(&rsp->barrier_cpu_count), done);
> +}
> +
> +/*
>   * RCU callback function for _rcu_barrier().  If we are last, wake
>   * up the task executing _rcu_barrier().
>   */
> @@ -2265,8 +2276,12 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
>  	struct rcu_data *rdp = container_of(rhp, struct rcu_data, barrier_head);
>  	struct rcu_state *rsp = rdp->rsp;
>  
> -	if (atomic_dec_and_test(&rsp->barrier_cpu_count))
> +	if (atomic_dec_and_test(&rsp->barrier_cpu_count)) {
> +		_rcu_barrier_trace(rsp, "LastCB", -1, rsp->n_barrier_done);
>  		complete(&rsp->barrier_completion);
> +	} else {
> +		_rcu_barrier_trace(rsp, "CB", -1, rsp->n_barrier_done);
> +	}
>  }
>  
>  /*
> @@ -2277,6 +2292,7 @@ static void rcu_barrier_func(void *type)
>  	struct rcu_state *rsp = type;
>  	struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
>  
> +	_rcu_barrier_trace(rsp, "IRQ", -1, rsp->n_barrier_done);
>  	atomic_inc(&rsp->barrier_cpu_count);
>  	rsp->call(&rdp->barrier_head, rcu_barrier_callback);
>  }
> @@ -2295,6 +2311,7 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  	unsigned long snap_done;
>  
>  	init_rcu_head_on_stack(&rd.barrier_head);
> +	_rcu_barrier_trace(rsp, "Begin", -1, snap);
>  
>  	/* Take mutex to serialize concurrent rcu_barrier() requests. */
>  	mutex_lock(&rsp->barrier_mutex);
> @@ -2307,7 +2324,9 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  
>  	/* Recheck ->n_barrier_done to see if others did our work for us. */
>  	snap_done = ACCESS_ONCE(rsp->n_barrier_done);
> +	_rcu_barrier_trace(rsp, "Check", -1, snap_done);
>  	if (ULONG_CMP_GE(snap_done, ((snap + 1) & ~0x1) + 2)) {
> +		_rcu_barrier_trace(rsp, "EarlyExit", -1, snap_done);
>  		smp_mb();
>  		mutex_unlock(&rsp->barrier_mutex);
>  		return;
> @@ -2316,6 +2335,7 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  	/* Increment ->n_barrier_done to avoid duplicate work. */
>  	ACCESS_ONCE(rsp->n_barrier_done)++;
>  	WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 1);
> +	_rcu_barrier_trace(rsp, "Inc1", -1, rsp->n_barrier_done);
>  	smp_mb(); /* Order ->n_barrier_done increment with below mechanism. */
>  
>  	/*
> @@ -2352,13 +2372,19 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  		preempt_disable();
>  		rdp = per_cpu_ptr(rsp->rda, cpu);
>  		if (cpu_is_offline(cpu)) {
> +			_rcu_barrier_trace(rsp, "Offline", cpu,
> +					   rsp->n_barrier_done);
>  			preempt_enable();
>  			while (cpu_is_offline(cpu) && ACCESS_ONCE(rdp->qlen))
>  				schedule_timeout_interruptible(1);
>  		} else if (ACCESS_ONCE(rdp->qlen)) {
> +			_rcu_barrier_trace(rsp, "OnlineQ", cpu,
> +					   rsp->n_barrier_done);
>  			smp_call_function_single(cpu, rcu_barrier_func, rsp, 1);
>  			preempt_enable();
>  		} else {
> +			_rcu_barrier_trace(rsp, "OnlineNQ", cpu,
> +					   rsp->n_barrier_done);
>  			preempt_enable();
>  		}
>  	}
> @@ -2391,6 +2417,7 @@ static void _rcu_barrier(struct rcu_state *rsp)
>  	smp_mb(); /* Keep increment after above mechanism. */
>  	ACCESS_ONCE(rsp->n_barrier_done)++;
>  	WARN_ON_ONCE((rsp->n_barrier_done & 0x1) != 0);
> +	_rcu_barrier_trace(rsp, "Inc2", -1, rsp->n_barrier_done);
>  	smp_mb(); /* Keep increment before caller's subsequent code. */
>  
>  	/* Wait for all rcu_barrier_callback() callbacks to be invoked. */
> -- 
> 1.7.8
>