[DTrace-devel] [PATCH 2/2] Snapshot aggregations just in time

[Kris Van Hees]

I am testing this a bit and seeing if we can centralize the call to
dtrace_aggregate_snap() a bit more (unlikely), but I would be inclined
to suggest to keep the aggrate code and only do the on-demand call to
dtrace_aggregate_snap() portion of this patch.  I could see some use
for using aggrate to rate-limit retrieving aggregate snapshots in
cases where the probe firing frequency is sufficiently high to make
aggregate retrieval a noticable cost, and where therefore limiting it
can be useful.  Of course, that would generally mean that the script
is poorly written and should be fixed to get the desired results.

But it has the added benefit of not introducing a new global variable
whole not getting rid of (a now obsolete) one.  Since aggrate is by
default 0, keeping that code (for now) is safe and will still result
in the default behaviour being 100% on-demand.

On Tue, May 27, 2025 at 01:43:13AM -0400, eugene.loh at oracle.com wrote:
> From: Eugene Loh <eugene.loh at oracle.com>
> 
> Currently, dtrace periodically calls dtrace_work(), which in turn calls
> dtrace_consume(), which among other things calls dtrace_aggregate_snap().
> But aggregations are kept in entirety in the kernel's BPF maps.  There
> is no need to snapshot the aggregations into user space unless we're
> actually going to do something with aggregations.
> 
> Snapshot aggregations just in time -- that is, if there is a clear(),
> trunc(), or printa() or if aggregations are to be printed at the end
> of a dtrace session.
> 
> Skip the aggrate-slow test.  Just-in-time snapshots mean the semantics
> of aggrate have changed.  A fast aggrate means nothing.  A slow aggrate
> means we are supposed to use stale aggregation data, which would be
> baffling.  A future patch is advised to deprecate aggrate entirely.
> 
> Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
> ---
>  libdtrace/dt_aggregate.c                 | 7 +++++++
>  libdtrace/dt_consume.c                   | 9 +++++++--
>  libdtrace/dt_impl.h                      | 1 +
>  test/unittest/options/tst.aggrate-slow.d | 1 +
>  4 files changed, 16 insertions(+), 2 deletions(-)
> 
> diff --git a/libdtrace/dt_aggregate.c b/libdtrace/dt_aggregate.c
> index 40c1ae44f..6c1b642ff 100644
> --- a/libdtrace/dt_aggregate.c
> +++ b/libdtrace/dt_aggregate.c
> @@ -817,6 +817,10 @@ dtrace_aggregate_snap(dtrace_hdl_t *dtp)
>  			dtp->dt_lastagg = now;
>  	}
>  
> +	if (dtp->dt_haveagg)
> +		return DTRACE_WORKSTATUS_OKAY;
> +	dtp->dt_haveagg = 1;
> +
>  	dtrace_aggregate_clear(dtp);
>  
>  	for (i = 0; i < dtp->dt_conf.num_online_cpus; i++) {
> @@ -1901,6 +1905,9 @@ dtrace_aggregate_print(dtrace_hdl_t *dtp, FILE *fp,
>  {
>  	dtrace_print_aggdata_t pd;
>  
> +	dtp->dt_haveagg = 0;
> +	dtrace_aggregate_snap(dtp);
> +
>  	if (dtp->dt_maxaggdsize == 0)
>  		return 0;
>  
> diff --git a/libdtrace/dt_consume.c b/libdtrace/dt_consume.c
> index 8f50ebefc..a91413672 100644
> --- a/libdtrace/dt_consume.c
> +++ b/libdtrace/dt_consume.c
> @@ -2353,11 +2353,15 @@ dt_consume_one_probe(dtrace_hdl_t *dtp, FILE *fp, char *data, uint32_t size,
>  				i++;
>  				continue;
>  			case DT_ACT_CLEAR:
> +				if (dtrace_aggregate_snap(dtp) == DTRACE_WORKSTATUS_ERROR)
> +					return DTRACE_WORKSTATUS_ERROR;
>  				if (dt_clear(dtp, data, rec) != 0)
>  					return DTRACE_WORKSTATUS_ERROR;
>  
>  				continue;
>  			case DT_ACT_TRUNC:
> +				if (dtrace_aggregate_snap(dtp) == DTRACE_WORKSTATUS_ERROR)
> +					return DTRACE_WORKSTATUS_ERROR;
>  				if (i == epd->dtdd_nrecs - 1)
>  					return dt_set_errno(dtp, EDT_BADTRUNC);
>  
> @@ -2519,6 +2523,8 @@ dt_consume_one_probe(dtrace_hdl_t *dtp, FILE *fp, char *data, uint32_t size,
>  			func = dtrace_fprintf;
>  			break;
>  		case DTRACEACT_PRINTA:
> +			if (dtrace_aggregate_snap(dtp) == DTRACE_WORKSTATUS_ERROR)
> +				return DTRACE_WORKSTATUS_ERROR;
>  			if (rec->dtrd_format != NULL)
>  				func = dtrace_fprinta;
>  			else
> @@ -3113,8 +3119,7 @@ dtrace_consume(dtrace_hdl_t *dtp, FILE *fp, dtrace_consume_probe_f *pf,
>  		}
>  	}
>  
> -	if (dtrace_aggregate_snap(dtp) == DTRACE_WORKSTATUS_ERROR)
> -		return DTRACE_WORKSTATUS_ERROR;
> +	dtp->dt_haveagg = 0;
>  
>  	/*
>  	 * If dtp->dt_beganon is not -1, we did not process the BEGIN probe
> diff --git a/libdtrace/dt_impl.h b/libdtrace/dt_impl.h
> index 1033154d9..2c376f59a 100644
> --- a/libdtrace/dt_impl.h
> +++ b/libdtrace/dt_impl.h
> @@ -405,6 +405,7 @@ struct dtrace_hdl {
>  	dt_percpu_drops_t *dt_drops; /* per-CPU drop counters cache */
>  	uint64_t dt_specdrops;	/* consumer-side spec drops counter */
>  	int dt_statusgen;	/* current status generation */
> +	int dt_haveagg;		/* FIXME: figure out a good name for this */
>  	hrtime_t dt_laststatus;	/* last status */
>  	hrtime_t dt_lastswitch;	/* last switch of buffer data */
>  	hrtime_t dt_lastagg;	/* last snapshot of aggregation data */
> diff --git a/test/unittest/options/tst.aggrate-slow.d b/test/unittest/options/tst.aggrate-slow.d
> index e2a0f2cb2..cd91c0a6c 100644
> --- a/test/unittest/options/tst.aggrate-slow.d
> +++ b/test/unittest/options/tst.aggrate-slow.d
> @@ -9,6 +9,7 @@
>   * When the aggrate is slower than the switchrate and the pace of printa()
>   * actions, multiple printa() should all reflect the same stale count.
>   */
> +/* @@skip: aggrate makes no sense */
>  /* @@trigger: periodic_output */
>  /* @@nosort */
>  
> -- 
> 2.43.5
>