Re: [PATCH 3/5] coresight: trbe: Keep TRBE disabled on overflow irq

[Suzuki K Poulose]

On 15/07/2021 04:54, Anshuman Khandual wrote:
> On 7/12/21 5:08 PM, Suzuki K Poulose wrote:
> > When an AUX buffer is marked TRUNCATED, the kernel will disable
> > the event (via irq_work) and can only be re-enabled by the
> > userspace tool.
>
> Will it be renabled via normal perf event enable callback OR an
> explicit perf event restart is required upon the TRUNCATED flag ?

The perf event moves to a DISABLED state. At this state an
explicit restart from the userspace tool is required, via
PERF_EVENT_IOC_ENABLE.

> > Also, since we *always* mark the buffer as TRUNCATED (which is
> > needs to be reconsidered, see below), we need not re-enable the
> > TRBE as the event is going to be now disabled. This follows the
> > SPE driver behavior.
> >
> > Without this change, we could leave the event disabled for
> > ever under certain conditions. i.e, when we try to re-enable
> > in the IRQ handler, if there is no space left in the buffer,
> > we "TRUNCATE" the buffer and create a record of size 0.
> > The perf tool skips such records and the event will remain
> > disabled forever.
>
> Why ? Should not the user space tool explicitly start back the
> tracing event after detecting zero sized record with buffer
> marked with TRUNCATE flag ? What prevents it to restart the
> event ?

The perf tool discards a 0 sized packet. While I agree that
this is something that should be fixed in perf tool too, this
deviation from the "expected driver" behavior (see SPE driver
which this one was inspired from) will break the existing
perf tools (not an ABI break, but a functional issue
which is not nice and generally discouraged in the kernel).

> > Regarding the use of TRUNCATED flag:
> > With FILL mode, the TRBE stops collection when the buffer is
> > full, raising the IRQ. Now, since the IRQ is asynchronous,
> > we could loose some amount of trace, after the buffer was
> > full until the IRQ is handled. Also the last packet may
> > have been trimmed. The decoder can figure this out and
> > cope with this. The side effect of this approach is:
> >
> >   We always disable the event when there is an IRQ, even
> >   when the other half of the ring buffer is free ! This
> >   is not ideal.
> >
> > Now, we should switch to using PARTIAL to indicate that there
> > was potentially some partial trace packets in the buffer and
> > some data was lost. We should use TRUNCATED only when there
> > is absolutely no space in the ring buffer. This change would
> > also require some additional changes in the CoreSight PMU
> > framework to allow, sinks to "close" the handle (rather
> > than the PMU driver closing the handle upon event_stop).
> > So, until that is sorted, let us keep the TRUNCATED flag
> > and the rework can be addressed separately.
>
> But I guess this is a separate problem all together.

Yes, it is. As mentioned above, we need changes to the
coresight PMU framework to be able to use the available
buffer. And the message already is clear, that this
is fixing the "odd" behavior.

> > Fixes: 3fbf7f011f24 ("coresight: sink: Add TRBE driver")
> > Reported-by: Tamas Zsoldos <Tamas.Zsoldos@xxxxxxx>
> > Cc: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
> > Cc: Mike Leach <mike.leach@xxxxxxxxxx>
> > Cc: Leo Yan <leo.yan@xxxxxxxxxx>
> > Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > ---
> >   drivers/hwtracing/coresight/coresight-trbe.c | 34 +++++++-------------
> >   1 file changed, 12 insertions(+), 22 deletions(-)
> >
> > diff --git a/drivers/hwtracing/coresight/coresight-trbe.c b/drivers/hwtracing/coresight/coresight-trbe.c
> > index 176868496879..ec38cf17b693 100644
> > --- a/drivers/hwtracing/coresight/coresight-trbe.c
> > +++ b/drivers/hwtracing/coresight/coresight-trbe.c
> > @@ -696,10 +696,8 @@ static void trbe_handle_spurious(struct perf_output_handle *handle)
> >   
> >   static void trbe_handle_overflow(struct perf_output_handle *handle)
> >   {
> > -	struct perf_event *event = handle->event;
> >   	struct trbe_buf *buf = etm_perf_sink_config(handle);
> >   	unsigned long offset, size;
> > -	struct etm_event_data *event_data;
> >   
> >   	offset = get_trbe_limit_pointer() - get_trbe_base_pointer();
> >   	size = offset - PERF_IDX2OFF(handle->head, buf);
> > @@ -709,30 +707,22 @@ static void trbe_handle_overflow(struct perf_output_handle *handle)
> >   	/*
> >   	 * Mark the buffer as truncated, as we have stopped the trace
> >   	 * collection upon the WRAP event, without stopping the source.
> > +	 *
> > +	 * We don't re-enable the TRBE here, as the event is
> > +	 * bound to be disabled due to the TRUNCATED flag.
> > +	 * This is not ideal, as we could use the available space in
> > +	 * the ring buffer and continue the tracing.
> > +	 *
> > +	 * TODO: Revisit the use of TRUNCATED flag and may be instead use
> > +	 * PARTIAL, to indicate trace may contain partial packets.
> > +	 * And TRUNCATED can be used only if we do not have enough space
> > +	 * in the buffer. This would need additional changes in
> > +	 * etm_event_stop() to allow the sinks to leave a closed
> > +	 * aux_handle.
> >   	 */
> >   	perf_aux_output_flag(handle, PERF_AUX_FLAG_CORESIGHT_FORMAT_RAW |
> >   				     PERF_AUX_FLAG_TRUNCATED);
> >   	perf_aux_output_end(handle, size);
> > -	event_data = perf_aux_output_begin(handle, event);
> > -	if (!event_data) {
> > -		/*
> > -		 * We are unable to restart the trace collection,
> > -		 * thus leave the TRBE disabled. The etm-perf driver
> > -		 * is able to detect this with a disconnected handle
> > -		 * (handle->event = NULL).
> > -		 */
> > -		trbe_drain_and_disable_local();
> > -		*this_cpu_ptr(buf->cpudata->drvdata->handle) = NULL;
> > -		return;
> > -	}
> > -	buf->trbe_limit = compute_trbe_buffer_limit(handle);
> > -	buf->trbe_write = buf->trbe_base + PERF_IDX2OFF(handle->head, buf);
> > -	if (buf->trbe_limit == buf->trbe_base) {
> > -		trbe_stop_and_truncate_event(handle);
> > -		return;
> > -	}
> > -	*this_cpu_ptr(buf->cpudata->drvdata->handle) = handle;
> > -	trbe_enable_hw(buf);
> >   }
>
> The change here just stops the event restart after handling the IRQ
> and marking the buffer with PERF_AUX_FLAG_TRUNCATED which helps the
> event from being disabled for ever (still need to understand how !).

The real issue is unnecessary starting of the event with new buffer
after we have "TRUNCATED" the buffer. This can lead to occassionally
hitting "0" sized buffer, because the "irq_work_run()" could kick
in and disable the event, leaving no trace generated (because we
were tracing only userspace). So, we now have a 0 sized record
with the event in disabled state, which the perf tool ignores.

> I guess it might be better to separate out the problems with using
> PERF_AUX_FLAG_TRUNCATED and related aspects, in a subsequent patch
> which just updates the code with the above TODO comment ?

The problems with the driver using TRUNCATED flag must be addressed
in a different patch. This patch is only to comply with the behavior,
of "TRUNCATED" indicates the event is disabled. So do not start a
session.

Does that help ?

Suzuki