Messages in this thread | | | Subject | Re: [PATCH 3/5] coresight: trbe: Keep TRBE disabled on overflow irq | From | Anshuman Khandual <> | Date | Thu, 15 Jul 2021 09:24:05 +0530 |
| |
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 ?
> > 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 ?
> > 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.
> > Fixes: 3fbf7f011f24 ("coresight: sink: Add TRBE driver") > Reported-by: Tamas Zsoldos <Tamas.Zsoldos@arm.com> > Cc: Anshuman Khandual <anshuman.khandual@arm.com> > Cc: Mathieu Poirier <mathieu.poirier@linaro.org> > Cc: Mike Leach <mike.leach@linaro.org> > Cc: Leo Yan <leo.yan@linaro.org> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org> > Cc: Will Deacon <will@kernel.org> > Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com> > --- > 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 !).
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 ?
> > static bool is_perf_trbe(struct perf_output_handle *handle) >
| |