Hi James,
On Fri, May 07, 2021 at 01:02:35PM +0300, James Clark wrote:
On 07/05/2021 12:58, James Clark wrote:
There is an intermittent issue on Trogdor devices that results in all Coresight timestamps having a value of zero.
I've attached a file here that has the issue. From the dump you can see the zero timestamps:
Idx:69; ID:10; I_TIMESTAMP : Timestamp.; Updated val = 0x0 Idx:71; ID:10; I_ATOM_F1 : Atom format 1.; E Idx:72; ID:10; I_ADDR_S_IS0 : Address, Short, IS0.; Addr=0xFFFFFFE723C65824 ~[0x5824]This doesn't have an impact on decoding as they end up being decoded in file order as in with timeless mode.
Just remind, as Mike has mentioned that if the timestamp is zero, it means the hardware setting for timestamp is not enabled properly. So for system wide or per CPU mode tracing, it's better to double check what's the reason the timestamp is not enabled properly.
IIUC, this patch breaks the existed rational in the code. Let's think about there have 4 CPUs, every CPU has its own AUX trace buffer, and when decode the trace data, it will use 4 queues to track the packets and every queue has its timestamp.
CPU0: cs_etm_queue -> ... -> packet_queue->timestamp CPU1: cs_etm_queue -> ... -> packet_queue->timestamp CPU2: cs_etm_queue -> ... -> packet_queue->timestamp CPU3: cs_etm_queue -> ... -> packet_queue->timestamp
The issue is if all CPUs' timestamp are zero, it's impossible to find a way to synthesize samples in the right time order.
[...]
diff --git a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c index b01d363b9301..947e44413c6e 100644 --- a/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c +++ b/tools/perf/util/cs-etm-decoder/cs-etm-decoder.c @@ -320,7 +320,10 @@ cs_etm_decoder__do_hard_timestamp(struct cs_etm_queue *etmq, * which instructions started by subtracting the number of instructions * executed to the timestamp. */
- packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
- if (packet_queue->instr_count >= elem->timestamp)
packet_queue->cs_timestamp = 0;- else
packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
Actually here have two situations: one case is "elem->timestamp" is zero, another case is the overflow for "elem->timestamp".
So the change should be like:
if (!elem->timestamp) packet_queue->cs_timestamp = 0; else if (packet_queue->instr_count >= elem->timestamp) /* handle overflow? */ else packet_queue->cs_timestamp = elem->timestamp - packet_queue->instr_count;
It's better to think about how to handle the overflow in this case.
Thanks, Leo