On 28/05/2021 19:15, Leo Yan wrote:
The perf tool records the Arm CoreSight trace data with snapshot mode with the option '-S', when receiving USR2 signal, it is observed the captured trace data size is very varied: from several MBs to ~20MBs. This can be reproduced with the command:
perf record -e cs_etm// -S \ -- dd if=/dev/zero of=/dev/null > /dev/null 2>&1 & PERFPID=$! sleep 1 kill -USR2 $PERFPID
It's different for only specifying option '-S' than options '-a -S'. If without option '-a', perf tool creates separate AUX buffers for every CPU, but the tracer will be enabled only when the profiled program is scheduled onto the corresponding CPU, this might lead to record very old trace data when snapshot.
Let's see below diagram: snapshot CPU0: ______###P1###__________________________________________| CPU1: __________________________###P3###____________###P5###__| CPU2: ____________________________________###P4###____________| CPU3: ________________###P2###________________________________V
In this diagram, the program runs for 5 periods (from P1 to P5), these 5 periods show the task run on different CPUs, e.g. during P1 period the program runs on CPU0, and during P2 period the program is migrated to CPU1, and so on. At the end of P1 period when the program is switched out from CPU0, the ETR trace data is saved into AUX trace buffer, this AUX buffer is a dedicated buffer for CPU0's tracer. With the same logic, P2's trace data is saved into CPU3's tracer buffer, P4's trace data is saved into CPU2's buffer, P3 and P5's trace data is saved into CPU1's. Therefore, when snapshot, it saves the trace data from all AUX ring buffers (in this case, it have total 4 AUX ring buffers) into perf data file.
Hi Leo,
I was testing out snapshot mode (without your patch) and I noticed that it only ever collects from the last CPU. For example on a 4 core system, the CPU ID of the AUX records and the AUXTRACE buffers is always 3.
This is with systemwide tracing, and running "stress -m 2 -c 2". Is this something that your patch fixes, or am I doing something wrong, or is it just a coincidence?
Here's a snippet of the output:
./perf report -D | grep AUX 0 0 0x200 [0x168]: PERF_RECORD_AUXTRACE_INFO type: 3 0 0 0x152248 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0 ref: 0x75e0bdc44ea1bb65 idx: 3 tid: -1 cpu: 3 3 583600975364460 0x152160 [0x40]: PERF_RECORD_AUX offset: 0x400000 size: 0x400000 flags: 0x2 [O] 0 0 0x55c950 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x400000 ref: 0x6f506d2d02841da4 idx: 3 tid: -1 cpu: 3 3 583602209157460 0x55c908 [0x40]: PERF_RECORD_AUX offset: 0x800000 size: 0x400000 flags: 0x2 [O] 0 0 0x9624d8 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x800000 ref: 0x2d83d30161e1117a idx: 3 tid: -1 cpu: 3 3 583602526365800 0x962490 [0x40]: PERF_RECORD_AUX offset: 0xc00000 size: 0x400000 flags: 0x2 [O] 0 0 0xd65f00 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0xc00000 ref: 0x5013e6e2a3c97c9 idx: 3 tid: -1 cpu: 3 3 583602714310320 0xd65eb8 [0x40]: PERF_RECORD_AUX offset: 0x1000000 size: 0x400000 flags: 0x2 [O] 0 0 0x1169be8 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x1000000 ref: 0x67b37e157f843269 idx: 3 tid: -1 cpu: 3 3 583602874193840 0x1169ba0 [0x40]: PERF_RECORD_AUX offset: 0x1400000 size: 0x400000 flags: 0x2 [O] 0 0 0x156d550 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x1400000 ref: 0x3cb268b926f22d41 idx: 3 tid: -1 cpu: 3 3 583603044203980 0x156d508 [0x40]: PERF_RECORD_AUX offset: 0x1800000 size: 0x400000 flags: 0x2 [O] 0 0 0x1971238 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x1800000 ref: 0x4905e0a21d5d35d7 idx: 3 tid: -1 cpu: 3 3 583603211393440 0x19711f0 [0x40]: PERF_RECORD_AUX offset: 0x1c00000 size: 0x400000 flags: 0x2 [O] 0 0 0x1d747e0 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x1c00000 ref: 0x4f8f48007f7d70e7 idx: 3 tid: -1 cpu: 3 3 583603362643100 0x1d74798 [0x40]: PERF_RECORD_AUX offset: 0x2000000 size: 0x400000 flags: 0x2 [O] 0 0 0x2178368 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x2000000 ref: 0x770d07213be2d29a idx: 3 tid: -1 cpu: 3 3 583603526029900 0x2178320 [0x40]: PERF_RECORD_AUX offset: 0x2400000 size: 0x400000 flags: 0x2 [O] 0 0 0x257bfb0 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x2400000 ref: 0x2e4ed1454815c13e idx: 3 tid: -1 cpu: 3 3 583603687951260 0x257bf68 [0x40]: PERF_RECORD_AUX offset: 0x2800000 size: 0x400000 flags: 0x2 [O] 0 0 0x297fb18 [0x30]: PERF_RECORD_AUXTRACE size: 0x400000 offset: 0x2800000 ref: 0x644eba01d391129 idx: 3 tid: -1 cpu: 3
Thanks James