On Mon, Apr 29, 2024 at 04:21:45PM +0100, James Clark wrote:
This will allow sessions with more than CORESIGHT_TRACE_IDS_MAX ETMs as long as there are fewer than that many ETMs connected to each sink.
Each sink owns its own trace ID map, and any Perf session connecting to that sink will allocate from it, even if the sink is currently in use by other users. This is similar to the existing behavior where the dynamic trace IDs are constant as long as there is any concurrent Perf session active. It's not completely optimal because slightly more IDs will be used than necessary, but the optimal solution involves tracking the PIDs of each session and allocating ID maps based on the session owner. This is difficult to do with the combination of per-thread and per-cpu modes and some scheduling issues. The complexity of this isn't likely to worth it because even with multiple users they'd just see a difference in the ordering of ID allocations rather than hitting any limits (unless the hardware does have too many ETMs connected to one sink).
Per-thread mode works but only until there are any overlapping IDs, at which point Perf will error out. Both per-thread mode and sysfs mode are left to future changes, but both can be added on top of this initial implementation and only sysfs mode requires further driver changes.
The HW_ID version field hasn't been bumped in order to not break Perf which already has an error condition for other values of that field. Instead a new minor version has been added which signifies that there are new fields but the old fields are backwards compatible.
I guess I can pick the tooling part now, right? Further reviewing would be nice tho.
- Arnaldo