Re: [PATCH 00/17] coresight: Use per-sink trace ID maps for Perf sessions

[Arnaldo Carvalho de Melo <acme@kernel.org>]

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