Re: [PATCH v9 7/9] perf cs-etm: Support call indentation

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Thread stacks for per-thread traces (CS_ETM_PIDFMT_NONE) are not flushed upon a trace discontinuity, leading to stale callindent depths.
--

commit ccf60466ef97abd9a958c518b0578321fda247d0
Author: Leo Yan <leo.yan@linux.dev>

perf cs-etm: Support call indentation
    
The perf script callindent is derived from call stack in thread context,
CS ETM ignores the requirement for callindent without pushing and poping
call stack.

Enable thread-stack when either itrace thread-stack support or last branch
entries are requested, allocate the branch stack storage accordingly, and
feed taken branches to thread_stack__event() whenever thread-stack state
is needed.

> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index d41aad583a26f..e18d2dec79431 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c

[ ... ]

> @@ -2027,7 +2030,7 @@ static void cs_etm__flush_all_stack(struct cs_etm_queue *etmq)
>  {
>  	enum cs_etm_pid_fmt pid_fmt = cs_etm__get_pid_fmt(etmq);
>  
> -	if (!etmq->etm->synth_opts.last_branch)
> +	if (!etmq->etm->use_thread_stack)
>  		return;
>  
>  	switch (pid_fmt) {

[Severity: Medium]
Will this skip flushing thread stacks for per-thread traces upon a trace
discontinuity?

In cs_etm__flush_all_stack(), the switch statement explicitly skips flushing
the stack if the format is CS_ETM_PIDFMT_NONE:

    switch (pid_fmt) {
    ...
    case CS_ETM_PIDFMT_NONE:
    default:
        break;
    }

Because the frontend thread for per-thread mode is still attached to the host
machine, the flush is completely skipped.

As a result, could instruction samples generated before the first branch in the
new buffer inherit the stale thread stack depth from before the discontinuity,
and break the newly added callindent output?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260616-b4-arm_cs_callchain_support_v1-v9-0-f8fad931c413@arm.com?part=7