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Date:   Tue, 22 Oct 2019 13:03:04 +0800
From:   Leo Yan <leo.yan@linaro.org>
To:     Mathieu Poirier <mathieu.poirier@linaro.org>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Mike Leach <mike.leach@linaro.org>,
        Coresight ML <coresight@lists.linaro.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH v3 3/6] perf cs-etm: Support thread stack
Message-ID: <20191022050304.GB32731@leoy-ThinkPad-X240s>
References: <20191005091614.11635-1-leo.yan@linaro.org>
 <20191005091614.11635-4-leo.yan@linaro.org>
 <20191011175353.GA13688@xps15>
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Hi Mathieu,

On Fri, Oct 11, 2019 at 11:53:53AM -0600, Mathieu Poirier wrote:
> On Sat, Oct 05, 2019 at 05:16:11PM +0800, Leo Yan wrote:
> > Since Arm CoreSight doesn't support thread stack, the decoding cannot
> > display symbols with indented spaces to reflect the stack depth.
> > 
> > This patch adds support thread stack for Arm CoreSight, this allows
> > 'perf script' to display properly for option '-F,+callindent'.
> > 
> > Before:
> > 
> >   # perf script -F,+callindent
> >             main  2808          1          branches: coresight_test1                      ffff8634f5c8 coresight_test1+0x3c (/root/coresight_test/libcstest.so)
> >             main  2808          1          branches: printf@plt                           aaaaba8d37ec main+0x28 (/root/coresight_test/main)
> >             main  2808          1          branches: printf@plt                           aaaaba8d36bc printf@plt+0xc (/root/coresight_test/main)
> >             main  2808          1          branches: _init                                aaaaba8d3650 _init+0x30 (/root/coresight_test/main)
> >             main  2808          1          branches: _dl_fixup                            ffff86373b4c _dl_runtime_resolve+0x40 (/lib/aarch64-linux-gnu/ld-2.28.so)
> >             main  2808          1          branches: _dl_lookup_symbol_x                  ffff8636e078 _dl_fixup+0xb8 (/lib/aarch64-linux-gnu/ld-2.28.so)
> >   [...]
> > 
> > After:
> > 
> >   # perf script -F,+callindent
> >             main  2808          1          branches:                 coresight_test1                                      ffff8634f5c8 coresight_test1+0x3c (/root/coresight_test/libcstest.so)
> >             main  2808          1          branches:                 printf@plt                                           aaaaba8d37ec main+0x28 (/root/coresight_test/main)
> >             main  2808          1          branches:                     printf@plt                                       aaaaba8d36bc printf@plt+0xc (/root/coresight_test/main)
> >             main  2808          1          branches:                     _init                                            aaaaba8d3650 _init+0x30 (/root/coresight_test/main)
> >             main  2808          1          branches:                     _dl_fixup                                        ffff86373b4c _dl_runtime_resolve+0x40 (/lib/aarch64-linux-gnu/ld-2.28.s
> >             main  2808          1          branches:                         _dl_lookup_symbol_x                          ffff8636e078 _dl_fixup+0xb8 (/lib/aarch64-linux-gnu/ld-2.28.so)
> >   [...]
> > 
> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
> > ---
> >  tools/perf/util/cs-etm.c | 44 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 44 insertions(+)
> > 
> > diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> > index 58ceba7b91d5..780abbfd1833 100644
> > --- a/tools/perf/util/cs-etm.c
> > +++ b/tools/perf/util/cs-etm.c
> > @@ -1117,6 +1117,45 @@ static void cs_etm__copy_insn(struct cs_etm_queue *etmq,
> >  			   sample->insn_len, (void *)sample->insn);
> >  }
> >  
> > +static void cs_etm__add_stack_event(struct cs_etm_queue *etmq,
> > +				    struct cs_etm_traceid_queue *tidq)
> > +{
> > +	struct cs_etm_auxtrace *etm = etmq->etm;
> > +	u8 trace_chan_id = tidq->trace_chan_id;
> > +	int insn_len;
> > +	u64 from_ip, to_ip;
> > +
> > +	if (etm->synth_opts.thread_stack) {
> > +		from_ip = cs_etm__last_executed_instr(tidq->prev_packet);
> > +		to_ip = cs_etm__first_executed_instr(tidq->packet);
> > +
> > +		insn_len = cs_etm__instr_size(etmq, trace_chan_id,
> > +					      tidq->prev_packet->isa, from_ip);
> > +
> > +		/*
> > +		 * Create thread stacks by keeping track of calls and returns;
> > +		 * any call pushes thread stack, return pops the stack, and
> > +		 * flush stack when the trace is discontinuous.
> > +		 */
> > +		thread_stack__event(tidq->thread, tidq->prev_packet->cpu,
> > +				    tidq->prev_packet->flags,
> > +				    from_ip, to_ip, insn_len,
> > +				    etmq->buffer->buffer_nr);
> 
> Details are a little fuzzy in my head but I'm pretty sure
> we want trace_chan_id here.  

I spent some time to look into this question, and I think we don't
need to add extra info for trace_chan_id.

The main reason is for CPU wide tracing, if one task is migrated from
CPU_a to CPU_b, if we append 'trace_chan_id' for the buffer number, then
it will tell the thread_stack that the buffer has been changed (or it
will be considered the trace is discontinuous), then thread stack will
be flushed.  Actually, this is not what we want; if a task is migrated
from one CPU to another, we still need to keep its thread stack if the
trace data comes from the same buffer_nr.

To be honest, I struggled to understand what's the purpose for
'buffer->buffer_nr', from the code, I think 'buffer->buffer_nr' is
mainly used to trace the splitted buffers (e.g. the buffers are splitted
into different queues so the trace data coming from different trace
chunk?).  Now I observe 'buffer->buffer_nr' is always zero since the
buffer is not used with splitted mode.  If later we support 1:1 map
between tracers and sinks, then we need to set 'buffer->buffer_nr' so
can reflect the correct buffer mapping, but we don't need to use
trace_chan_id as extra info at here.

Please let me know what you think about this?  If you agree with this,
I will send out patch v4 soon with addressing other comments.

Thanks,
Leo Yan

> > +	} else {
> > +		/*
> > +		 * The thread stack can be output via thread_stack__process();
> > +		 * thus the detailed information about paired calls and returns
> > +		 * will be facilitated by Python script for the db-export.
> > +		 *
> > +		 * Need to set trace buffer number and flush thread stack if the
> > +		 * trace buffer number has been alternate.
> > +		 */
> > +		thread_stack__set_trace_nr(tidq->thread,
> > +					   tidq->prev_packet->cpu,
> > +					   etmq->buffer->buffer_nr);
> 
> Same here.
> 
> > +	}
> > +}
> > +
> >  static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq,
> >  					    struct cs_etm_traceid_queue *tidq,
> >  					    u64 addr, u64 period)
> > @@ -1393,6 +1432,9 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
> >  		tidq->period_instructions = instrs_over;
> >  	}
> >  
> > +	if (tidq->prev_packet->last_instr_taken_branch)
> > +		cs_etm__add_stack_event(etmq, tidq);
> > +
> >  	if (etm->sample_branches) {
> >  		bool generate_sample = false;
> >  
> > @@ -2593,6 +2635,8 @@ int cs_etm__process_auxtrace_info(union perf_event *event,
> >  		itrace_synth_opts__set_default(&etm->synth_opts,
> >  				session->itrace_synth_opts->default_no_sample);
> >  		etm->synth_opts.callchain = false;
> > +		etm->synth_opts.thread_stack =
> > +				session->itrace_synth_opts->thread_stack;
> >  	}
> >  
> >  	err = cs_etm__synth_events(etm, session);
> > -- 
> > 2.17.1
> >