Re: [PATCH v3 6/6] perf cs-etm: Synchronize instruction sample with the thread stack

Leo Yan
2019-10-22T04:51:18Z

Hi Mathieu,

On Fri, Oct 11, 2019 at 02:17:50PM -0600, Mathieu Poirier wrote:
> On Sat, Oct 05, 2019 at 05:16:14PM +0800, Leo Yan wrote:
> > The synthesized flow use 'tidq->packet' for instruction samples; on the
> > other hand, 'tidp->prev_packet' is used to generate the thread stack and
> > the branch samples, this results in the instruction samples using one
> > packet ahead than thread stack and branch samples ('tidp->prev_packet'
> > vs 'tidq->packet').
> >
> > This leads to an instruction's callchain error as shows in below
> > example:
> >
> > main 1579 100 instructions:
> > ffff000010214854 perf_event_update_userpage+0x4c ([kernel.kallsyms])
> > ffff000010214850 perf_event_update_userpage+0x48 ([kernel.kallsyms])
> > ffff000010219360 perf_swevent_add+0x88 ([kernel.kallsyms])
> > ffff0000102135f4 event_sched_in.isra.57+0xbc ([kernel.kallsyms])
> > ffff0000102137a0 group_sched_in+0x60 ([kernel.kallsyms])
> > ffff000010213b84 flexible_sched_in+0xfc ([kernel.kallsyms])
> > ffff00001020c0b4 visit_groups_merge+0x12c ([kernel.kallsyms])
> >
> > In the callchain log, for the two continuous lines the up line contains
> > one child function info and the followed line contains the caller
> > function info, and so forth. So the first two lines are:
> >
> > perf_event_update_userpage+0x4c => the sampled instruction
> > perf_event_update_userpage+0x48 => the parent function's calling
> >
> > The child function and parent function both are the same function
> > perf_event_update_userpage(), but this isn't a recursive function, thus
> > the sequence for perf_event_update_userpage() calling itself shouldn't
> > never happen. This callchain error is caused by the instruction sample
> > using an ahead packet than the thread stack, the thread stack is deferred
> > to process the new packet and misses to pop stack if it is just a return
> > packet.
> >
> > To fix this issue, we can simply change to use 'tidq->prev_packet' to
> > generate the instruction samples, this allows the thread stack to push
> > and pop synchronously with instruction sample. Finally, the callchain
> > can be displayed correctly as below:
> >
> > main 1579 100 instructions:
> > ffff000010214854 perf_event_update_userpage+0x4c ([kernel.kallsyms])
> > ffff000010219360 perf_swevent_add+0x88 ([kernel.kallsyms])
> > ffff0000102135f4 event_sched_in.isra.57+0xbc ([kernel.kallsyms])
> > ffff0000102137a0 group_sched_in+0x60 ([kernel.kallsyms])
> > ffff000010213b84 flexible_sched_in+0xfc ([kernel.kallsyms])
> > ffff00001020c0b4 visit_groups_merge+0x12c ([kernel.kallsyms])
> >
> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
> > ---
> > tools/perf/util/cs-etm.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> > index 56e501cd2f5f..fa969dcb45d2 100644
> > --- a/tools/perf/util/cs-etm.c
> > +++ b/tools/perf/util/cs-etm.c
> > @@ -1419,7 +1419,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
> > struct cs_etm_packet *tmp;
> > int ret;
> > u8 trace_chan_id = tidq->trace_chan_id;
> > - u64 instrs_executed = tidq->packet->instr_count;
> > + u64 instrs_executed = tidq->prev_packet->instr_count;
> >
> > tidq->period_instructions += instrs_executed;
> >
> > @@ -1450,7 +1450,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
> > */
> > s64 offset = (instrs_executed - instrs_over - 1);
> > u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
> > - tidq->packet, offset);
> > + tidq->prev_packet, offset);
>
> I have tested this set in --per-thread mode and things are working as
> advertised. Did you see how things look like in CPU-wide scenarios?

After some testing, I can confirm this patch set can works well for
CPU-wide trace; the reason is in the arch/arm/util/cs-etm.c, function
cs_etm_recording_options() has enabled option 'ETM_OPT_CTXTID' for
CPU-wide trace:

/*
* In the case of per-cpu mmaps, we need the CPU on the
* AUX event. We also need the contextID in order to be notified
* when a context switch happened.
*/
if (!perf_cpu_map__empty(cpus)) {
perf_evsel__set_sample_bit(cs_etm_evsel, CPU);

err = cs_etm_set_option(itr, cs_etm_evsel,
ETM_OPT_CTXTID | ETM_OPT_TS);
if (err)
goto out;
}

As result, we don't need to specify extra option to enable CTXID
configuration. So below two commands have the same behaviour:

# perf record -e cs_etm/@tmc_etr0/ -a -- sh test.sh
# perf record -e cs_etm/@tmc_etr0,config=0x4000/ -a -- sh test.sh
`-> bit 14: for ETM_OPT_CTXTID

Since the decoding will set tid when receive the packet
'OCSD_GEN_TRC_ELEM_PE_CONTEXT', thus it can give the correct tid/pid
info for threads. This allows to generate per thread stack base on
thread->tid and avoid mixing info cross different threads.

Thanks,
Leo Yan

> >
> > ret = cs_etm__synth_instruction_sample(
> > etmq, tidq, addr, etm->instructions_sample_period);
> > --
> > 2.17.1
> >