Re: [PATCH v4 5/5] perf cs-etm: Synchronize instruction sample with the thread stack

From: Leo Yan
Date: Thu Feb 13 2020 - 04:08:45 EST

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Hi Mike,

On Thu, Feb 06, 2020 at 03:01:52PM +0000, Mike Leach wrote:
> Hi Leo,
>
> On Mon, 3 Feb 2020 at 02:08, Leo Yan <leo.yan@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> > tools/perf/util/cs-etm.c | 8 +++++---
> > 1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> > index 8f805657658d..410e40ce19f2 100644
> > --- a/tools/perf/util/cs-etm.c
> > +++ b/tools/perf/util/cs-etm.c
> > @@ -1414,7 +1414,7 @@ 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;
> >
> > @@ -1505,7 +1505,8 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
> > * instruction)
> > */
> > addr = cs_etm__instr_addr(etmq, trace_chan_id,
> > - tidq->packet, offset - 1);
> > + tidq->prev_packet,
> > + offset - 1);
> > ret = cs_etm__synth_instruction_sample(
> > etmq, tidq, addr,
> > etm->instructions_sample_period);
> > @@ -1525,7 +1526,8 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
> > * instruction)
> > */
> > addr = cs_etm__instr_addr(etmq, trace_chan_id,
> > - tidq->packet, offset - 1);
> > + tidq->prev_packet,
> > + offset - 1);
> > ret = cs_etm__synth_instruction_sample(
> > etmq, tidq, addr,
> > etm->instructions_sample_period);
> > --
> > 2.17.1
> >
> I am really not convinced that this is a correct solution.
>
> Consider a set of trace range packet inputs:
> current: 0x3000-0x3050
> prev: 0x2000-0x2100
> prev-1: 0x1020-0x1080
>
> Before your modification.....
> cs_etm__sample() processes the current packet....
>
> On entry, the branch stack will contain:0x1080=>0x2000;
>
> We add to this from the current packet to get: 0x1080=>0x2000; 0x2100=>0x3000;
>
> This is then copied by cs_etm__copy_last_branch_rb()
>
> We find the instruction sample address in the range 0x3000 to 0x3050,
> e.g. 0x3010.
> cs_etm__synth_instruction_sample() will then generate a sample with values
>
> sample.ip = 0x3010
> sample.branch_stack = 0x1080=>0x2000; 0x2100=>0x3000;
>
> to be passed to the perf session / injected as required.
> This sample has the correct branch context for the sampled address -
> i.e. how the code arrived @0x3010
>
> After the modification.....
> The branch stack will be the same, but the sample address will be from
> the range 0x2000-0x2010, e.g. 0x2008 to give a sample in
> cs_etm__synth_instruction_sample() of
> sample.ip = 0x2008
> sample.branch_stack = 0x1080=>0x2000; 0x2100=>0x3000;
>
> This really does not make much sense - the branch stack no longer
> relates to the sample.ip.
>
> Further - cs_etm__synth_instruction_sample() calls cs_etm__copy_insn()
> using the _current_ packet and sample.ip. This is a clear mismatch.
>
> I don't know what is causing the apparent error in the callchain, but
> given that the previous features added in this set, work without this
> alteration, I feel there must be another solution.

Good catch! Thanks a lot for very detailed analysis.

I root caused this issue is relevant with the sequence between two
functions thread_stack__event() and thread_stack__sample().

In this series, thread_stack__sample() is prior to thread_stack__event(),
thus the thread stack event cannot be handled before thread stack
generation.

If move the function thread_stack__event() up and place it before
instruction sample synthesizing; thread_stack__event() can be invoked
prior to thread_stack__sample(), then I can see the thread stack can
be popped properly and the issue can be fixed. Simply to say, patch
0002 should change the code as below:

/*
* Record a branch when the last instruction in
* PREV_PACKET is a branch.
*/
if (etm->synth_opts.last_branch &&
tidq->prev_packet->sample_type == CS_ETM_RANGE &&
tidq->prev_packet->last_instr_taken_branch)
cs_etm__update_last_branch_rb(etmq, tidq);

/*
* The stack event must be processed prior to synthesizing
* instruction sample; this can ensure the instruction samples
* to generate correct thread stack.
*/
if (tidq->prev_packet->last_instr_taken_branch)
cs_etm__add_stack_event(etmq, tidq);

if (etm->sample_instructions &&
tidq->period_instructions >= etm->instructions_sample_period) {

cs_etm__synth_instruction_sample();
`-> thread_stack__sample();

}

Does this make sense for you?

Thanks,
Leo Yan

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