Messages in this thread Patch in this message | | | Date | Thu, 13 Feb 2020 17:08:27 +0800 | From | Leo Yan <> | Subject | Re: [PATCH v4 5/5] perf cs-etm: Synchronize instruction sample with the thread stack |
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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@linaro.org> 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 | 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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