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Thu, 04 Jun 2026 07:09:27 -0700 (PDT) Message-ID: <9d1e0448-27d7-42d3-aaa3-2d09489f18d9@linaro.org> Date: Thu, 4 Jun 2026 15:09:26 +0100 Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 3/8] perf cs-etm: Use thread-stack for last branch entries To: Leo Yan Cc: linux-arm-kernel@lists.infradead.org, coresight@lists.linaro.org, linux-perf-users@vger.kernel.org, Arnaldo Carvalho de Melo , John Garry , Will Deacon , Mike Leach , Suzuki K Poulose , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Al Grant , Paschalis Mpeis , Amir Ayupov References: <20260526-b4-arm_cs_callchain_support_v1-v6-0-f9f49f53c9dd@arm.com> <20260526-b4-arm_cs_callchain_support_v1-v6-3-f9f49f53c9dd@arm.com> Content-Language: en-US From: James Clark In-Reply-To: <20260526-b4-arm_cs_callchain_support_v1-v6-3-f9f49f53c9dd@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 26/05/2026 5:59 pm, Leo Yan wrote: > CS ETM maintains its own circular array for last branch entries, with > local helpers to update, copy and reset the branch stack. This duplicates > logic already provided by the common code. > > Record branch with thread_stack__event() and synthesize branch stack > with thread_stack__br_sample(). This removes the local last_branch_rb > buffer and position tracking. Keep the buffer number updated via > thread_stack__set_trace_nr(), which is used when exporting samples to > Python scripts. > > The output should remain same, except that be->flags.predicted is no > longer set. Since CoreSight trace does not provide branch prediction > information, clearing the flag avoids confusion. > > Signed-off-by: Leo Yan > --- > tools/perf/util/cs-etm.c | 152 +++++++++++++---------------------------------- > 1 file changed, 41 insertions(+), 111 deletions(-) > > diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c > index 5bff8811d61e423463b7bd4e20d599d5b5307a1a..398ab3b7a429d402cc8e5f6cccb35c0b7c253732 100644 > --- a/tools/perf/util/cs-etm.c > +++ b/tools/perf/util/cs-etm.c > @@ -83,14 +83,13 @@ struct cs_etm_auxtrace { > struct cs_etm_traceid_queue { > u8 trace_chan_id; > u64 period_instructions; > - size_t last_branch_pos; > union perf_event *event_buf; > struct thread *thread; > struct thread *prev_packet_thread; > ocsd_ex_level prev_packet_el; > ocsd_ex_level el; > + unsigned int br_stack_sz; > struct branch_stack *last_branch; > - struct branch_stack *last_branch_rb; > struct cs_etm_packet *prev_packet; > struct cs_etm_packet *packet; > struct cs_etm_packet_queue packet_queue; > @@ -635,9 +634,8 @@ static int cs_etm__init_traceid_queue(struct cs_etm_queue *etmq, > tidq->last_branch = zalloc(sz); > if (!tidq->last_branch) > goto out_free; > - tidq->last_branch_rb = zalloc(sz); > - if (!tidq->last_branch_rb) > - goto out_free; > + > + tidq->br_stack_sz = etm->synth_opts.last_branch_sz; > } > > tidq->event_buf = malloc(PERF_SAMPLE_MAX_SIZE); > @@ -647,7 +645,6 @@ static int cs_etm__init_traceid_queue(struct cs_etm_queue *etmq, > return 0; > > out_free: > - zfree(&tidq->last_branch_rb); > zfree(&tidq->last_branch); > zfree(&tidq->prev_packet); > zfree(&tidq->packet); > @@ -941,7 +938,6 @@ static void cs_etm__free_traceid_queues(struct cs_etm_queue *etmq) > thread__zput(tidq->prev_packet_thread); > zfree(&tidq->event_buf); > zfree(&tidq->last_branch); > - zfree(&tidq->last_branch_rb); > zfree(&tidq->prev_packet); > zfree(&tidq->packet); > zfree(&tidq); > @@ -1281,57 +1277,6 @@ static int cs_etm__queue_first_cs_timestamp(struct cs_etm_auxtrace *etm, > return ret; > } > > -static inline > -void cs_etm__copy_last_branch_rb(struct cs_etm_queue *etmq, > - struct cs_etm_traceid_queue *tidq) > -{ > - struct branch_stack *bs_src = tidq->last_branch_rb; > - struct branch_stack *bs_dst = tidq->last_branch; > - size_t nr = 0; > - > - /* > - * Set the number of records before early exit: ->nr is used to > - * determine how many branches to copy from ->entries. > - */ > - bs_dst->nr = bs_src->nr; > - > - /* > - * Early exit when there is nothing to copy. > - */ > - if (!bs_src->nr) > - return; > - > - /* > - * As bs_src->entries is a circular buffer, we need to copy from it in > - * two steps. First, copy the branches from the most recently inserted > - * branch ->last_branch_pos until the end of bs_src->entries buffer. > - */ > - nr = etmq->etm->synth_opts.last_branch_sz - tidq->last_branch_pos; > - memcpy(&bs_dst->entries[0], > - &bs_src->entries[tidq->last_branch_pos], > - sizeof(struct branch_entry) * nr); > - > - /* > - * If we wrapped around at least once, the branches from the beginning > - * of the bs_src->entries buffer and until the ->last_branch_pos element > - * are older valid branches: copy them over. The total number of > - * branches copied over will be equal to the number of branches asked by > - * the user in last_branch_sz. > - */ > - if (bs_src->nr >= etmq->etm->synth_opts.last_branch_sz) { > - memcpy(&bs_dst->entries[nr], > - &bs_src->entries[0], > - sizeof(struct branch_entry) * tidq->last_branch_pos); > - } > -} > - > -static inline > -void cs_etm__reset_last_branch_rb(struct cs_etm_traceid_queue *tidq) > -{ > - tidq->last_branch_pos = 0; > - tidq->last_branch_rb->nr = 0; > -} > - > static inline int cs_etm__t32_instr_size(struct cs_etm_queue *etmq, > u8 trace_chan_id, u64 addr) > { > @@ -1400,38 +1345,6 @@ static inline u64 cs_etm__instr_addr(struct cs_etm_queue *etmq, > return addr; > } > > -static void cs_etm__update_last_branch_rb(struct cs_etm_queue *etmq, > - struct cs_etm_traceid_queue *tidq) > -{ > - struct branch_stack *bs = tidq->last_branch_rb; > - struct branch_entry *be; > - > - /* > - * The branches are recorded in a circular buffer in reverse > - * chronological order: we start recording from the last element of the > - * buffer down. After writing the first element of the stack, move the > - * insert position back to the end of the buffer. > - */ > - if (!tidq->last_branch_pos) > - tidq->last_branch_pos = etmq->etm->synth_opts.last_branch_sz; > - > - tidq->last_branch_pos -= 1; > - > - be = &bs->entries[tidq->last_branch_pos]; > - be->from = cs_etm__last_executed_instr(tidq->prev_packet); > - be->to = cs_etm__first_executed_instr(tidq->packet); > - /* No support for mispredict */ > - be->flags.mispred = 0; > - be->flags.predicted = 1; > - > - /* > - * Increment bs->nr until reaching the number of last branches asked by > - * the user on the command line. > - */ > - if (bs->nr < etmq->etm->synth_opts.last_branch_sz) > - bs->nr += 1; > -} > - > static int cs_etm__inject_event(struct cs_etm_auxtrace *etm, union perf_event *event, > struct perf_sample *sample, u64 type) > { > @@ -1579,6 +1492,37 @@ static inline u64 cs_etm__resolve_sample_time(struct cs_etm_queue *etmq, > return etm->latest_kernel_timestamp; > } > > +static void cs_etm__add_stack_event(struct cs_etm_queue *etmq, > + struct cs_etm_traceid_queue *tidq) > +{ > + u64 from, to; > + int size; > + > + if (!tidq->prev_packet->last_instr_taken_branch) > + return; > + > + if (tidq->prev_packet->sample_type != CS_ETM_RANGE || > + tidq->packet->sample_type != CS_ETM_RANGE) > + return; > + > + if (etmq->etm->synth_opts.last_branch) { > + from = cs_etm__last_executed_instr(tidq->prev_packet); > + to = cs_etm__first_executed_instr(tidq->packet); > + > + size = cs_etm__instr_size(etmq, tidq->trace_chan_id, > + tidq->prev_packet->isa, from); > + > + /* Enable callchain so thread stack entry can be allocated */ > + thread_stack__event(tidq->thread, tidq->prev_packet->cpu, > + tidq->prev_packet->flags, from, to, size, > + etmq->buffer->buffer_nr + 1, true, > + tidq->br_stack_sz, 0); > + } else { > + thread_stack__set_trace_nr(tidq->thread, tidq->prev_packet->cpu, > + etmq->buffer->buffer_nr + 1); > + } > +} > + > static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq, > struct cs_etm_traceid_queue *tidq, > u64 addr, u64 period) > @@ -1608,8 +1552,12 @@ static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq, > > cs_etm__copy_insn(etmq, tidq->trace_chan_id, tidq->packet, &sample); > > - if (etm->synth_opts.last_branch) > + if (etm->synth_opts.last_branch) { > + thread_stack__br_sample(tidq->thread, tidq->packet->cpu, > + tidq->last_branch, > + tidq->br_stack_sz); > sample.branch_stack = tidq->last_branch; > + } > > if (etm->synth_opts.inject) { > ret = cs_etm__inject_event(etm, event, &sample, > @@ -1798,14 +1746,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq, > > tidq->period_instructions += tidq->packet->instr_count; > > - /* > - * 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); > + cs_etm__add_stack_event(etmq, tidq); Would it be cleaner to call this whenever a branch sample is generated? Seems like the conditions for calling thread_stack__event() and cs_etm__synth_branch_sample() are slightly different (ignoring the fact that branches are only generated when the user asks for them). Maybe the conditions should be different, but maybe a comment why or if they're the same, a shared function for the conditions would help. For example, we don't push a branch to the stack for CS_ETM_DISCONTINUITY, but we do generate a branch sample from 0. > > if (etm->synth_opts.instructions && > tidq->period_instructions >= etm->instructions_sample_period) { > @@ -1864,10 +1805,6 @@ static int cs_etm__sample(struct cs_etm_queue *etmq, > u64 offset = etm->instructions_sample_period - instrs_prev; > u64 addr; > > - /* Prepare last branches for instruction sample */ > - if (etm->synth_opts.last_branch) > - cs_etm__copy_last_branch_rb(etmq, tidq); > - > while (tidq->period_instructions >= > etm->instructions_sample_period) { > /* > @@ -1947,10 +1884,6 @@ static int cs_etm__flush(struct cs_etm_queue *etmq, > etmq->etm->synth_opts.instructions && > tidq->prev_packet->sample_type == CS_ETM_RANGE) { > u64 addr; > - > - /* Prepare last branches for instruction sample */ > - cs_etm__copy_last_branch_rb(etmq, tidq); > - > /* > * Generate a last branch event for the branches left in the > * circular buffer at the end of the trace. > @@ -1982,7 +1915,7 @@ static int cs_etm__flush(struct cs_etm_queue *etmq, > > /* Reset last branches after flush the trace */ > if (etm->synth_opts.last_branch) > - cs_etm__reset_last_branch_rb(tidq); > + thread_stack__flush(tidq->thread); > > return err; > } > @@ -2006,9 +1939,6 @@ static int cs_etm__end_block(struct cs_etm_queue *etmq, > tidq->prev_packet->sample_type == CS_ETM_RANGE) { > u64 addr; > > - /* Prepare last branches for instruction sample */ > - cs_etm__copy_last_branch_rb(etmq, tidq); > - > /* > * Use the address of the end of the last reported execution > * range. >