linux-kernel - Re: [RFT v3 1/4] perf cs-etm: Generate branch sample for missed packets

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Message-ID: <CANLsYkzPShMj1cbXZPb7Lzio+gKcmvhD2+KTYr7vP9ec1d6bmg@mail.gmail.com>
Date:   Wed, 30 May 2018 08:45:46 -0600
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Robert Walker <Robert.Walker@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Kim Phillips <kim.phillips@....co>,
        Tor Jeremiassen <tor@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        coresight@...ts.linaro.org
Subject: Re: [RFT v3 1/4] perf cs-etm: Generate branch sample for missed packets

On 29 May 2018 at 18:28, Leo Yan <leo.yan@...aro.org> wrote:
> Hi Mathieu,
>
> On Tue, May 29, 2018 at 10:04:49AM -0600, Mathieu Poirier wrote:
>
> [...]
>
>> > As now this patch is big with more complex logic, so I consider to
>> > split it into small patches:
>> >
>> > - Define CS_ETM_INVAL_ADDR;
>> > - Fix for CS_ETM_TRACE_ON packet;
>> > - Fix for exception packet;
>> >
>> > Does this make sense for you?  I have concern that this patch is a
>> > fixing patch, so not sure after spliting patches will introduce
>> > trouble for applying them for other stable kernels ...
>>
>> Reverse the order:
>>
>> - Fix for CS_ETM_TRACE_ON packet;
>> - Fix for exception packet;
>> - Define CS_ETM_INVAL_ADDR;
>>
>> But you may not need to - see next comment.
>
> From the discussion context, I think here 'you may not need to' is
> referring to my concern for applying patches on stable kernel, so I
> should take this patch series as an enhancement and don't need to
> consider much for stable kernel.

Yes, that is what I meant.

>
> On the other hand, your suggestion is possible to mean 'not need
> to' split into small patches (though I guess this is misunderstanding
> for your meaning).
>
> Could you clarify which is your meaning?
>
>> >> > +
>> >> > +   /*
>> >> >      * The packet records the execution range with an exclusive end address
>> >> >      *
>> >> >      * A64 instructions are constant size, so the last executed
>> >> > @@ -505,6 +519,18 @@ static inline u64 cs_etm__last_executed_instr(struct cs_etm_packet *packet)
>> >> >     return packet->end_addr - A64_INSTR_SIZE;
>> >> >  }
>> >> >
>> >> > +static inline u64 cs_etm__first_executed_instr(struct cs_etm_packet *packet)
>> >> > +{
>> >> > +   /*
>> >> > +    * The packet is the CS_ETM_TRACE_ON packet if the start_addr is
>> >> > +    * magic number 0xdeadbeefdeadbeefUL, returns 0 for this case.
>> >> > +    */
>> >> > +   if (packet->start_addr == 0xdeadbeefdeadbeefUL)
>> >> > +           return 0;
>> >>
>> >> Same comment as above.
>> >
>> > Will do this.
>> >
>> >> > +
>> >> > +   return packet->start_addr;
>> >> > +}
>> >> > +
>> >> >  static inline u64 cs_etm__instr_count(const struct cs_etm_packet *packet)
>> >> >  {
>> >> >     /*
>> >> > @@ -546,7 +572,7 @@ static void cs_etm__update_last_branch_rb(struct cs_etm_queue *etmq)
>> >> >
>> >> >     be       = &bs->entries[etmq->last_branch_pos];
>> >> >     be->from = cs_etm__last_executed_instr(etmq->prev_packet);
>> >> > -   be->to   = etmq->packet->start_addr;
>> >> > +   be->to   = cs_etm__first_executed_instr(etmq->packet);
>> >> >     /* No support for mispredict */
>> >> >     be->flags.mispred = 0;
>> >> >     be->flags.predicted = 1;
>> >> > @@ -701,7 +727,7 @@ static int cs_etm__synth_branch_sample(struct cs_etm_queue *etmq)
>> >> >     sample.ip = cs_etm__last_executed_instr(etmq->prev_packet);
>> >> >     sample.pid = etmq->pid;
>> >> >     sample.tid = etmq->tid;
>> >> > -   sample.addr = etmq->packet->start_addr;
>> >> > +   sample.addr = cs_etm__first_executed_instr(etmq->packet);
>> >> >     sample.id = etmq->etm->branches_id;
>> >> >     sample.stream_id = etmq->etm->branches_id;
>> >> >     sample.period = 1;
>> >> > @@ -897,13 +923,28 @@ static int cs_etm__sample(struct cs_etm_queue *etmq)
>> >> >             etmq->period_instructions = instrs_over;
>> >> >     }
>> >> >
>> >> > -   if (etm->sample_branches &&
>> >> > -       etmq->prev_packet &&
>> >> > -       etmq->prev_packet->sample_type == CS_ETM_RANGE &&
>> >> > -       etmq->prev_packet->last_instr_taken_branch) {
>> >> > -           ret = cs_etm__synth_branch_sample(etmq);
>> >> > -           if (ret)
>> >> > -                   return ret;
>> >> > +   if (etm->sample_branches && etmq->prev_packet) {
>> >> > +           bool generate_sample = false;
>> >> > +
>> >> > +           /* Generate sample for start tracing packet */
>> >> > +           if (etmq->prev_packet->sample_type == 0 ||
>> >>
>> >> What kind of packet is sample_type == 0 ?
>> >
>> > Just as explained above, sample_type == 0 is the packet which
>> > initialized in the function cs_etm__alloc_queue().
>> >
>> >> > +               etmq->prev_packet->sample_type == CS_ETM_TRACE_ON)
>> >> > +                   generate_sample = true;
>> >> > +
>> >> > +           /* Generate sample for exception packet */
>> >> > +           if (etmq->prev_packet->exc == true)
>> >> > +                   generate_sample = true;
>> >>
>> >> Please don't do that.  Exception packets have a type of their own and can be
>> >> added to the decoder packet queue the same way INST_RANGE and TRACE_ON packets
>> >> are.  Moreover exception packet containt an address that, if I'm reading the
>> >> documenation properly, can be used to keep track of instructions that were
>> >> executed between the last address of the previous range packet and the address
>> >> executed just before the exception occurred.  Mike and Rob will have to confirm
>> >> this as the decoder may be doing all that hard work for us.
>> >
>> > Sure, will wait for Rob and Mike to confirm for this.
>> >
>> > At my side, I dump the packet, the exception packet isn't passed to
>> > cs-etm.c layer, the decoder layer only sets the flag
>> > 'packet->exc = true' when exception packet is coming [1].
>>
>> That's because we didn't need the information.  Now that we do a
>> function that will insert a packet in the decoder packet queue and
>> deal with the new packet type in the main decoder loop [2].  At that
>> point your work may not be eligible for stable anymore and I think it
>> is fine.  Robert's work was an enhancement over mine and yours is an
>> enhancement over his.
>>
>> [2]. https://elixir.bootlin.com/linux/v4.17-rc7/source/tools/perf/util/cs-etm.c#L999
>
> Agree, will look into for exception packet and try to add new packet
> type for this.
>
> [...]
>
> Thanks,
> Leo Yan