linux-kernel - Re: [PATCH] perf script python: Fix buffer size to report iregs in perf script

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Date:   Tue, 29 Jun 2021 12:39:24 +0530
From:   kajoljain <kjain@...ux.ibm.com>
To:     "Paul A. Clarke" <pc@...ibm.com>
Cc:     acme@...nel.org, maddy@...ux.vnet.ibm.com,
        atrajeev@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
        jolsa@...hat.com, ravi.bangoria@...ux.ibm.com,
        linux-perf-users@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        rnsastry@...ux.ibm.com
Subject: Re: [PATCH] perf script python: Fix buffer size to report iregs in
 perf script



On 6/28/21 8:19 PM, Paul A. Clarke wrote:
> On Mon, Jun 28, 2021 at 11:53:41AM +0530, Kajol Jain wrote:
>> Commit 48a1f565261d ("perf script python: Add more PMU fields
>> to event handler dict") added functionality to report fields like
>> weight, iregs, uregs etc via perf report.
>> That commit predefined buffer size to 512 bytes to print those fields.
>>
>> But incase of powerpc, since we added extended regs support
>> in commits:
>>
>> Commit 068aeea3773a ("perf powerpc: Support exposing Performance Monitor
>> Counter SPRs as part of extended regs")
>> Commit d735599a069f ("powerpc/perf: Add extended regs support for
>> power10 platform")
>>
>> Now iregs can carry more bytes of data and this predefined buffer size
>> can result to data loss in perf script output.
>>
>> Patch resolve this issue by making buffer size dynamic based on number
>> of registers needed to print. It also changed return type for function
>> "regs_map" from int to void, as the return value is not being used by
>> the caller function "set_regs_in_dict".
>>
>> Fixes: 068aeea3773a ("perf powerpc: Support exposing Performance Monitor
>> Counter SPRs as part of extended regs")
>> Signed-off-by: Kajol Jain <kjain@...ux.ibm.com>
>> ---
>>  .../util/scripting-engines/trace-event-python.c | 17 ++++++++++++-----
>>  1 file changed, 12 insertions(+), 5 deletions(-)
>>
>> diff --git a/tools/perf/util/scripting-engines/trace-event-python.c b/tools/perf/util/scripting-engines/trace-event-python.c
>> index 4e4aa4c97ac5..c8c9706b4643 100644
>> --- a/tools/perf/util/scripting-engines/trace-event-python.c
>> +++ b/tools/perf/util/scripting-engines/trace-event-python.c
> [...]
>> @@ -713,7 +711,16 @@ static void set_regs_in_dict(PyObject *dict,
>>  			     struct evsel *evsel)
>>  {
>>  	struct perf_event_attr *attr = &evsel->core.attr;
>> -	char bf[512];
>> +
>> +	/*
>> +	 * Here value 28 is a constant size which can be used to print
>> +	 * one register value and its corresponds to:
>> +	 * 16 chars is to specify 64 bit register in hexadecimal.
>> +	 * 2 chars is for appending "0x" to the hexadecimal value and
>> +	 * 10 chars is for register name.
>> +	 */
>> +	int size = __sw_hweight64(attr->sample_regs_intr) * 28;
>> +	char bf[size];
> 
> I propose using a template rather than a magic number here. Something like:
> const char reg_name_tmpl[] = "10 chars  ";
> const char reg_value_tmpl[] = "0x0123456789abcdef";
> const int size = __sw_hweight64(attr->sample_regs_intr) +
>                  sizeof reg_name_tmpl + sizeof reg_value_tmpl;
> 

Hi Paul,
   Thanks for reviewing the patch. Yes these are
some standardization we can do by creating macros for different
fields.
The basic idea is, we want to provide significant buffer size
based on number of registers present in sample_regs_intr to accommodate
all data.

But before going to optimizing code, Arnaldo/Jiri, is this approach looks good to you?

> Pardon my ignorance, but is there no separation/whitespace between the name
> and the value?

This is how we will get data via perf script

r0:0xc000000000112008
r1:0xc000000023b37920
r2:0xc00000000144c900
r3:0xc0000000bc566120
r4:0xc0000000c5600000
r5:0x2606c6506ca
r6:0xc000000023b378f8
r7:0xfffffd9f93a48f0e
.....

 And is there some significance to 10 characters for the
> register name, or is that a magic number?

Most of the register name are within 10 characters, basically we are giving this
magic number to make sure we have enough space in buffer to contain all registers
name with colon.

Thanks,
Kajol Jain
 
> 
> PC
>