linux-kernel - Re: [PATCH] perf record: Remove unnecessary warning for missing sysfs entry

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Message-Id: <53a2face-d26c-8851-d594-5f78da5814c3@linux.vnet.ibm.com>
Date:   Thu, 12 Apr 2018 15:19:31 +0200
From:   Thomas-Mich Richter <tmricht@...ux.vnet.ibm.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Thomas Richter <tmricht@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        brueckner@...ux.vnet.ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com
Subject: Re: [PATCH] perf record: Remove unnecessary warning for missing sysfs
 entry

On 04/12/2018 03:05 PM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Apr 12, 2018 at 01:47:23PM +0200, Thomas Richter escreveu:
>> Using perf on 4.16.0 kernel on s390 shows warning
>>    failed: can't open node sysfs data
>> each time I run command perf record ... for example:
>>
>> [root@...lp76 perf]# ./perf record -e rB0000 -- sleep 1
>> [ perf record: Woken up 1 times to write data ]
>> failed: can't open node sysfs data
>> [ perf record: Captured and wrote 0.001 MB perf.data (4 samples) ]
>> [root@...lp76 perf]#
>>
>> BTW: I find this error message not very informative.
> 
> What an understatement :-)
>  
>> It turns out commit e2091cedd51bf ("perf tools: Add MEM_TOPOLOGY
>> feature to perf data file") tries to open directory named
>> /sys/devices/system/node/ which does not exist on s390.
>>
>> This is the call stack:
>>  __cmd_record
>>  +---> perf_session__write_header
>>        +---> perf_header__adds_write
>>              +---> do_write_feat
>> 	           +---> write_mem_topology
>> 		         +---> build_mem_topology
>> 			       prints warning
>> The issue starts in do_write_feat() which unconditionally
>> loops over all features and now includes HEADER_MEM_TOPOLOGY and calls
>> write_mem_topology().
>> Function record__init_features() at the beginning of __cmd_record()
>> sets all features and then turns off some.
>>
>> Fix this by removed the warning, if the directory is not present
>> memory node information is not available and won't be displayed.
> 
> Can't we instead improve the error message and turn this into a
> pr_debug2? Isn't it a reasonable scenario that the user expects this
> topology information to be present and then ends up without it?
> 
> Perhaps something like:
> 
> 	pr_debug2("%s: could't read %s, does this arch have topology information?\n", __func__, path);
> 
> - Arnaldo
>  

Fine with me, I will provide a version 2....

-- 
Thomas Richter, Dept 3303, IBM LTC Boeblingen Germany
--
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