linux-kernel - Re: [PATCH 3/4] perf tool: Fix build when sysconf doesn't support cache line size

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Message-ID: <577AFD08.7090303@gmail.com>
Date:	Mon, 4 Jul 2016 17:19:20 -0700
From:	Chris Phlipot <cphlipot0@...il.com>
To:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
Cc:	peterz@...radead.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] perf tool: Fix build when sysconf doesn't support
 cache line size



On 07/04/2016 03:48 PM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Jun 30, 2016 at 10:12:34PM -0700, Chris Phlipot escreveu:
>> Enable perf to build on libc implementations where sysconf() doesn't
>> support _SC_LEVEL1_DCACHE_LINESIZE as a parameter.
>>
>> For example, the Bionic implementation does not support this as a
>> paremter. Older versions of Bionic will throw an error when this is passed
>> in as a parameter, and more recent versions will just return 0 as the
>> cache line size.
>>
>> Signed-off-by: Chris Phlipot <cphlipot0@...il.com>
>> ---
>>   tools/perf/perf.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/tools/perf/perf.c b/tools/perf/perf.c
>> index 8f21922..113ca5b 100644
>> --- a/tools/perf/perf.c
>> +++ b/tools/perf/perf.c
>> @@ -509,7 +509,11 @@ int main(int argc, const char **argv)
>>
>>   	/* The page_size is placed in util object. */
>>   	page_size = sysconf(_SC_PAGE_SIZE);
>> +#ifdef _SC_LEVEL1_DCACHE_LINESIZE
>>   	cacheline_size = sysconf(_SC_LEVEL1_DCACHE_LINESIZE);
>> +#else
>> +	cacheline_size = 0;
>> +#endif
>
> Couldn't we instead fallback to:
>
> sysfs__read_int("devices/system/cpu/cpu0/cache/index0/coherency_line_size", &cacheline_size)
>
> ?

I agree that in general this would be a better fallback, but in  all 
Android images I have tested so far, "devices/system/cpu/cpu0/cache" 
does not exist. I know not know of a good way to retrieve cache line 
size in this case.

I would be ok with attempting to get cacheline size using using the 
following methods, unless you have other ideas:

1. attempt to use sysconf(_SC_LEVEL1_DCACHE_LINESIZE)
2. attempt to use 
sysfs__read_int("devices/system/cpu/cpu0/cache/index0/coherency_line_size", 
&cacheline_size)
3. set to zero if both of the above fail.


>
>>   	if (sysctl__read_int("kernel/perf_event_max_stack", &value) == 0)
>>   		sysctl_perf_event_max_stack = value;
>> --
>> 2.7.4