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

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Date:	Mon, 4 Jul 2016 17:47:12 -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 05:26 PM, Arnaldo Carvalho de Melo wrote:
> Em Mon, Jul 04, 2016 at 05:19:20PM -0700, Chris Phlipot escreveu:
>>
>>
>> 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.
>
> Ok, but perhaps we should have some sort of warning in places using
> this?
>
> - Arnaldo

Such as printing a warning when cacheline_size is set to zero, or simply 
adding comments to the code in areas where cacheline_size is used?

-Chris

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