linux-kernel - Re: [PATCH v4 09/16] perf tools: Enable indices setting syntax for BPF maps

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Date:	Fri, 11 Dec 2015 20:39:35 +0800
From:	pi3orama <pi3orama@....com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Wang Nan <wangnan0@...wei.com>, namhyung@...nel.org,
	linux-kernel@...r.kernel.org, mingo@...nel.org, lizefan@...wei.com,
	Alexei Starovoitov <ast@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: [PATCH v4 09/16] perf tools: Enable indices setting syntax for BPF maps



发自我的 iPhone

> 在 2015年12月11日，下午8:15，Arnaldo Carvalho de Melo <acme@...nel.org> 写道：
> 
> Em Fri, Dec 11, 2015 at 09:11:45AM -0300, Arnaldo Carvalho de Melo escreveu:
>> Em Tue, Dec 08, 2015 at 02:25:37AM +0000, Wang Nan escreveu:
>>> This patch introduce a new syntax to perf event parser:
>>> 
>>> # perf record -e bpf_file.c/maps.mymap.value[0,3...5,7]=1234/ ...
>> 
>> Is the above example valid? Wouldn't this be "maps:mymap.value" ?
>> 
>>> 
>>> By utilizing the basic facilities in bpf-loader.c which allow setting
>>> different slots in a BPF map separately, the newly introduced syntax
>>> allows perf to control specific elements in a BPF map.
>>> 
>>> Test result:
>>> 
>>> # cat ./test_bpf_map_3.c
>>> /************************ BEGIN **************************/
>>> #define SEC(NAME) __attribute__((section(NAME), used))
>>> enum bpf_map_type {
>>>     BPF_MAP_TYPE_ARRAY = 2,
>>> };
>>> struct bpf_map_def {
>>>     unsigned int type;
>>>     unsigned int key_size;
>>>     unsigned int value_size;
>>>     unsigned int max_entries;
>>> };
>>> static void *(*map_lookup_elem)(struct bpf_map_def *, void *) =
>>>     (void *)1;
>>> static int (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
>>>     (void *)6;
>> 
>> Can you explain the above a bit more? What are the magic 1 and 6 values?
> 
> So, from another patch:
> 
> static u64 (*bpf_ktime_get_ns)(void) =
>     (void *)5;
> static int (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
>     (void *)6;
> static int (*bpf_get_smp_processor_id)(void) =
>     (void *)8;
> static int (*bpf_perf_event_output)(void *, struct bpf_map_def *, int,
> void *, unsigned long) =
>     (void *)23;
> 
> Where can I get this magical mistery table? Could this be hidden away in
> some .h file automagically included in bpf scriptlets so that n00bies
> like me don't have to be wtf'ing?
> 

They are function numbers defined in bpf.h and bpf-common.h, but they are Linux
headers. Directly include them causes many error for llvm. Also, the function
prototypes are BPF specific and can't included in Linux source. We should have
a place holds those indices and prototypes together.

> - Arnaldo

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