linux-kernel - Re: [PATCH] perf record: Add snapshot mode support for perf's regular events

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Date:	Wed, 25 Nov 2015 10:27:33 +0200
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	David Ahern <dsahern@...il.com>,
	Yunlong Song <yunlong.song@...wei.com>, a.p.zijlstra@...llo.nl,
	paulus@...ba.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
	namhyung@...nel.org, ast@...nel.org,
	masami.hiramatsu.pt@...achi.com, kan.liang@...el.com,
	jolsa@...nel.org, bp@...en8.de, jean.pihet@...aro.org,
	rric@...nel.org, xiakaixu@...wei.com, hekuang@...wei.com
Subject: Re: [PATCH] perf record: Add snapshot mode support for perf's regular
 events

On 25/11/15 09:47, Wangnan (F) wrote:
> 
> 
> On 2015/11/25 15:22, Adrian Hunter wrote:
>> On 25/11/15 05:50, Wangnan (F) wrote:
>>>
>>> On 2015/11/24 23:20, Arnaldo Carvalho de Melo wrote:
>>>> Em Tue, Nov 24, 2015 at 08:06:41AM -0700, David Ahern escreveu:
>>>>> On 11/24/15 7:00 AM, Yunlong Song wrote:
>>>>>> +static int record__write(struct record *rec, void *bf, size_t size)
>>>>>> +{
>>>>>> +    if (rec->memory.size && memory_enabled) {
>>>>>> +        if (perf_memory__write(&rec->memory, bf, size) < 0) {
>>>>>> +            pr_err("failed to write memory data, error: %m\n");
>>>>>> +            return -1;
>>>>>> +        }
>>>>>> +    } else {
>>>>>> +        if (perf_data_file__write(rec->session->file, bf, size) < 0) {
>>>>>> +            pr_err("failed to write perf data, error: %m\n");
>>>>>> +            return -1;
>>>>>> +        }
>>>>>> +        rec->bytes_written += size;
>>>>>>        }
>>>>>>
>>>>>> -    rec->bytes_written += size;
>>>>>>        return 0;
>>>>>>    }
>>>>>>
>>>>>> @@ -86,6 +214,8 @@ static int record__mmap_read(struct record *rec, int
>>>>>> idx)
>>>>>>        if (old == head)
>>>>>>            return 0;
>>>>>>
>>>>>> +    memory_enabled = 1;
>>>>>> +
>>>>>>        rec->samples++;
>>>>>>
>>>>>>        size = head - old;
>>>>>> @@ -113,6 +243,7 @@ static int record__mmap_read(struct record *rec, int
>>>>>> idx)
>>>>>>        md->prev = old;
>>>>>>        perf_evlist__mmap_consume(rec->evlist, idx);
>>>>>>    out:
>>>>>> +    memory_enabled = 0;
>>>>>>        return rc;
>>>>>>    }
>>>>>>
>>>>> So you are basically ignoring all samples until SIGUSR2 is received. That
>>>> No, he is not, its just that his code is difficult to follow, has to be
>>>> rewritten, but he is ignoring just PERF_RECORD_SAMPLE events, so it
>>>> will..
>>>>
>>>>> means the resulting data file will have limited history of task events for
>>>> ... have a complete history of task events, since PERF_RECORD_FORK, etc
>>>> are not being ignored.
>>>>
>>>> No?
>>> Actually we are discussing about this problem.
>>>
>>> For such tracking events (PERF_RECORD_FORK...), we have dummy event so
>>> it is possible for us to receive tracking events from a separated
>>> channel, therefore we don't have to parse every events to pick those
>>> events out. Instead, we can process tracking events differently, then
>>> more interesting things can be done. For example, squashing those tracking
>>> events if it takes too much memory...
>>>
>>> Furthermore, there's another problem being discussed: if userspace
>>> ringbuffer
>>> is bytes based, parsing event is unavoidable. Without parsing event we are
>>> unable to find the new 'head' pointer when overwriting.
>> Have you considered trying to find the head by trial-and-error at the time
>> you make the snapshot i.e. look at the first 8 bytes (event records are 8
>> byte aligned) and see if it is a valid record header, if not try the next 8
>> bytes.  When you find a real event record it should parse without error and
>> the subsequent events should all parse without error too, all the way to the
>> tail.  Then you can use timestamps and compare the events byte-by-byte to
>> avoid overlaps between 2 snapshots.
> 
> It seems not work. Now we have BPF output event, it is possible that a
> BPF program output anything through that event. Even if we have a magic
> in head of each event, we can't prevent BPF output event output that
> magic, except we introduce some 'escape' method to prevent BPF output
> event output some data pattern. So although might work in reallife,
> this solution is logically incorrect. Or am I miss someting?

When you find the head, all the events will parse correctly.  It seems to me
highly unlikely that would happen if you guessed the head wrongly.
It is only incorrect if it gives the wrong result.

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