linux-kernel - Re: [PATCH v11 2/3]: perf record: enable asynchronous trace writing

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Date:   Mon, 8 Oct 2018 18:39:21 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 2/3]: perf record: enable asynchronous trace writing

Hi,
On 08.10.2018 18:11, Jiri Olsa wrote:
> On Mon, Oct 08, 2018 at 05:42:18PM +0300, Alexey Budankov wrote:
>> Hi,
>> On 08.10.2018 15:53, Jiri Olsa wrote:
>>> On Mon, Oct 08, 2018 at 03:24:31PM +0300, Alexey Budankov wrote:
>>>> Hi,
>>>>
>>>> On 08.10.2018 13:52, Jiri Olsa wrote:
>>>>> On Mon, Oct 08, 2018 at 09:17:11AM +0300, Alexey Budankov wrote:
>>>> <SNIP>
>>>>>> +#ifdef HAVE_AIO_SUPPORT
>>>>>> +	off = lseek(trace_fd, 0, SEEK_CUR);
>>>>>> +#endif
>>>>>
>>>>> I'm still little puzzled why we need to do this,
>>>>> when the aio write takes the offset value, but
>>>>
>>>> pwrite() syscall [1] which is the base for aio_write() doesn't 
>>>> advance file pos value so it requires to be calculated and 
>>>> updated by callers of aio_write() API.
>>>
>>> ok, so aio_write does not need the offset to be updated,
>>> who needs it then?
>>
>> aio_write() needs this offset as an input parameter.
>> aio_write() gets offset as a part of cblock object.
> 
> yes, it's an 'arg' to aio_write syscall
> 
>> Adjacent aio_write() records should not overlap in the trace file so
>> off value is incremented by size in every loop iteration after 
>> successful aio_write() call.
> 
> but does the aio_write need the lseek 'set' call? if not, we could

Yes, lseek(,, SEEK_SET) is still required after the loop, so the next 
synchronous records would go after AIO written parts that might be
still *in-flight* by that time.

Thanks,
Alexey

> keep the 'offset' value within perf (like in the struct perf_data_file
> or struct record) without any need to call lseek
> 
> jirka
>