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

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Date:   Mon, 8 Oct 2018 17:11:08 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.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

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

jirka