linux-kernel - Re: [PATCH v2 2/2]: perf record: enable asynchronous trace writing

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Date:   Mon, 27 Aug 2018 10:38:52 +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>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 2/2]: perf record: enable asynchronous trace writing

On Thu, Aug 23, 2018 at 07:47:01PM +0300, Alexey Budankov wrote:

SNIP

>  static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evlist,
>  				    bool overwrite)
>  {
>  	u64 bytes_written = rec->bytes_written;
> -	int i;
> -	int rc = 0;
> +	int i, rc = 0;
>  	struct perf_mmap *maps;
> +	int trace_fd = rec->session->data->file.fd;
> +	struct aiocb **mmap_aio = rec->evlist->mmap_aio;
> +	int mmap_aio_size = 0;
> +	off_t off;
>  
>  	if (!evlist)
>  		return 0;
> @@ -546,14 +620,17 @@ static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evli
>  	if (overwrite && evlist->bkw_mmap_state != BKW_MMAP_DATA_PENDING)
>  		return 0;
>  
> +	off = lseek(trace_fd, 0, SEEK_CUR);
> +

with async write, do we need to query/set the offset like this
all the time?

could we just keep/update the offset value in the 'struct perf_data_file'
and skip both lseek calls?

jirka

SNIP

>  
> +	record__mmap_read_sync(trace_fd, mmap_aio, mmap_aio_size, rec);
> +
> +	lseek(trace_fd, off, SEEK_SET);
> +