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

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Date:   Mon, 27 Aug 2018 10:34:57 +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

> diff --git a/tools/perf/util/mmap.c b/tools/perf/util/mmap.c
> index e71d46cb01cc..c8b921c88a5d 100644
> --- a/tools/perf/util/mmap.c
> +++ b/tools/perf/util/mmap.c
> @@ -292,11 +292,11 @@ int perf_mmap__read_init(struct perf_mmap *map)
>  }
>  
>  int perf_mmap__push(struct perf_mmap *md, void *to,
> -		    int push(void *to, void *buf, size_t size))
> +		    int push(void *to, void *buf, size_t size, off_t), off_t *off)
>  {
>  	u64 head = perf_mmap__read_head(md);
>  	unsigned char *data = md->base + page_size;
> -	unsigned long size;
> +	unsigned long size, size0 = 0;
>  	void *buf;
>  	int rc = 0;
>  
> @@ -308,23 +308,22 @@ int perf_mmap__push(struct perf_mmap *md, void *to,
>  
>  	if ((md->start & md->mask) + size != (md->end & md->mask)) {
>  		buf = &data[md->start & md->mask];
> -		size = md->mask + 1 - (md->start & md->mask);
> -		md->start += size;
> -
> -		if (push(to, buf, size) < 0) {
> -			rc = -1;
> -			goto out;
> -		}
> +		size0 = md->mask + 1 - (md->start & md->mask);
> +		md->start += size0;
> +		memcpy(md->data, buf, size0);
>  	}
>  
>  	buf = &data[md->start & md->mask];
>  	size = md->end - md->start;
>  	md->start += size;
> +	memcpy(md->data + size0, buf, size);

this will need more comments.. and explanation why we copy the data
over to another buffer.. it's interesting, it's still faster 

jirka