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

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Date:   Mon, 27 Aug 2018 12:12:34 +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>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 2/2]: perf record: enable asynchronous trace writing

Hi,

On 27.08.2018 11:34, Jiri Olsa wrote:
> 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 

Sure. Comments will follow in v3. 
We copy data into a buffer to release space in the kernel buffer 
as fast as possible. That lets the kernel to store more data earlier 
than other per-cpu buffers are handled.

> 
> jirka
>