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SubjectRe: [PATCH v2 2/2]: perf record: enable asynchronous trace writing
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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
>

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