Re: [PATCH v3 2/2]: perf record: enable asynchronous trace writing

Alexey Budankov
2018-08-28T10:28:36Z

Hi,

On 28.08.2018 11:53, Jiri Olsa wrote:
> On Mon, Aug 27, 2018 at 09:16:55PM +0300, Alexey Budankov wrote:
>
> SNIP
>
>> 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);
>>
>> - if (push(to, buf, size) < 0) {
>> - rc = -1;
>> + rc = push(to, md, size0 + size, *off) < 0 ? -1 : 1;
>> + if (rc == -1)
>> goto out;
>> - }
>> +
>> + perf_mmap__get(md);
>> + *off += size0 + size;
>
> this get is for the perf_mmap pointer storage in the mmap_aio array right?

Right. perf_mmap__get() here guards whole perf_mmap object and memory
referenced thru it from premature deallocation because mmap->base kernel
buffer can be released earlier than aio requests started on mmap->data
complete and the both buffers are referenced thru the same perf_mmap object.

>
> I see it's released in record__mmap_read_sync, which might also return
> without releasing it.. this needs to be fixed and explained in here,
> why we take the reference in the first place

So we increment after successful push() from map->base to map->data
with following aio_write() and decrement when aio_write() is fully
complete, because it may require restart if the kernel didn't write
whole chunk at once.

Probably we are still missing one more perf_mmap__put() after:
pr_err("failed to write perf data, error: %m\n");
prior nullifying the appropriate cblock.

Updated [PATCH v4 2/2]:
- Written comment in perf_mmap__push() just before perf_mmap__get();
- Written comment in record__mmap_read_sync() on possible restarting
of aio_write() operation and releasing perf_mmap object after all;
- added perf_mmap__put() for the cases of failed aio_write();

Thanks!

>
> thanks,
> jirka
>