linux-kernel - Re: [PATCH v9 2/3]: perf record: enable asynchronous trace writing

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Date:   Fri, 5 Oct 2018 11:31:11 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Namhyung Kim <namhyung@...nel.org>
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>,
        Jiri Olsa <jolsa@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, kernel-team@....com
Subject: Re: [PATCH v9 2/3]: perf record: enable asynchronous trace writing

Hi,

On 05.10.2018 10:16, Namhyung Kim wrote:
> On Wed, Oct 03, 2018 at 07:12:10PM +0300, Alexey Budankov wrote:
<SNIP>
>> +static void record__aio_sync(struct perf_mmap *md)
>> +{
>> +	struct aiocb *cblock = &md->cblock;
>> +	struct timespec timeout = { 0, 1000 * 1000  * 1 }; // 1ms
>> +
>> +	do {
>> +		if (cblock->aio_fildes == -1 || record__aio_complete(md, cblock))
>> +			return;
>> +
>> +		while (aio_suspend((const struct aiocb**)&cblock, 1, &timeout)) {
>> +			if (!(errno == EAGAIN || errno == EINTR))
>> +				pr_err("failed to sync perf data, error: %m\n");
> 
> Is there somthing we can do in this error case?  Any chance it gets
> stuck in the loop?

Not really. Currently, in glibc, it can block on a mutex only.

> 
> 
>> +		}
>> +	} while (1);
>> +}
>> +
>> +static int record__aio_pushfn(void *to, struct aiocb *cblock, void *bf, size_t size)
>> +{
>> +	off_t off;
>> +	struct record *rec = to;
>> +	int ret, trace_fd = rec->session->data->file.fd;
>> +
>> +	rec->samples++;
>> +
>> +	off = lseek(trace_fd, 0, SEEK_CUR);
>> +	lseek(trace_fd, off + size, SEEK_SET);
> 
> It'd be nice if these lseek() could be removed and use
> rec->bytes_written instead.

Well, this could be implemented like this avoiding lseek() in else branch:

	off = lseek(trace_fd, 0, SEEK_CUR);
	ret = record__aio_write(cblock, trace_fd, bf, size, off);
	if (!ret) {
		lseek(trace_fd, off + size, SEEK_SET);
		rec->bytes_written += size;

		if (switch_output_size(rec))
			trigger_hit(&switch_output_trigger);
	}
> 
> 
>> +	ret = record__aio_write(cblock, trace_fd, bf, size, off);
>> +	if (!ret) {
>> +		rec->bytes_written += size;
>> +
>> +		if (switch_output_size(rec))
>> +			trigger_hit(&switch_output_trigger);
> 
> Doesn't it need the _sync() before the trigger?  Maybe it should be
> moved to record__mmap_read_evlist() or so..

Currently trigger just updates variable state.
The state is then checked thru separate API at __cmd_record() where 
record__mmap_read_sync() is called prior switching to a new trace file 
or finishing collection.

>>  
<SNIP>
>>  		if (map->base) {
>> +#ifndef HAVE_AIO_SUPPORT
>>  			if (perf_mmap__push(map, rec, record__pushfn) != 0) {
>>  				rc = -1;
>>  				goto out;
>>  			}
>> +#else
>> +			if (!rec->opts.nr_cblocks) {
>> +				if (perf_mmap__push(map, rec, record__pushfn) != 0) {
>> +					rc = -1;
>> +					goto out;
>> +				}
>> +			} else {
>> +				/*
>> +				 * Call record__aio_sync() to wait till map->data buffer
>> +				 * becomes available after previous aio write request.
>> +				 */
>> +				record__aio_sync(map);
>> +				if (perf_mmap__aio_push(map, rec, record__aio_pushfn) != 0) {
>> +					rc = -1;
>> +					goto out;
>> +				}
>> +			}
>> +#endif
> 
> If dummy aio functions are provided, the #ifdef can be removed and
> just use the #else part assuming opts.nr_cblocks == 0.

Yes, it looks a little bit cumbersome. Would this be more compact?

		if (map->base) {
#ifdef HAVE_AIO_SUPPORT
			if (!rec->opts.nr_cblocks) {
#endif
				if (perf_mmap__push(map, rec, record__pushfn) != 0) {
					rc = -1;
					goto out;
				}
#ifdef HAVE_AIO_SUPPORT
			} else {
				int idx;
				/*
				 * Call record__aio_sync() to wait till map->data buffer
				 * becomes available after previous aio write request.
				 */
				idx = record__aio_sync(map, false);
				if (perf_mmap__aio_push(map, rec, idx, record__aio_pushfn) != 0) {
					rc = -1;
					goto out;
				}
			}
#endif
		}

Thanks,
Alexey