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

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Date:   Tue, 28 Aug 2018 14:21:16 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.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>
Subject: Re: [PATCH v3 2/2]: perf record: enable asynchronous trace writing

On Tue, Aug 28, 2018 at 02:31:04PM +0300, Alexey Budankov wrote:
> Hi,
> 
> On 28.08.2018 11:57, Jiri Olsa wrote:
> > On Mon, Aug 27, 2018 at 09:16:55PM +0300, Alexey Budankov wrote:
> > 
> > SNIP
> > 
> >> +	int trace_fd = rec->session->data->file.fd;
> >> +	struct aiocb **mmap_aio = rec->evlist->mmap_aio;
> >> +	int mmap_aio_size = 0;
> >> +	off_t off;
> >>  
> >>  	if (!evlist)
> >>  		return 0;
> >> @@ -528,14 +632,17 @@ static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evli
> >>  	if (overwrite && evlist->bkw_mmap_state != BKW_MMAP_DATA_PENDING)
> >>  		return 0;
> >>  
> >> +	off = lseek(trace_fd, 0, SEEK_CUR);
> >> +
> >>  	for (i = 0; i < evlist->nr_mmaps; i++) {
> >>  		struct auxtrace_mmap *mm = &maps[i].auxtrace_mmap;
> >>  
> >>  		if (maps[i].base) {
> >> -			if (perf_mmap__push(&maps[i], rec, record__pushfn) != 0) {
> >> -				rc = -1;
> >> +			rc = perf_mmap__push(&maps[i], rec, record__pushfn, &off);
> >> +			if (rc < 0)
> >>  				goto out;
> >> -			}
> >> +			else if (rc > 0)
> >> +				mmap_aio[mmap_aio_size++] = &maps[i].cblock;
> > 
> > I understand the purpose of mmap_aio array, but I don't see a reason
> > to fill it in every time we call record__mmap_read_evlist
> 
> The cycle trips the same number of iterations over kernel buffers 
> for every call of record__mmap_read_evlist(). Called perf_mmap__push() 
> checks if there is data ready for spill in the corresponding buffer 
> and if there is no such data returns 0. So every time we execute 
> the cycle we get different set of buffers to spill and in this 
> circumstances dynamic filling of mmap_aio looks preferable.
> Lifetime management of perf_mmap object and referenced memory 
> is not related another thing.

argh.. the perf_mmap__push returns 0 for empty buffer, ok

jirka