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

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Date:   Tue, 28 Aug 2018 10:57:03 +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 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 way I see it, when 'pushing the data' it's either all or nothing,

if there's an error in pushing one map, we bail out completely..
so the mmap_aio array could be preallocated (it is now) and
pre-filled with cblock pointers

that would probably ease up the reference counting I mentioned
in the previous email

thanks,
jirka