linux-kernel - Re: [PATCH] perf record: Add snapshot mode support for perf's regular events

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Date:	Tue, 24 Nov 2015 08:06:41 -0700
From:	David Ahern <dsahern@...il.com>
To:	Yunlong Song <yunlong.song@...wei.com>, a.p.zijlstra@...llo.nl,
	paulus@...ba.org, mingo@...hat.com, acme@...nel.org
Cc:	linux-kernel@...r.kernel.org, wangnan0@...wei.com,
	namhyung@...nel.org, ast@...nel.org,
	masami.hiramatsu.pt@...achi.com, kan.liang@...el.com,
	adrian.hunter@...el.com, jolsa@...nel.org, bp@...en8.de,
	jean.pihet@...aro.org, rric@...nel.org, xiakaixu@...wei.com,
	hekuang@...wei.com
Subject: Re: [PATCH] perf record: Add snapshot mode support for perf's regular
 events

On 11/24/15 7:00 AM, Yunlong Song wrote:
> +static int record__write(struct record *rec, void *bf, size_t size)
> +{
> +	if (rec->memory.size && memory_enabled) {
> +		if (perf_memory__write(&rec->memory, bf, size) < 0) {
> +			pr_err("failed to write memory data, error: %m\n");
> +			return -1;
> +		}
> +	} else {
> +		if (perf_data_file__write(rec->session->file, bf, size) < 0) {
> +			pr_err("failed to write perf data, error: %m\n");
> +			return -1;
> +		}
> +		rec->bytes_written += size;
>   	}
>
> -	rec->bytes_written += size;
>   	return 0;
>   }
>
> @@ -86,6 +214,8 @@ static int record__mmap_read(struct record *rec, int idx)
>   	if (old == head)
>   		return 0;
>
> +	memory_enabled = 1;
> +
>   	rec->samples++;
>
>   	size = head - old;
> @@ -113,6 +243,7 @@ static int record__mmap_read(struct record *rec, int idx)
>   	md->prev = old;
>   	perf_evlist__mmap_consume(rec->evlist, idx);
>   out:
> +	memory_enabled = 0;
>   	return rc;
>   }
>

So you are basically ignoring all samples until SIGUSR2 is received. 
That means the resulting data file will have limited history of task 
events for example. And for other events the quantity is random as to 
when the mmaps were last scanned.

Your cover letter mentioned my code "just makes some count when the 
signal triggers perf sched, with no sample recording and has nothing to 
do with perf.data". That is not correct. If you look at the perf-daemon 
code I pointed you to it processes task events as they are received and 
saves the last N-events after time sorting (limited by memory or time). 
When a signal is received it processes the saved events and dumps them 
to stdout versus writing a perf.data file.

David
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