linux-kernel - Re: [PATCH V2 1/1] perf,tools: add time out to force stop endless mmap processing

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Date:	Thu, 11 Jun 2015 12:45:44 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	kan.liang@...el.com
Cc:	linux-kernel@...r.kernel.org, ying.huang@...el.com,
	andi@...stfloor.org
Subject: Re: [PATCH V2 1/1] perf,tools: add time out to force stop endless
 mmap processing

Em Thu, Jun 11, 2015 at 03:49:04AM -0400, kan.liang@...el.com escreveu:
> perf top reads all threads' /proc/xxx/maps. If there is any threads
> which generating a keeping growing huge /proc/xxx/maps, perf will do
> infinite loop in perf_event__synthesize_mmap_events.
> This patch fixes this issue by adding a time out to force stop this kind
> of endless mmap processing.
> 
> Reported-by: Huang, Ying <ying.huang@...el.com>
> Signed-off-by: Kan Liang <kan.liang@...el.com>
> 
> ---
> 
> Changes since V1
>  - Add warning message for time out.

<SNIP>

> +		if ((rdclock() - t) > MMAP_TIMEOUT) {
> +			pr_warning("Reading %s time out."
> +				   "The file may be too huge or keep growing.\n",
> +				   filename);
> +			break;
> +		}
> +
>  		/* ensure null termination since stack will be reused. */
>  		strcpy(execname, "");

Have you tried this? I.e. pr_warning, IIRC, will make it appear in the
logs that this happened, but this will get probably lost in the noise
and only if you have a suspicion that something may have went wrong you
will try to look in the, possibly long, logs for a possible explanation.

So, perhaps we need to do something like what is done in
perf_session__process_events(), i.e. at the end of the synthesizing,
look at a counter for such situations and use ui__warning(), that in the
TUI case will show a window message that will show the warning and wait
for the user to acknowledge it by pressing a Ok button.

This is all done in perf_session__warn_about_errors().

This is how I think this should be done, but as this is such a corner
case, and this patch fixes these long loops, this may be applied now and
then what I suggest may be done on top.

Anyway, please try to reply to David Ahern question about an example for
this case, because he is working on a netlink based replacement to the
synthesizing of PERF_RECORD_ events for existing tasks and will have to
take this case into account there as well...

- Arnaldo
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