Re: [PATCH v1 0/2] perf: Drop leaked kernel samples

[Jin, Yao]

Hi All,

This patch raised many questions, I was prepared. :)

I'd like to try another proposal that it adds a special flag in the 
returned perf_sample_data to indicate the perf binary that this sample 
is a leaked sample.

For example, create a new PERF_SAMPLE_RETURN_LEAKAGE in 
perf_event_sample_format.

In perf_prepare_sample(),

if (event->attr.exclude_kernel && !user_mode(regs))
	data->type |= PERF_SAMPLE_RETURN_LEAKAGE;

Now all the samples are kept and the leaked kernel samples are tagged 
with PERF_SAMPLE_RETURN_LEAKAGE.

In perf binary, it filters out the samples with 
PERF_SAMPLE_RETURN_LEAKAGE. It needs perf binary modification but rr 
doesn't need to be changed.

I don't 0-stuffing some fields because:

1. Keeping the skid info should allow us to look at that if we have 
interesting later.

2. Not sure if 0-stuffing some fields has potential conflicts with some 
applications.

Is this proposal reasonable?

Thanks
Jin Yao

On 6/16/2018 1:34 AM, Robert O'Callahan wrote:
> On Fri, Jun 15, 2018 at 10:16 AM, Kyle Huey <me@xxxxxxxxxxxx> wrote:
> > If you want a sysctl for your own reasons that's fine. But we don't
> > want a sysctl. We want to work without any further configuration.
>
> Also toggling a sysctl would require root privileges, but rr does not
> currently need to run as root. Thus rr users would have to either
> permanently change their system configuration (and every extra
> configuration step is a pain), or run rr as root so rr can toggle the
> sysctl itself. Running rr as root is highly undesirable.
>
> Rob