Re: [PATCH] perf/core: generate overflow signal when samples are dropped (WAS: Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region)

From: Mark Rutland
Date: Tue Jul 04 2017 - 05:34:46 EST

On Tue, Jul 04, 2017 at 11:03:13AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 28, 2017 at 03:55:07PM -0700, Kyle Huey wrote:
> > > Having thought about this some more, I think Vince does make a good
> > > point that throwing away samples is liable to break stuff, e.g. that
> > > which only relies on (non-sensitive) samples.
> > >
> > > It still seems wrong to make up data, though.
> It is something we do in other places as well though. For example the
> printk() %pK thing fakes NULL pointers when kptr_restrict is set.

It looks like I'm outnumbered on that, then. :)

I'd still argue it's worth somehow indicating which samples were thrown
away, so that (updated) userspace can choose to ignore them, but I guess
that can come later.

> Faking data gets a wee bit tricky in how much data we need to clear
> through, its not only IP, pretty much everything we get from the
> interrupt context, like the branch stack and registers is also suspect.

Indeed. I'll take a run through __perf_event_output() and callees, and
see what we need to drop.

> > > Maybe for exclude_kernel && !exclude_user events we can always generate
> > > samples from the user regs, rather than the exception regs. That's going
> > > to be closer to what the user wants, regardless. I'll take a look
> > > tomorrow.
> >
> > I'm not very familiar with the kernel internals, but the reason I
> > didn't suggest this originally is it seems like it will be difficult
> > to determine what the "correct" userspace registers are. For example,
> > what happens if a performance counter is fixed to a given tid, the
> > interrupt fires during a context switch from that task to another that
> > is not being monitored, and the kernel is far enough along in the
> > context switch that the current task struct has been switched out?
> > Reporting the new task's registers seems as bad as reporting the
> > kernel's registers. But maybe this is easier than I imagine for
> > whatever reason.
> If the counter is fixed to a task then its scheduled along with the
> task. We'll schedule out the event before doing the actual task switch
> and switch in the new event after.
> That said, with a per-cpu event the TID sample value is indeed subject
> to skid like you describe.

For per-cpu events, does that matter? Those don't have TID filters in
the first place, no?