Re: [PATCH v4 2/2] ThunderX2: Add Cavium ThunderX2 SoC UNCORE PMU driver

[Kim Phillips <kim.phillips@arm.com>]

On Fri, 27 Apr 2018 17:09:14 +0100
Will Deacon <will.deacon@arm.com> wrote:

> Kim,
> 
> [Ganapat: please don't let this discussion disrupt your PMU driver
>  development. You can safely ignore it for now :)]
> 
> On Fri, Apr 27, 2018 at 10:46:29AM -0500, Kim Phillips wrote:
> > On Fri, 27 Apr 2018 15:37:20 +0100
> > Will Deacon <will.deacon@arm.com> wrote:
> > 
> > > On Fri, Apr 27, 2018 at 08:15:25AM -0500, Kim Phillips wrote:
> > > > On Fri, 27 Apr 2018 10:30:27 +0100
> > > > Mark Rutland <mark.rutland@arm.com> wrote:
> > > > > On Thu, Apr 26, 2018 at 05:06:24PM -0500, Kim Phillips wrote:
> > > > > > On Wed, 25 Apr 2018 14:30:47 +0530
> > > > > > Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com> wrote:
> > > > > > 
> > > > > > > +static int thunderx2_uncore_event_init(struct perf_event *event)
> > > > > 
> > > > > > This PMU driver can be made more user-friendly by not just silently
> > > > > > returning an error code such as -EINVAL, but by emitting a useful
> > > > > > message describing the specific error via dmesg.
> > > > > 
> > > > > As has previously been discussed on several occasions, patches which log
> > > > > to dmesg in a pmu::event_init() path at any level above pr_debug() are
> > > > > not acceptable -- dmesg is not intended as a mechanism to inform users
> > > > > of driver-specific constraints.
> > > > 
> > > > I disagree - drivers do it all the time, using dev_err(), dev_warn(), etc.
> > > > 
> > > > > I would appreciate if in future you could qualify your suggestion with
> > > > > the requirement that pr_debug() is used.
> > > > 
> > > > It shouldn't - the driver isn't being debugged, it's in regular use.
> > > 
> > > For anything under drivers/perf/, I'd prefer not to have these prints
> > > and instead see efforts to improve error reporting via the perf system
> > > call interface.
> > 
> > We'd all prefer that, and for all PMU drivers, why should ones under
> > drivers/perf be treated differently?
> 
> Because they're the ones I maintain...

You represent a minority on your opinion on this matter though.

> > As you are already aware, I've personally tried to fix this problem -
> > that has existed since before the introduction of the perf tool (I
> > consider it a syscall-independent enhanced error interface), multiple
> > times, and failed.
> 
> Why is that my problem? Try harder?

It's your problem because we're here reviewing a patch that happens to
fall under your maintainership.  I'll be the first person to tell you
I'm obviously incompetent and haven't been able to come up with a
solution that is acceptable for everyone up to and including Linus
Torvalds.  I'm just noticing a chronic usability problem that can be
easily alleviated in the context of this patch review.

> > So until someone comes up with a solution that works for everyone
> > up to and including Linus Torvalds (who hasn't put up a problem
> > pulling PMU drivers emitting things to dmesg so far, by the way), this
> > keep PMU drivers' errors silent preference of yours is unnecessarily
> > impeding people trying to measure system performance on Arm based
> > machines - all other archs' maintainers are fine with PMU drivers using
> > dmesg.
> 
> Good for them, although I'm pretty sure that at least the x86 folks are
> against this crap too.

Unfortunately, it doesn't affect them nearly as much as it does our
more diverse platforms, which is why I don't think they care to do
much about it.

> > > Anyway, I think this driver has bigger problems that need addressing.
> > 
> > To me it represents yet another PMU driver submission - as the years go
> > by - that is lacking in the user messaging area.  Which reminds me, can
> > you take another look at applying this?:
> 
> As I said before, I'm not going to take anything that logs above pr_debug
> for things that are directly triggerable from userspace. Spin a version

Why?  There are plenty of things that emit stuff into dmesg that are
directly triggerable from userspace.  Is it because it upsets fuzzing
tests?  How about those be run with a patched kernel that somehow
mitigates the printing?

> using pr_debug and I'll queue it.

How about using a ratelimited dev_err variant?

> Have a good weekend,

You too.

Kim