Re: [PATCH bpf-next v8 0/4] bpf: add cpu cycles kfuncss

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 26, 2024 at 10:12:57AM -0800, Andrii Nakryiko wrote:
> On Fri, Nov 22, 2024 at 3:34 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Wed, Nov 20, 2024 at 04:08:10PM -0800, Vadim Fedorenko wrote:
> > > This patchset adds 2 kfuncs to provide a way to precisely measure the
> > > time spent running some code. The first patch provides a way to get cpu
> > > cycles counter which is used to feed CLOCK_MONOTONIC_RAW. On x86
> > > architecture it is effectively rdtsc_ordered() function while on other
> > > architectures it falls back to __arch_get_hw_counter(). The second patch
> > > adds a kfunc to convert cpu cycles to nanoseconds using shift/mult
> > > constants discovered by kernel. The main use-case for this kfunc is to
> > > convert deltas of timestamp counter values into nanoseconds. It is not
> > > supposed to get CLOCK_MONOTONIC_RAW values as offset part is skipped.
> > > JIT version is done for x86 for now, on other architectures it falls
> > > back to slightly simplified version of vdso_calc_ns.
> >
> > So having now read this. I'm still left wondering why you would want to
> > do this.
> >
> > Is this just debug stuff, for when you're doing a poor man's profile
> > run? If it is, why do we care about all the precision or the ns. And why
> > aren't you using perf?
> 
> No, it's not debug stuff. It's meant to be used in production for
> measuring durations of whatever is needed. Like uprobe entry/exit
> duration, or time between scheduling switches, etc.
> 
> Vadim emphasizes benchmarking at scale, but that's a bit misleading.
> It's not "benchmarking", it's measuring durations of relevant pairs of
> events. In production and at scale, so the unnecessary overhead all
> adds up. We'd like to have the minimal possible overhead for this time
> passage measurement. And some durations are very brief,

You might want to consider leaving out the LFENCE before the RDTSC on
some of those, LFENCE isn't exactly cheap.

> so precision
> matters as well. And given this is meant to be later used to do
> aggregation and comparison across large swaths of production hosts, we
> have to have comparable units, which is why nanoseconds and not some
> abstract "time cycles".
> 
> Does this address your concerns?

Well, it's clearly useful for you guys, but I do worry about it. Even on
servers DVFS is starting to play a significant role. And the TSC is
unaffected by it.

Directly comparing these numbers, esp. across different systems makes no
sense to me. Yes putting them all in [ns] allows for comparison, but
you're still comparing fundamentally different things.

How does it make sense to measure uprobe entry/exit in wall-clock when
it can vary by at least a factor of 2 depending on DVFS. How does it
make sense to compare an x86-64 uprobe entry/exit to an aaargh64 one?

Or are you trying to estimate the fraction of overhead spend on
instrumentation instead of real work? Like, this machine spends 5% of
its wall-time in instrumentation, which is effectively not doing work?

The part I'm missing is how using wall-time for these things makes
sense.

I mean, if all you're doing is saying, hey, we appear to be spending X
on this action on this particular system Y doing workload Z (irrespecive
of you then having like a million Ys) and this patch reduces X by half
given the same Y and Z. So patch must be awesome.

Then you don't need the conversion to [ns], and the DVFS angle is more
or less mitigated by the whole 'same workload' thing.