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@xxxxxxxxxxxxx> 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.
