On Wed, 21 Nov 2018 16:28:56 -0800 Daniel Colascione <dancol@xxxxxxxxxx> wrote: > > > The problem here is the possibility of confusion, even if it's rare. > > > Does the naive approach of just walking /proc and ignoring the > > > possibility of PID reuse races work most of the time? Sure. But "most > > > of the time" isn't good enough. It's not that there are tons of sob > > > stories: it's that without completely robust reporting, we can't rule > > > out of the possibility that weirdness we observe in a given trace is > > > actually just an artifact from a kinda-sort-working best-effort trace > > > collection system instead of a real anomaly in behavior. Tracing, > > > essentially, gives us deltas for system state, and without an accurate > > > baseline, collected via some kind of scan on trace startup, it's > > > impossible to use these deltas to robustly reconstruct total system > > > state at a given time. And this matters, because errors in > > > reconstruction (e.g., assigning a thread to the wrong process because > > > the IDs happen to be reused) can affect processing of the whole trace. > > > If it's 3am and I'm analyzing the lone trace from a dogfooder > > > demonstrating a particularly nasty problem, I don't want to find out > > > that the trace I'm analyzing ended up being useless because the > > > kernel's trace system is merely best effort. It's very cheap to be > > > 100% reliable here, so let's be reliable and rule out sources of > > > error. > > > > So we're solving a problem which isn't known to occur, but solving it > > provides some peace-of-mind? Sounds thin! > > So you want to reject a cheap fix for a problem that you know occurs > at some non-zero frequency? There's a big difference between "may or > may not occur" and "will occur eventually, given enough time, and so > must be taken into account in analysis". Would you fix a refcount race > that you knew was possible, but didn't observe? What, exactly, is your > threshold for accepting a fix that makes tracing more reliable? Well for a start I'm looking for a complete patch changelog. One which permits readers to fully understand the user-visible impact of the problem. If it is revealed that is a theoretical problem which has negligible end-user impact then sure, it is rational to leave things as they are. That's what "negligible" means!
