On Wed, Nov 21, 2018 at 4:22 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Wed, 21 Nov 2018 15:21:40 -0800 Daniel Colascione <dancol@xxxxxxxxxx> wrote: > > > On Wed, Nov 21, 2018 at 2:50 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Wed, 21 Nov 2018 14:40:28 -0800 Daniel Colascione <dancol@xxxxxxxxxx> wrote: > > > > > > > On Wed, Nov 21, 2018 at 2:12 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > ... > > > > > > I wouldn't call tracing a specialized thing: it's important enough to > > > > justify its own summit and a whole ecosystem of trace collection and > > > > analysis tools. We use it in every day in Android. It's tremendously > > > > helpful for understanding system behavior, especially in cases where > > > > multiple components interact in ways that we can't readily predict or > > > > replicate. Reliability and precision in this area are essential: > > > > retrospective analysis of difficult-to-reproduce problems involves > > > > puzzling over trace files and testing hypothesis, and when the trace > > > > system itself is occasionally unreliable, the set of hypothesis to > > > > consider grows. I've tried to keep the amount of kernel infrastructure > > > > needed to support this precision and reliability to a minimum, pushing > > > > most of the complexity to userspace. But we do need, from the kernel, > > > > reliable process disambiguation. > > > > > > > > Besides: things like checkpoint and restart are also non-core > > > > features, but the kernel has plenty of infrastructure to support them. > > > > We're talking about a very lightweight feature in this thread. > > > > > > I'm still not understanding the seriousness of the problem. Presumably > > > you've hit problems in real-life which were serious and frequent enough > > > to justify getting down and writing the code. Please share some sob stories > > > with us! > > > > 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?
