Re: [PATCH v2] Add /proc/pid_gen

[Andy Lutomirski <luto@amacapital.net>]

> On Nov 21, 2018, at 4:21 PM, Daniel Colascione <dancol@google.com> wrote:
> 
>> On Wed, Nov 21, 2018 at 2:50 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>> 
>>> On Wed, 21 Nov 2018 14:40:28 -0800 Daniel Colascione <dancol@google.com> wrote:
>>> 
>>>> On Wed, Nov 21, 2018 at 2:12 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>>>> 
>>>>> On Wed, 21 Nov 2018 12:54:20 -0800 Daniel Colascione <dancol@google.com> wrote:
>>>>> 
>>>>> Trace analysis code needs a coherent picture of the set of processes
>>>>> and threads running on a system. While it's possible to enumerate all
>>>>> tasks via /proc, this enumeration is not atomic. If PID numbering
>>>>> rolls over during snapshot collection, the resulting snapshot of the
>>>>> process and thread state of the system may be incoherent, confusing
>>>>> trace analysis tools. The fundamental problem is that if a PID is
>>>>> reused during a userspace scan of /proc, it's impossible to tell, in
>>>>> post-processing, whether a fact that the userspace /proc scanner
>>>>> reports regarding a given PID refers to the old or new task named by
>>>>> that PID, as the scan of that PID may or may not have occurred before
>>>>> the PID reuse, and there's no way to "stamp" a fact read from the
>>>>> kernel with a trace timestamp.
>>>>> 
>>>>> This change adds a per-pid-namespace 64-bit generation number,
>>>>> incremented on PID rollover, and exposes it via a new proc file
>>>>> /proc/pid_gen. By examining this file before and after /proc
>>>>> enumeration, user code can detect the potential reuse of a PID and
>>>>> restart the task enumeration process, repeating until it gets a
>>>>> coherent snapshot.
>>>>> 
>>>>> PID rollover ought to be rare, so in practice, scan repetitions will
>>>>> be rare.
>>>> 
>>>> In general, tracing is a rather specialized thing.  Why is this very
>>>> occasional confusion a sufficiently serious problem to warrant addition
>>>> of this code?
>>> 
>>> 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.
> 
>>>> Which userspace tools will be using pid_gen?  Are the developers of
>>>> those tools signed up to use pid_gen?
>>> 
>>> I'll be changing Android tracing tools to capture process snapshots
>>> using pid_gen, using the algorithm in the commit message.
>> 
>> Which other tools could use this and what was the feedback from their
>> developers?
> 
> I'm going to have Android's systrace and Perfetto use this approach.
> Exactly how many tools signed up to use this feature do you need?
> 
>> Those people are the intended audience and the
>> best-positioned reviewers so let's hear from them?
> 
> I'm writing plenty of trace analysis tools myself, so I'm part of this
> intended audience. Other tracing tool authors have told me about
> out-of-tree hacks for process atomic snapshots via ftrace events. This
> approach avoids the necessity of these more-invasive hacks.

Would a tracepoint for pid reuse solve your problem?