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From:   Daniel Colascione <dancol@google.com>
Date:   Wed, 21 Nov 2018 16:28:56 -0800
Message-ID: <CAKOZuet7ipw8zoXCDQUB5PKs3kdSye-zesBbEv7jVDSByhXgkw@mail.gmail.com>
Subject: Re: [PATCH v2] Add /proc/pid_gen
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Primiano Tucci <primiano@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>, Roman Gushchin <guro@fb.com>,
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On Wed, Nov 21, 2018 at 4:22 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 21 Nov 2018 15:21:40 -0800 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:
> >
> > ...
> >
> > > > 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?