Re: [PATCH v4 1/5] tracing: Introduce faultable tracepoints

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 21, 2023 at 05:50:29PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 21, 2023 at 11:00:13AM -0500, Mathieu Desnoyers wrote:
> > On 2023-11-21 10:52, Peter Zijlstra wrote:
> > > On Tue, Nov 21, 2023 at 03:46:43PM +0100, Peter Zijlstra wrote:
> > > 
> > > > Why is this such a hard question?
> > > 
> > > Anyway, recapping from IRC:
> > > 
> > > preemptible, SRCU:
> > >    counter-array based, GP advances by increasing array index
> > >    and waiting for previous index to drop to 0.
> > > 
> > >    notably, a GP can pass while a task is preempted but not within a
> > >    critical section.
> > > 
> > >    SRCU has smp_mb() in the critical sections to improve GP.
> > 
> > Also:
> > 
> > preemptible only allows blocking when priority inheritance is
> > guarantees, which excludes doing I/O, and thus page faults.
> > Otherwise a long I/O could cause the system to OOM.
> > 
> > SRCU allows all kind of blocking, as long as the entire SRCU
> > domain does not mind waiting for a while before readers complete.
> 
> Well, no. Fundamentally both SRCU and preemptible (and many other
> flavours) are just a counter-array. The non-blocking for preempt comes
> from the fact that it is the main global rcu instance and allowing all
> that would make GPs too rare and cause you memory trouble.
> 
> But that's not because of how it's implemented, but because of it being
> the main global instance.
> 
> > > tasks:
> > >    waits for every task to pass schedule()
> > > 
> > >    ensures that any pieces of text rendered unreachable before, is
> > >    actually unused after.
> > > 
> > > tasks-rude:
> > >    like tasks, but different? build to handle tracing while rcu-idle,
> > >    even though that was already deemed bad?
> > > 
> > > tasks-tracing-rcu:
> > >    extention of tasks to have critical-sections ? Should this simply be
> > >    tasks?
> > 
> > tasks-trace-rcu is meant to allow tasks to block/take a page fault within
> > the read-side. It is specialized for tracing and has a single domain. It
> > does not need the smp_mb on the read-side, which makes it lower-overhead
> > than SRCU.
> 
> That's what it's meant for, not what it is.
> 
> Turns out that tasks-tracing is a per-task counter based thing, and as
> such does not require all tasks to pass through schedule() and does not
> imply the tasks flavour (nor the tasks-rude) despite the similarity in
> naming.
> 
> But now I am again left wondering what the fundamental difference is
> between a per-task counter and a per-cpu counter.
> 
> At the end of the day, you still have to wait for the thing to hit 0.
> 
> So I'm once again confused, ...

Updating myself.. so task-tracing-rcu is in fact *very* similar to
regular preemptible-rcu but is slightly different mostly because it is
*not* the main global instance.

Both are a single per-task counter (and not the per-cpu summing that I
remember from many many *many* years ago; OLS'07), mostly because this
helps identify which task is to blame when things go sideways.