Re: [PATCH v3] tracing: Guard __DECLARE_TRACE() use of __DO_TRACE_CALL() with SRCU-fast

[Paul E. McKenney]

On Fri, Dec 12, 2025 at 09:28:37AM +0000, Joel Fernandes wrote:
> 
> 
> > On Dec 12, 2025, at 4:50 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > 
> > On Fri, Dec 12, 2025 at 03:43:07AM +0000, Joel Fernandes wrote:
> >> 
> >> 
> >>>> On Dec 12, 2025, at 9:47 AM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >>> 
> >>> On Fri, Dec 12, 2025 at 09:12:07AM +0900, Joel Fernandes wrote:
> >>>> 
> >>>> 
> >>>>> On 12/11/2025 3:23 PM, Paul E. McKenney wrote:
> >>>>> On Thu, Dec 11, 2025 at 08:02:15PM +0000, Joel Fernandes wrote:
> >>>>>> 
> >>>>>> 
> >>>>>>> On Dec 8, 2025, at 1:20 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> >>>>>>> 
> >>>>>>> The current use of guard(preempt_notrace)() within __DECLARE_TRACE()
> >>>>>>> to protect invocation of __DO_TRACE_CALL() means that BPF programs
> >>>>>>> attached to tracepoints are non-preemptible.  This is unhelpful in
> >>>>>>> real-time systems, whose users apparently wish to use BPF while also
> >>>>>>> achieving low latencies.  (Who knew?)
> >>>>>>> 
> >>>>>>> One option would be to use preemptible RCU, but this introduces
> >>>>>>> many opportunities for infinite recursion, which many consider to
> >>>>>>> be counterproductive, especially given the relatively small stacks
> >>>>>>> provided by the Linux kernel.  These opportunities could be shut down
> >>>>>>> by sufficiently energetic duplication of code, but this sort of thing
> >>>>>>> is considered impolite in some circles.
> >>>>>>> 
> >>>>>>> Therefore, use the shiny new SRCU-fast API, which provides somewhat faster
> >>>>>>> readers than those of preemptible RCU, at least on Paul E. McKenney's
> >>>>>>> laptop, where task_struct access is more expensive than access to per-CPU
> >>>>>>> variables.  And SRCU-fast provides way faster readers than does SRCU,
> >>>>>>> courtesy of being able to avoid the read-side use of smp_mb().  Also,
> >>>>>>> it is quite straightforward to create srcu_read_{,un}lock_fast_notrace()
> >>>>>>> functions.
> >>>>>>> 
> >>>>>>> While in the area, SRCU now supports early boot call_srcu().  Therefore,
> >>>>>>> remove the checks that used to avoid such use from rcu_free_old_probes()
> >>>>>>> before this commit was applied:
> >>>>>>> 
> >>>>>>> e53244e2c893 ("tracepoint: Remove SRCU protection")
> >>>>>>> 
> >>>>>>> The current commit can be thought of as an approximate revert of that
> >>>>>>> commit, with some compensating additions of preemption disabling.
> >>>>>>> This preemption disabling uses guard(preempt_notrace)().
> >>>>>>> 
> >>>>>>> However, Yonghong Song points out that BPF assumes that non-sleepable
> >>>>>>> BPF programs will remain on the same CPU, which means that migration
> >>>>>>> must be disabled whenever preemption remains enabled.  In addition,
> >>>>>>> non-RT kernels have performance expectations that would be violated by
> >>>>>>> allowing the BPF programs to be preempted.
> >>>>>>> 
> >>>>>>> Therefore, continue to disable preemption in non-RT kernels, and protect
> >>>>>>> the BPF program with both SRCU and migration disabling for RT kernels,
> >>>>>>> and even then only if preemption is not already disabled.
> >>>>>> 
> >>>>>> Hi Paul,
> >>>>>> 
> >>>>>> Is there a reason to not make non-RT also benefit from SRCU fast and trace points for BPF? Can be a follow up patch though if needed.
> >>>>> 
> >>>>> Because in some cases the non-RT benefit is suspected to be negative
> >>>>> due to increasing the probability of preemption in awkward places.
> >>>> 
> >>>> Since you mentioned suspected, I am guessing there is no concrete data collected
> >>>> to substantiate that specifically for BPF programs, but correct me if I missed
> >>>> something. Assuming you're referring to latency versus tradeoffs issues, due to
> >>>> preemption, Android is not PREEMPT_RT but is expected to be low latency in
> >>>> general as well. So is this decision the right one for Android as well,
> >>>> considering that (I heard) it uses BPF? Just an open-ended question.
> >>>> 
> >>>> There is also issue of 2 different paths for PREEMPT_RT versus otherwise,
> >>>> complicating the tracing side so there better be a reason for that I guess.
> >>> 
> >>> You are advocating a change in behavior for non-RT workloads.  Why do
> >>> you believe that this change would be OK for those workloads?
> >> 
> >> Same reasons I provided in my last email. If we are saying SRCU-fast is required for lower latency, I find it strange that we are leaving out Android which has low latency audio usecases, for instance.
> > 
> > If Android provides numbers showing that it helps them, then it is easy
> > to provide a Kconfig option that defaults to PREEMPT_RT, but that Android
> > can override.  Right?
> 
> Sure, but my suspicion is Android or others are not going to look into every PREEMPT_RT specific optimization (not just this one) and see if it benefits their interactivity usecases. They will simply miss out on it without knowing they are.
> 
> It might be a good idea (for me) to explore how many such optimizations exist though, that we take for granted. I will look into exploring this on my side. :)

One workload's optimization is another workload's pessimization, in
part because there are a lot of different measures of performance that
different workloads care about..

But as a practical matter, this is Steven's decision.

Though if he does change the behavior on non-RT setups, I would thank
him to remove my name from the commit, or at least record in the commit
log that I object to changing other workloads' behaviors.

							Thanx, Paul

> thanks,
> 
>  - Joel 
> 
> > 
> >                            Thanx, Paul
> > 
> >> Thanks,
> >> 
> >> - Joel
> >> 
> >> 
> >>> 
> >>>                           Thanx, Paul