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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2023-11-21 03:47, Peter Zijlstra wrote:
> On Mon, Nov 20, 2023 at 03:56:30PM -0800, Paul E. McKenney wrote:
>> On Mon, Nov 20, 2023 at 11:23:11PM +0100, Peter Zijlstra wrote:
>>> On Mon, Nov 20, 2023 at 02:18:29PM -0800, Paul E. McKenney wrote:
>>>> On Mon, Nov 20, 2023 at 10:47:42PM +0100, Peter Zijlstra wrote:
>>>>> On Mon, Nov 20, 2023 at 03:54:14PM -0500, Mathieu Desnoyers wrote:
>>>>>> When invoked from system call enter/exit instrumentation, accessing
>>>>>> user-space data is a common use-case for tracers. However, tracepoints
>>>>>> currently disable preemption around iteration on the registered
>>>>>> tracepoint probes and invocation of the probe callbacks, which prevents
>>>>>> tracers from handling page faults.
>>>>>>
>>>>>> Extend the tracepoint and trace event APIs to allow defining a faultable
>>>>>> tracepoint which invokes its callback with preemption enabled.
>>>>>>
>>>>>> Also extend the tracepoint API to allow tracers to request specific
>>>>>> probes to be connected to those faultable tracepoints. When the
>>>>>> TRACEPOINT_MAY_FAULT flag is provided on registration, the probe
>>>>>> callback will be called with preemption enabled, and is allowed to take
>>>>>> page faults. Faultable probes can only be registered on faultable
>>>>>> tracepoints and non-faultable probes on non-faultable tracepoints.
>>>>>>
>>>>>> The tasks trace rcu mechanism is used to synchronize read-side
>>>>>> marshalling of the registered probes with respect to faultable probes
>>>>>> unregistration and teardown.
>>>>>
>>>>> What is trace-trace rcu and why is it needed here? What's wrong with
>>>>> SRCU ?
>>>>
>>>> Tasks Trace RCU avoids SRCU's full barriers and the array accesses in the
>>>> read-side primitives.  This can be important when tracing low-overhead
>>>> components of fast paths.
>>>
>>> So why wasn't SRCU improved? That is, the above doesn't much explain.
>>>
>>> What is the trade-off made to justify adding yet another RCU flavour?
>>
>> We didn't think you would be all that happy about having each and
>> every context switch iterating through many tens or even hundreds of
>> srcu_struct structures.  For that matter, we didn't think that anyone
>> else would be all that happy either.  Us included.
> 
> So again, what is task-trace RCU ? How does it differ from say
> preemptible rcu, which AFAICT could be used here too, no?

Task trace RCU fits a niche that has the following set of requirements/tradeoffs:

- Allow page faults within RCU read-side (like SRCU),
- Has a low-overhead read lock-unlock (without the memory barrier overhead of SRCU),
- The tradeoff: Has a rather slow synchronize_rcu(), but tracers should not care about
   that. Hence, this is not meant to be a generic replacement for SRCU.

Based on my reading of https://lwn.net/Articles/253651/ , preemptible RCU is not a good
fit for the following reasons:

- It disallows blocking within a RCU read-side on non-CONFIG_PREEMPT kernels,
- AFAIU the mmap_sem used within the page fault handler does not have priority inheritance.

Please let me know if I'm missing something.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com