linux-kernel - Re: [PATCH v1 5/8] tracing: Allow system call tracepoints to handle page faults

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Message-ID: <eacb18fc-7066-40f3-9d83-4602e946c225@efficios.com>
Date: Thu, 3 Oct 2024 20:35:03 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org,
 Peter Zijlstra <peterz@...radead.org>, Alexei Starovoitov <ast@...nel.org>,
 Yonghong Song <yhs@...com>, "Paul E . McKenney" <paulmck@...nel.org>,
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Namhyung Kim <namhyung@...nel.org>,
 Andrii Nakryiko <andrii.nakryiko@...il.com>, bpf@...r.kernel.org,
 Joel Fernandes <joel@...lfernandes.org>, linux-trace-kernel@...r.kernel.org,
 Michael Jeanson <mjeanson@...icios.com>
Subject: Re: [PATCH v1 5/8] tracing: Allow system call tracepoints to handle
 page faults

On 2024-10-04 00:29, Steven Rostedt wrote:
> On Thu,  3 Oct 2024 11:16:35 -0400
[...[
>> -#define __DO_TRACE(name, args, cond, rcuidle)				\
>> +#define __DO_TRACE(name, args, cond, rcuidle, syscall)			\
>>   	do {								\
>>   		int __maybe_unused __idx = 0;				\
>>   									\
>> @@ -222,8 +224,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>>   			      "Bad RCU usage for tracepoint"))		\
>>   			return;						\
>>   									\
>> -		/* keep srcu and sched-rcu usage consistent */		\
>> -		preempt_disable_notrace();				\
>> +		if (syscall) {						\
>> +			rcu_read_lock_trace();				\
>> +		} else {						\
>> +			/* keep srcu and sched-rcu usage consistent */	\
>> +			preempt_disable_notrace();			\
>> +		}							\
>>   									\
> 
> I'm thinking we just use rcu_read_lock_trace() and get rid of the
> preempt_disable and srcu locks for all tracepoints. Oh crap! I should get
> rid of srcu locking too, as it was only needed for the rcuidle code :-p

How about we do it one step at a time ? First introduce use of the
(lightly tested) rcu_read_lock_trace() (at least in comparison with
preempt disable RCU) only for syscalls, and if this works well,
then eventually consider moving the preempt off users to
rcu_read_lock_trace as well ?

Of course it should all work well, in theory. But considering the
vast number of tracepoints we have in the kernel, I am reluctant
to change too many things at once in that area. We may very well
be bitten by unforeseen corner-cases.

Thanks,

Mathieu

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