Re: [lttng-dev] [PATCH] sched/tracing: fix __trace_sched_switch_state (5.18-rc7+)

[Mathieu Desnoyers via lttng-dev <lttng-dev@lists.lttng.org>]

----- On May 19, 2022, at 11:02 AM, Bruce Ashfield via lttng-dev lttng-dev@lists.lttng.org wrote:

> From: Bruce Ashfield <bruce.ashfield@gmail.com>
> 
> The commit [fix: sched/tracing: Don't re-read p->state when emitting
> sched_switch event (v5.18)] was correct, but the kernel changed their
> mind with the following commit:
> 
>   commit 9c2136be0878c88c53dea26943ce40bb03ad8d8d
>   Author: Delyan Kratunov <delyank@fb.com>
>   Date:   Wed May 11 18:28:36 2022 +0000
> 
>       sched/tracing: Append prev_state to tp args instead
> 
>       Commit fa2c3254d7cf (sched/tracing: Don't re-read p->state when emitting
>       sched_switch event, 2022-01-20) added a new prev_state argument to the
>       sched_switch tracepoint, before the prev task_struct pointer.
> 
>       This reordering of arguments broke BPF programs that use the raw
>       tracepoint (e.g. tp_btf programs). The type of the second argument has
>       changed and existing programs that assume a task_struct* argument
>       (e.g. for bpf_task_storage access) will now fail to verify.
> 
>       If we instead append the new argument to the end, all existing programs
>       would continue to work and can conditionally extract the prev_state
>       argument on supported kernel versions.
> 
>       Fixes: fa2c3254d7cf (sched/tracing: Don't re-read p->state when emitting
>       sched_switch event, 2022-01-20)
>       Signed-off-by: Delyan Kratunov <delyank@fb.com>
>       Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>       Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
>       Link:
>       https://lkml.kernel.org/r/c8a6930dfdd58a4a5755fc01732675472979732b.camel@fb.com
> 
> By reordering the parameters (again) we can get back up and building.
> 
> Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
> ---
> 
> Hi all,
> 
> This is more than likely NOT a correct fix, but while working on the
> yocto -dev reference kernel, I ran into this build failure against
> 5.18-rc7.
> 
> I didn't see any sign of another fix on the mailing list, so I wanted
> to send this in case anyone else runs into the failure and to check
> to see if there's a better fix in progress.

Hi Bruce,

Please see: https://review.lttng.org/c/lttng-modules/+/8045

I was planning to merge it today, unless you have objections.

I notice that in your own patch, you modify the arguments passed to
__trace_sched_switch_state(), althrough I don't see them changing in
the upstream kernel. Am I missing something ?

Thanks,

Mathieu

> 
> Cheers,
> 
> Bruce
> 
> 
> include/instrumentation/events/sched.h | 15 ++++++++-------
> 1 file changed, 8 insertions(+), 7 deletions(-)
> 
> diff --git a/include/instrumentation/events/sched.h
> b/include/instrumentation/events/sched.h
> index 339bec9..f9e9c38 100644
> --- a/include/instrumentation/events/sched.h
> +++ b/include/instrumentation/events/sched.h
> @@ -23,8 +23,9 @@
> #if (LTTNG_LINUX_VERSION_CODE >= LTTNG_KERNEL_VERSION(5,18,0))
> 
> static inline long __trace_sched_switch_state(bool preempt,
> -		unsigned int prev_state,
> -		struct task_struct *p)
> +		struct task_struct *p,
> +		struct task_struct *n,
> +		unsigned int prev_state )
> {
>         unsigned int state;
> 
> @@ -356,20 +357,20 @@ LTTNG_TRACEPOINT_EVENT_INSTANCE(sched_wakeup_template,
> sched_wakeup_new,
> LTTNG_TRACEPOINT_EVENT(sched_switch,
> 
> 	TP_PROTO(bool preempt,
> -		unsigned int prev_state,
> 		struct task_struct *prev,
> -		struct task_struct *next),
> +		struct task_struct *next,
> +		unsigned int prev_state),
> 
> -	TP_ARGS(preempt, prev_state, prev, next),
> +	TP_ARGS(preempt, prev, next, prev_state),
> 
> 	TP_FIELDS(
> 		ctf_array_text(char, prev_comm,	prev->comm, TASK_COMM_LEN)
> 		ctf_integer(pid_t, prev_tid, prev->pid)
> 		ctf_integer(int, prev_prio, prev->prio - MAX_RT_PRIO)
> #ifdef CONFIG_LTTNG_EXPERIMENTAL_BITWISE_ENUM
> -		ctf_enum(task_state, long, prev_state, __trace_sched_switch_state(preempt,
> prev_state, prev))
> +		ctf_enum(task_state, long, prev_state, __trace_sched_switch_state(preempt,
> prev, next, prev_state))
> #else
> -		ctf_integer(long, prev_state, __trace_sched_switch_state(preempt, prev_state,
> prev))
> +		ctf_integer(long, prev_state, __trace_sched_switch_state(preempt, prev, next,
> prev_state))
> #endif
> 		ctf_array_text(char, next_comm, next->comm, TASK_COMM_LEN)
> 		ctf_integer(pid_t, next_tid, next->pid)
> --
> 2.19.1
> 
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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