Re: [PATCH v4 32/39] perf: Remove get_perf_callchain() 'crosstask' argument

[Andrii Nakryiko]

On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> get_perf_callchain() doesn't support cross-task unwinding, so it doesn't
> make much sense to have 'crosstask' as an argument.
>
> Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> ---
>  include/linux/perf_event.h |  2 +-
>  kernel/bpf/stackmap.c      | 12 ++++--------
>  kernel/events/callchain.c  |  6 +-----
>  kernel/events/core.c       |  9 +++++----
>  4 files changed, 11 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 4c8ff7258c6a..1563dc2cd979 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1590,7 +1590,7 @@ extern void perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct p
>  extern void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs);
>  extern struct perf_callchain_entry *
>  get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
> -                  u32 max_stack, bool crosstask, bool add_mark);
> +                  u32 max_stack, bool add_mark);
>  extern int get_callchain_buffers(int max_stack);
>  extern void put_callchain_buffers(void);
>  extern struct perf_callchain_entry *get_callchain_entry(int *rctx);
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index ec3a57a5fba1..ee9701337912 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -314,8 +314,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
>         if (max_depth > sysctl_perf_event_max_stack)
>                 max_depth = sysctl_perf_event_max_stack;
>
> -       trace = get_perf_callchain(regs, kernel, user, max_depth,
> -                                  false, false);
> +       trace = get_perf_callchain(regs, kernel, user, max_depth, false);
>
>         if (unlikely(!trace))
>                 /* couldn't fetch the stack trace */
> @@ -430,10 +429,8 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>         if (task && user && !user_mode(regs))
>                 goto err_fault;
>
> -       /* get_perf_callchain does not support crosstask user stack walking
> -        * but returns an empty stack instead of NULL.
> -        */
> -       if (crosstask && user) {
> +       /* get_perf_callchain() does not support crosstask stack walking */
> +       if (crosstask) {

crosstask stack trace is supported for kernel stack traces (see
get_callchain_entry_for_task() call), so this is breaking that case

>                 err = -EOPNOTSUPP;
>                 goto clear;
>         }
> @@ -451,8 +448,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
>         else if (kernel && task)
>                 trace = get_callchain_entry_for_task(task, max_depth);
>         else
> -               trace = get_perf_callchain(regs, kernel, user, max_depth,
> -                                          crosstask, false);
> +               trace = get_perf_callchain(regs, kernel, user, max_depth,false);

nit: missing space

>
>         if (unlikely(!trace) || trace->nr < skip) {
>                 if (may_fault)
> diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
> index 83834203e144..655fb25a725b 100644
> --- a/kernel/events/callchain.c
> +++ b/kernel/events/callchain.c

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