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

Andrii Nakryiko
2025-01-24T18:13:36Z

On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@kernel.org> 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@kernel.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
> 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 …