linux-kernel - Re: [PATCH bpf-next v8 2/3] perf: Refactor get_perf

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Message-ID: <CAEf4BzZq8g_BciT3JNXuYZpzq9zphaVsvOTuOVnqjDS=9w73Gw@mail.gmail.com>
Date: Tue, 27 Jan 2026 13:07:58 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Tao Chen <chen.dylane@...ux.dev>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org, 
	namhyung@...nel.org, mark.rutland@....com, alexander.shishkin@...ux.intel.com, 
	jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com, 
	kan.liang@...ux.intel.com, song@...nel.org, ast@...nel.org, 
	daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev, 
	eddyz87@...il.com, yonghong.song@...ux.dev, john.fastabend@...il.com, 
	kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v8 2/3] perf: Refactor get_perf_callchain

On Sun, Jan 25, 2026 at 11:45 PM Tao Chen <chen.dylane@...ux.dev> wrote:
>
> From BPF stack map, we want to ensure that the callchain buffer
> will not be overwritten by other preemptive tasks and we also aim
> to reduce the preempt disable interval, Based on the suggestions from Peter
> and Andrrii, export new API __get_perf_callchain and the usage scenarios
> are as follows from BPF side:
>
> preempt_disable()
> entry = get_callchain_entry()
> preempt_enable()
> __get_perf_callchain(entry)
> put_callchain_entry(entry)
>
> Suggested-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Tao Chen <chen.dylane@...ux.dev>
> ---
>  include/linux/perf_event.h |  5 +++++
>  kernel/events/callchain.c  | 34 ++++++++++++++++++++++------------
>  2 files changed, 27 insertions(+), 12 deletions(-)

[...]

> +struct perf_callchain_entry *
> +get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
> +                  u32 max_stack, bool crosstask, bool add_mark, u64 defer_cookie)
> +{
> +       struct perf_callchain_entry *entry;
> +       int ret;
> +
> +       entry = get_callchain_entry();
> +       if (!entry)
> +               return NULL;
>
> -exit_put:
> +       ret = __get_perf_callchain(entry, regs, kernel, user, max_stack, crosstask, add_mark,
> +                                  defer_cookie);
>         put_callchain_entry(entry);
> +       if (ret)
> +               entry = NULL;
>

purely stylistical nit, so this can be ignored if you disagree, but I
find code that modifies some variable before returning it slightly
less preferable to more explicit:


if (__get_perf_callchain(...)) {
    put_callchain_entry(entry);
    return NULL;
}

return entry;

>         return entry;
>  }

> --
> 2.48.1
>