linux-kernel - Re: [PATCH 2/5] ARM: stacktrace: Avoid duplicate saving of exception PC value

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Date:   Mon, 18 Jul 2022 11:01:33 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Li Huafei <lihuafei1@...wei.com>
Cc:     linux@...linux.org.uk, rmk+kernel@...linux.org.uk, ardb@...nel.org,
        will@...nel.org, mark.rutland@....com, broonie@...nel.org,
        peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        namhyung@...nel.org, arnd@...db.de, rostedt@...dmis.org,
        nick.hawkins@....com, john@...ozen.org, mhiramat@...nel.org,
        ast@...nel.org, linyujun809@...wei.com, ndesaulniers@...gle.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 2/5] ARM: stacktrace: Avoid duplicate saving of exception
 PC value

On Tue, Jul 12, 2022 at 4:18 AM Li Huafei <lihuafei1@...wei.com> wrote:

> Because an exception stack frame is not created in the exception entry,
> save_trace() does special handling for the exception PC, but this is
> only needed when CONFIG_FRAME_POINTER_UNWIND=y. When
> CONFIG_ARM_UNWIND=y, unwind annotations have been added to the exception
> entry and save_trace() will repeatedly save the exception PC:
>
>     [0x7f000090] hrtimer_hander+0x8/0x10 [hrtimer]
>     [0x8019ec50] __hrtimer_run_queues+0x18c/0x394
>     [0x8019f760] hrtimer_run_queues+0xbc/0xd0
>     [0x8019def0] update_process_times+0x34/0x80
>     [0x801ad2a4] tick_periodic+0x48/0xd0
>     [0x801ad3dc] tick_handle_periodic+0x1c/0x7c
>     [0x8010f2e0] twd_handler+0x30/0x40
>     [0x80177620] handle_percpu_devid_irq+0xa0/0x23c
>     [0x801718d0] generic_handle_domain_irq+0x24/0x34
>     [0x80502d28] gic_handle_irq+0x74/0x88
>     [0x8085817c] generic_handle_arch_irq+0x58/0x78
>     [0x80100ba8] __irq_svc+0x88/0xc8
>     [0x80108114] arch_cpu_idle+0x38/0x3c
>     [0x80108114] arch_cpu_idle+0x38/0x3c    <==== duplicate saved exception PC
>     [0x80861bf8] default_idle_call+0x38/0x130
>     [0x8015d5cc] do_idle+0x150/0x214
>     [0x8015d978] cpu_startup_entry+0x18/0x1c
>     [0x808589c0] rest_init+0xd8/0xdc
>     [0x80c00a44] arch_post_acpi_subsys_init+0x0/0x8
>
> We can move the special handling of the exception PC in save_trace() to
> the unwind_frame() of the frame pointer unwinder.
>
> Signed-off-by: Li Huafei <lihuafei1@...wei.com>

This is another very nice patch!
Reviewed-by: Linus Waleij <linus.walleij@...aro.org>

Nitpick:

> +               if ((unsigned long)&regs[1] > ALIGN(frame->sp, THREAD_SIZE))
> +                       return -EINVAL;

It'd be nice to add a comment saying what is in regs[1] at this point
so it is easier to understand the code. Not your fault as it is just
moved code, but if you have time please add a small comment.

Yours,
Linus Walleij