Re: [PATCH] rcu: record kasan stack before enter local_irq_save()/restore() critical area

[Marco Elver]

On Sat, 25 Dec 2021 at 09:39, Zhang, Qiang1 <qiang1.zhang@xxxxxxxxx> wrote:
[...]
> Hi Marco, Are the following modifications clear to you?

I understood now that the contention you're talking about is from
depot_lock, which wasn't clear before (I thought you intended to
reduce contention by shortening some other critical section).

> Subject: [PATCH] rcu: Reduce the consumption time of
>  local_irq_save()/restore() critical area

Subject: rcu, kasan: Record work creation stack trace with interrupts enabled

> In non-production KASAN kernel, a large number of call stacks are recorded,
> it takes some time to acquire the global spinlock(depot_lock) inside
> kasan_record_aux_stack_noalloc(), increased interrupts disable time,
> kasan_record_aux_stack_noalloc() doesn't care if interrupts are enabled or
> not when called, so move it outside the critical area.

I think this might be clearer:

"Recording the work creation stack trace for KASAN reports in
call_rcu() is expensive, due to unwinding the stack, but also due to
acquiring depot_lock inside stackdepot (which may be contended).
Because calling kasan_record_aux_stack_noalloc() does not require
interrupts to already be disabled, this may unnecessarily extend the
time with interrupts disabled.

Therefore, move calling kasan_record_aux_stack() before the section
with interrupts disabled."

> Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>

Acked-by: Marco Elver <elver@xxxxxxxxxx>

> ---
>  kernel/rcu/tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9b58bae0527a..36bd3f9e57b3 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3068,8 +3068,8 @@ void call_rcu(struct rcu_head *head, rcu_callback_t func)
>         }
>         head->func = func;
>         head->next = NULL;
> -       local_irq_save(flags);
>         kasan_record_aux_stack_noalloc(head);
> +       local_irq_save(flags);
>         rdp = this_cpu_ptr(&rcu_data);
>
> Thanks,
> Zqiang