linux-kernel - Re: [PATCH] sched, mm: Optimize current_gfp

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Date:   Thu, 18 Jun 2020 18:07:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Michel Lespinasse <walken@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched, mm: Optimize current_gfp_context()

On Thu, Jun 18, 2020 at 11:58:47AM -0400, Waiman Long wrote:
> The current_gfp_context() converts a number of PF_MEMALLOC_* per-process
> flags into the corresponding GFP_* flags for memory allocation. In
> that function, current->flags is accessed 3 times. That may lead to
> duplicated access of the same memory location.
> 
> This is not usually a problem with minimal debug config options on as the
> compiler can optimize away the duplicated memory accesses.  With most
> of the debug config options on, however, that may not be the case.
> For example, the x86-64 object size of the __need_fs_reclaim() in a
> debug kernel that calls current_gfp_context() was 309 bytes. With this
> patch applied, the object size is reduced to 202 bytes. This is a saving
> of 107 bytes and will probably be slightly faster too.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  include/linux/sched/mm.h | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 480a4d1b7dd8..09e01909d192 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -181,18 +181,20 @@ static inline bool in_vfork(struct task_struct *tsk)
>   */
>  static inline gfp_t current_gfp_context(gfp_t flags)
>  {
> -	if (unlikely(current->flags &
> +	unsigned int pflags = current->flags;

Per the above, I would've expected a READ_ONCE() there, otherwise the
compiler can still re-load.