Re: [PATCH v2] sched, mm: Optimize current_gfp_context()

From: Xu, Yanfei
Date: Sat Sep 19 2020 - 10:55:43 EST

On 9/18/20 11:18 PM, Waiman Long wrote:
On 9/18/20 2:44 AM, Xu, Yanfei wrote:
Hi Waiman,

On 8/12/20 6:29 AM, Andrew Morton wrote:
On Thu, 18 Jun 2020 17:29:36 -0400 Waiman Long <longman@xxxxxxxxxx> 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.

I have a puzzle about this comment, what's the meaning about "That may
lead to duplicated access of the same memory location". After using
variable 'pflags', will it not duplicated access the same memory
Looking forward to your reply :)

That condition usually won't happen on a non-debug kernel. However, if certain debugging capability is turned on, access to current will be compiled into a bunch of checks and memory accesses. So if current is used multiple times, the same set of codes will be duplicated the same number of times slowing down the operation and increasing code size. By accessing current once, we avoid this overhead in a debug kernel.


Hah, got it.
Thanks for your detailed explain!