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

[Xu, Yanfei]

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
> > location?
> > 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.
>
> Cheers,
> Longman
Hah， got it.
Thanks for your detailed explain！

cheers，
Yanfei