linux-kernel - Re: [PATCH v2] sched, mm: Optimize current_gfp

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Date:   Fri, 18 Sep 2020 11:18:31 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Xu, Yanfei" <yanfei.xu@...driver.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Michel Lespinasse <walken@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched, mm: Optimize current_gfp_context()

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@...hat.com> 
>> 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