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

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Date:   Sat, 19 Sep 2020 22:55:28 +0800
From:   "Xu, Yanfei" <yanfei.xu@...driver.com>
To:     Waiman Long <longman@...hat.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 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@...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
> 
Hah， got it.
Thanks for your detailed explain！

cheers，
Yanfei