From: "Xu, Yanfei" <yanfei.xu@windriver.com>
To: Waiman Long <longman@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Michel Lespinasse <walken@google.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] sched, mm: Optimize current_gfp_context()
Date: Sat, 19 Sep 2020 22:55:28 +0800 [thread overview]
Message-ID: <a6b1ea3c-a186-c807-a0af-19c0149f1402@windriver.com> (raw)
In-Reply-To: <c8f3c589-c4c4-f132-8759-24a44cfa73f4@redhat.com>
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@redhat.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
prev parent reply other threads:[~2020-09-19 14:55 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-18 21:29 [PATCH v2] sched, mm: Optimize current_gfp_context() Waiman Long
2020-08-11 22:29 ` Andrew Morton
2020-08-11 23:32 ` Waiman Long
2020-09-18 6:44 ` Xu, Yanfei
2020-09-18 15:18 ` Waiman Long
2020-09-19 14:55 ` Xu, Yanfei [this message]
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