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Subject: Re: [PATCH v2] sched, mm: Optimize current_gfp_context()
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     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
References: <20200618212936.9776-1-longman@redhat.com>
 <20200811152921.1d43e34ad7aff928c6c1267f@linux-foundation.org>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <3b353d6a-8430-b847-6ddd-21189c87ef6b@redhat.com>
Date:   Tue, 11 Aug 2020 19:32:52 -0400
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On 8/11/20 6:29 PM, 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.
>>
>> This is not usually a problem with minimal debug config options on as the
>> compiler can optimize away the duplicated memory accesses.  With most
>> of the debug config options on, however, that may not be the case.
>> For example, the x86-64 object size of the __need_fs_reclaim() in a
>> debug kernel that calls current_gfp_context() was 309 bytes. With this
>> patch applied, the object size is reduced to 202 bytes. This is a saving
>> of 107 bytes and will probably be slightly faster too.
>>
>> ...
>>
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -181,18 +181,20 @@ static inline bool in_vfork(struct task_struct *tsk)
>>    */
>>   static inline gfp_t current_gfp_context(gfp_t flags)
>>   {
>> -	if (unlikely(current->flags &
>> +	unsigned int pflags = READ_ONCE(current->flags);
> Why use READ_ONCE() here?

It was a change suggested by PeterZ. He said without the READ_ONCE(), 
the compiler may still choose to access current->flags multiple times 
instead of just reading it once. I think he is right and so I made the 
change.

Cheers,
Longman