linux-kernel - Re: [PATCH v2] locking/rwsem: Add __always_inline annotation to __down_read

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Message-ID: <88977fec-16f9-a507-c717-709d6288084a@redhat.com>
Date:   Mon, 17 Apr 2023 10:09:04 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Minchan Kim <minchan@...nel.org>,
        Tim Murray <timmurray@...gle.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>, kernel-team@...roid.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] locking/rwsem: Add __always_inline annotation to
 __down_read_common()


On 4/17/23 07:19, Peter Zijlstra wrote:
> On Wed, Apr 12, 2023 at 03:59:05AM +0000, John Stultz wrote:
>> Apparently despite it being marked inline, the compiler
>> may not inline __down_read_common() which makes it difficult
>> to identify the cause of lock contention, as the blocked
>> function will always be listed as __down_read_common().
>>
>> So this patch adds __always_inline annotation to the
>> function to force it to be inlines so the calling function
>> will be listed.
> I'm a wee bit confused; what are you looking at? Wchan? What is stopping
> the compiler from now handing you
> __down_read{,_interruptible,_killable}() instead? Is that fine?
>
My theory is that the compiler may refuse to inline __down_read_common() 
because it is called 3 times in order to reduce overall code size. The 
other __down_read*() functions you listed are only called once.

My 2 cents.

Cheers,
Longman