linux-kernel - Re: [PATCH] locking/local_lock: Make the empty local_lock

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Date:   Mon, 10 Jan 2022 08:43:53 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Waiman Long <longman@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] locking/local_lock: Make the empty local_lock_*()
 function a macro.

On 2022-01-05 22:34:31 [-0500], Waiman Long wrote:
> 
> I try out this patch and it indeed helps to reduce the object size of
> functions that use local_lock(). However, the extra code isn't an additional
> mov+add.
> 
> Using folio_add_lru() as an example,
> 
> Without the patch:
> 
> 466        local_lock(&lru_pvecs.lock);
>    0x00000000000032ee <+14>:    mov    $0x1,%edi
>    0x00000000000032f3 <+19>:    callq  0x32f8 <folio_add_lru+24>
>    0x00000000000032f8 <+24>:    callq  0x32fd <folio_add_lru+29>

The call here might be due to some debugging switches or compiler
optimisation. I have with no debug and gcc-11:
| # mm/swap.c:466:     local_lock(&lru_pvecs.lock);
|         movq    $lru_pvecs, %rbx        #, tmp135
|         movq    %rbx, %rax      # tmp135, tcp_ptr__
| #APP
| # 466 "mm/swap.c" 1
|         add %gs:this_cpu_off(%rip), %rax        # this_cpu_off, tcp_ptr__

so it is mov per-CPU variable, add per-CPU offset.

Sebastian