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From:   Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To:     Waiman Long <longman@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] locking/local_lock: Make the empty local_lock_*()
 function a macro.
Message-ID: <YdvjuZATR4727gaT@linutronix.de>
References: <20220105202623.1118172-1-bigeasy@linutronix.de>
 <00c4c55b-7fa6-d29c-4a80-c196922ef527@redhat.com>
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On 2022-01-05 22:34:31 [-0500], Waiman Long wrote:
>=20
> 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 additio=
nal
> mov+add.
>=20
> Using folio_add_lru() as an example,
>=20
> Without the patch:
>=20
> 466=C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 local_lock(&lru_pvecs.lock);
> =C2=A0=C2=A0 0x00000000000032ee <+14>:=C2=A0=C2=A0=C2=A0 mov=C2=A0=C2=A0=
=C2=A0 $0x1,%edi
> =C2=A0=C2=A0 0x00000000000032f3 <+19>:=C2=A0=C2=A0=C2=A0 callq=C2=A0 0x32=
f8 <folio_add_lru+24>
> =C2=A0=C2=A0 0x00000000000032f8 <+24>:=C2=A0=C2=A0=C2=A0 callq=C2=A0 0x32=
fd <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