Re: [PATCH] locking/local_lock: Reduce local_[un]lock_nested_bh() overhead

[Eric Dumazet]

On Mon, Mar 9, 2026 at 3:18 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Mon, Mar 9, 2026 at 3:03 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Mon, Mar 9, 2026 at 2:44 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >
> > > On Mon, Mar 09, 2026 at 12:20:55PM +0000, Eric Dumazet wrote:
> > >
> > > > diff --git a/include/linux/local_lock.h b/include/linux/local_lock.h
> > > > index b8830148a8591c17c22e36470fbc13ff5c354955..40c2da54a0b720265be7b6327e0922a49befd8fc 100644
> > > > --- a/include/linux/local_lock.h
> > > > +++ b/include/linux/local_lock.h
> > > > @@ -94,12 +94,19 @@ DEFINE_LOCK_GUARD_1(local_lock_irqsave, local_lock_t __percpu,
> > > >                   local_unlock_irqrestore(_T->lock, _T->flags),
> > > >                   unsigned long flags)
> > > >
> > > > +#if defined(WARN_CONTEXT_ANALYSIS) || defined(CONFIG_PREEMPT_RT) || \
> > > > +    defined(CONFIG_DEBUG_LOCK_ALLOC)
> > > >  #define local_lock_nested_bh(_lock)                          \
> > > >       __local_lock_nested_bh(__this_cpu_local_lock(_lock))
> > > >
> > > >  #define local_unlock_nested_bh(_lock)                                \
> > > >       __local_unlock_nested_bh(__this_cpu_local_lock(_lock))
> > > >
> > > > +#else
> > > > +static inline void local_lock_nested_bh(local_lock_t *_lock) {}
> > > > +static inline void local_unlock_nested_bh(local_lock_t *__lock) {}
> > > > +#endif
> > >
> > > This isn't going to work; WARN_CONTEXT_ANALYSIS is unconditional on
> > > clang >= 22.1
> > >
> > > How come that this isn't DCEd properly?
> >
> > It might be partially done.
> >
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index 0e217041958a83d2a3c18de2965808442546c49b..50455951dc38668b0cbbcccdb2c5ce726e3c4da9
> > 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -7498,3 +7498,12 @@ struct vlan_type_depth
> > __vlan_get_protocol_offset(const struct sk_buff *skb,
> >         };
> >  }
> >  EXPORT_SYMBOL(__vlan_get_protocol_offset);
> > +
> > +void ericeric(void);
> > +void ericeric(void)
> > +{
> > +       local_lock_nested_bh(&napi_alloc_cache.bh_lock);
> > +       local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
> > +       local_lock_nested_bh(&napi_alloc_cache.bh_lock);
> > +       local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
> > +}
> >
> > objdump --disassemble=ericeric -r net/core/skbuff.o
> >
> > net/core/skbuff.o:     file format elf64-x86-64
> >
> >
> > Disassembly of section .text:
> >
> > 000000000000fe40 <ericeric>:
> >     fe40: f3 0f 1e fa          endbr64
> >     fe44: e8 00 00 00 00        call   fe49 <ericeric+0x9>
> > fe45: R_X86_64_PLT32 __fentry__-0x4
> >     fe49: 65 48 8b 05 00 00 00 mov    %gs:0x0(%rip),%rax        # fe51
> > <ericeric+0x11>
> >     fe50: 00
> > fe4d: R_X86_64_PC32 this_cpu_off-0x4
> >     fe51: 2e e9 00 00 00 00    cs jmp fe57 <ericeric+0x17>
> > fe53: R_X86_64_PLT32 __x86_return_thunk-0x4
> >
> > Disassembly of section .init.text:
>
> Same for
>
> +
> +void ericeric(void);
> +void ericeric(void)
> +{
> +       raw_cpu_read_long(this_cpu_off);
> +       raw_cpu_read_long(this_cpu_off);
> +}
>
> I am guessing __raw_cpu_read() is forcing the asm ?

Might be a clang issue. Oh well.

clang --version
Debian clang version 19.1.7 (10.1+build1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin

Documentation/process/changes.rst mentions the minimum supported version is 15.0