[dpdk-dev] [PATCH 2/2] eal/x86: Use lock-prefixed instructions to reduce cost of rte_smp_mb()

[Stephen Hemminger]

On Fri, 1 Dec 2017 23:08:39 +0000
"Ananyev, Konstantin" <konstantin.ananyev at intel.com> wrote:

> Hi Stephen,
> 
> > -----Original Message-----
> > From: Stephen Hemminger [mailto:stephen at networkplumber.org]
> > Sent: Friday, December 1, 2017 6:04 PM
> > To: Ananyev, Konstantin <konstantin.ananyev at intel.com>
> > Cc: dev at dpdk.org
> > Subject: Re: [dpdk-dev] [PATCH 2/2] eal/x86: Use lock-prefixed instructions to reduce cost of rte_smp_mb()
> > 
> > On Fri,  1 Dec 2017 11:12:51 +0000
> > Konstantin Ananyev <konstantin.ananyev at intel.com> wrote:
> >   
> > > On x86 it  is possible to use lock-prefixed instructions to get
> > > the similar effect as mfence.
> > > As pointed by Java guys, on most modern HW that gives a better
> > > performance than using mfence:
> > > https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
> > > That patch adopts that technique for rte_smp_mb() implementation.
> > > On BDW 2.2 mb_autotest on single lcore reports 2X cycle reduction,
> > > i.e. from ~110 to ~55 cycles per operation.
> > >
> > > Signed-off-by: Konstantin Ananyev <konstantin.ananyev at intel.com>
> > > ---
> > >  .../common/include/arch/x86/rte_atomic.h           | 45 +++++++++++++++++++++-
> > >  1 file changed, 43 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/lib/librte_eal/common/include/arch/x86/rte_atomic.h b/lib/librte_eal/common/include/arch/x86/rte_atomic.h
> > > index 4eac66631..07b7fa7f7 100644
> > > --- a/lib/librte_eal/common/include/arch/x86/rte_atomic.h
> > > +++ b/lib/librte_eal/common/include/arch/x86/rte_atomic.h
> > > @@ -55,12 +55,53 @@ extern "C" {
> > >
> > >  #define	rte_rmb() _mm_lfence()
> > >
> > > -#define rte_smp_mb() rte_mb()
> > > -
> > >  #define rte_smp_wmb() rte_compiler_barrier()
> > >
> > >  #define rte_smp_rmb() rte_compiler_barrier()
> > >
> > > +/*
> > > + * From Intel Software Development Manual; Vol 3;
> > > + * 8.2.2 Memory Ordering in P6 and More Recent Processor Families:
> > > + * ...
> > > + * . Reads are not reordered with other reads.
> > > + * . Writes are not reordered with older reads.
> > > + * . Writes to memory are not reordered with other writes,
> > > + *   with the following exceptions:
> > > + *   . streaming stores (writes) executed with the non-temporal move
> > > + *     instructions (MOVNTI, MOVNTQ, MOVNTDQ, MOVNTPS, and MOVNTPD); and
> > > + *   . string operations (see Section 8.2.4.1).
> > > + *  ...
> > > + * . Reads may be reordered with older writes to different locations but not
> > > + * with older writes to the same location.
> > > + * . Reads or writes cannot be reordered with I/O instructions,
> > > + * locked instructions, or serializing instructions.
> > > + * . Reads cannot pass earlier LFENCE and MFENCE instructions.
> > > + * . Writes ... cannot pass earlier LFENCE, SFENCE, and MFENCE instructions.
> > > + * . LFENCE instructions cannot pass earlier reads.
> > > + * . SFENCE instructions cannot pass earlier writes ...
> > > + * . MFENCE instructions cannot pass earlier reads, writes ...
> > > + *
> > > + * As pointed by Java guys, that makes possible to use lock-prefixed
> > > + * instructions to get the same effect as mfence and on most modern HW
> > > + * that gives a better perfomarnce than using mfence:
> > > + * https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
> > > + * So below we use that technique for rte_smp_mb() implementation.
> > > + */
> > > +
> > > +#ifdef RTE_ARCH_I686
> > > +#define	RTE_SP	RTE_STR(esp)
> > > +#else
> > > +#define	RTE_SP	RTE_STR(rsp)
> > > +#endif
> > > +
> > > +#define RTE_MB_DUMMY_MEMP	"-128(%%" RTE_SP ")"
> > > +
> > > +static __rte_always_inline void
> > > +rte_smp_mb(void)
> > > +{
> > > +	asm volatile("lock addl $0," RTE_MB_DUMMY_MEMP "; " ::: "memory");
> > > +}
> > > +
> > >  #define rte_io_mb() rte_mb()
> > >
> > >  #define rte_io_wmb() rte_compiler_barrier()  
> > 
> > The lock instruction is a stronger barrier than the compiler barrier
> > and has worse performance impact. Are you sure it is necessary to use it in DPDK.
> > Linux kernel has successfully used simple compiler reodering barrier for years.  
> 
> Where do you see compiler barrier?
> Right now for x86 rte_smp_mb()==rte_mb()==mfence.
> So I am replacing mfence with 'lock add'.
> As comment above says - on most modern x86 systems it is faster,
> while allow to preserve memory ordering.

There are cases like virtio/vhost where we could be using compiler_barrier.
The mfence to lock add conversion makes sense.