Re: [dpdk-dev] [EXT] [PATCH v3 5/6] spinlock: reimplement with atomic one-way barrier builtins

[Jerin Jacob Kollanukkaran <jerinj@marvell.com>]

On Mon, 2019-01-14 at 05:54 +0000, Honnappa Nagarahalli wrote:
> > > On Thu, 2018-12-27 at 12:13 +0800, Gavin Hu wrote:
> > > ---------------------------------------------------------------
> > > ----
> > > > ---
> > > > The __sync builtin based implementation generates full memory
> > > > barriers ('dmb ish') on Arm platforms. Using C11 atomic
> > > > builtins to
> > > > generate one way barriers.
> > > > 
> > > > Here is the assembly code of __sync_compare_and_swap builtin.
> > > > __sync_bool_compare_and_swap(dst, exp, src);
> > > >    0x000000000090f1b0 <+16>:    e0 07 40 f9 ldr x0, [sp, #8]
> > > >    0x000000000090f1b4 <+20>:    e1 0f 40 79 ldrh    w1, [sp,
> > > > #6]
> > > >    0x000000000090f1b8 <+24>:    e2 0b 40 79 ldrh    w2, [sp,
> > > > #4]
> > > >    0x000000000090f1bc <+28>:    21 3c 00 12 and w1, w1, #0xffff
> > > >    0x000000000090f1c0 <+32>:    03 7c 5f 48 ldxrh   w3, [x0]
> > > >    0x000000000090f1c4 <+36>:    7f 00 01 6b cmp w3, w1
> > > >    0x000000000090f1c8 <+40>:    61 00 00 54 b.ne    0x90f1d4
> > > > <rte_atomic16_cmpset+52>  // b.any
> > > >    0x000000000090f1cc <+44>:    02 fc 04 48 stlxrh  w4, w2,
> > > > [x0]
> > > >    0x000000000090f1d0 <+48>:    84 ff ff 35 cbnz    w4,
> > > > 0x90f1c0
> > > > <rte_atomic16_cmpset+32>
> > > >    0x000000000090f1d4 <+52>:    bf 3b 03 d5 dmb ish
> > > >    0x000000000090f1d8 <+56>:    e0 17 9f 1a cset    w0, eq  //
> > > > eq =
> > > > none
> > > > 
> > > > The benchmarking results showed 3X performance gain on Cavium
> > > > ThunderX2 and
> > > > 13% on Qualcomm Falmon and 3.7% on 4-A72 Marvell macchiatobin.
> > > > Here is the example test result on TX2:
> > > > 
> > > > *** spinlock_autotest without this patch *** Core [123] Cost
> > > > Time =
> > > > 639822 us Core [124] Cost Time = 633253 us Core [125] Cost Time
> > > > =
> > > > 646030 us Core [126] Cost Time = 643189 us Core [127] Cost Time
> > > > =
> > > > 647039 us Total Cost Time = 95433298 us
> > > > 
> > > > *** spinlock_autotest with this patch *** Core [123] Cost Time
> > > > =
> > > > 163615 us Core [124] Cost Time = 166471 us Core [125] Cost Time
> > > > =
> > > > 189044 us Core [126] Cost Time = 195745 us Core [127] Cost Time
> > > > =
> > > > 78423 us Total Cost Time = 27339656 us
> > > > 
> > > > Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> > > > Reviewed-by: Phil Yang <phil.yang@arm.com>
> > > > Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com
> > > > >
> > > > Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
> > > > Reviewed-by: Steve Capper <Steve.Capper@arm.com>
> > > > ---
> > > >  lib/librte_eal/common/include/generic/rte_spinlock.h | 18
> > > > +++++++++++++-----
> > > >  1 file changed, 13 insertions(+), 5 deletions(-)
> > > > 
> > > > diff --git
> > > > a/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > > b/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > > index c4c3fc31e..87ae7a4f1 100644
> > > > --- a/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > > +++ b/lib/librte_eal/common/include/generic/rte_spinlock.h
> > > > @@ -61,9 +61,14 @@ rte_spinlock_lock(rte_spinlock_t
> > > > *sl);  static
> > > > inline void  rte_spinlock_lock(rte_spinlock_t *sl)  {
> > > > -	while (__sync_lock_test_and_set(&sl->locked, 1))
> > > > -		while(sl->locked)
> > > > +	int exp = 0;
> > > > +
> > > > +	while (!__atomic_compare_exchange_n(&sl->locked, &exp,
> > > > 1, 0,
> > > > +				__ATOMIC_ACQUIRE,
> > > > __ATOMIC_RELAXED))
> > > {
> > > 
> > > How about remove explict exp = 0 and change to
> > > __atomic_test_and_set(flag, __ATOMIC_ACQUIRE);
> > 
> > Yes, __atomic_test_and_set means simpler code and better, but
> > __atomic_test_and_set takes the first argument as a pointer to type
> > bool or
> > char, in our case, sl->locked is of type uint32.
> > We can force it to uint8, or just pass in the 32bit pointer, only
> > one byte/bit is
> > really used in this case, is that ok?
> > 
> > "It should be only used for operands of type bool or char. For
> > other types only
> > part of the value may be set."
> > https://gcc.gnu.org/onlinedocs/gcc-6.1.0/gcc/_005f_005fatomic-
> > Builtins.html
> > 
> > From performance perspective, in our testing, the performance was
> > very close,
> > compared to __atomic.
> If performance is close, I suggest we go with the existing patch.
> Changing sl->locked to bool/char would be an ABI change and will
> affect x86 TM based implementation as well.
> Jerin, what do you think?

Looks good to me.