Hi Konstantin,
<snip>
> >
> > > Subject: RE: [PATCH v3 06/12] ipsec: optimize with c11 atomic for sa
> outbound
> > > sqn update
> > >
> > > Hi Phil,
> > >
> > > >
> > > > For SA outbound packets, rte_atomic64_add_return is used to
> generate
> > > > SQN atomically. This introduced an unnecessary full barrier by calling
> > > > the '__sync' builtin implemented rte_atomic_XX API on aarch64. This
> > > > patch optimized it with c11 atomic and eliminated the expensive
> > > > barrier for aarch64.
> > > >
> > > > Signed-off-by: Phil Yang <phil.y...@arm.com>
> > > > Reviewed-by: Ruifeng Wang <ruifeng.w...@arm.com>
> > > > Reviewed-by: Gavin Hu <gavin...@arm.com>
> > > > ---
> > > > lib/librte_ipsec/ipsec_sqn.h | 3 ++-
> > > > lib/librte_ipsec/sa.h | 2 +-
> > > > 2 files changed, 3 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/lib/librte_ipsec/ipsec_sqn.h
> > > > b/lib/librte_ipsec/ipsec_sqn.h index 0c2f76a..e884af7 100644
> > > > --- a/lib/librte_ipsec/ipsec_sqn.h
> > > > +++ b/lib/librte_ipsec/ipsec_sqn.h
> > > > @@ -128,7 +128,8 @@ esn_outb_update_sqn(struct rte_ipsec_sa *sa,
> > > > uint32_t *num)
> > > >
> > > > n = *num;
> > > > if (SQN_ATOMIC(sa))
> > > > - sqn = (uint64_t)rte_atomic64_add_return(&sa-
> > > >sqn.outb.atom, n);
> > > > + sqn = __atomic_add_fetch(&sa->sqn.outb.atom, n,
> > > > + __ATOMIC_RELAXED);
> > >
> > > One generic thing to note:
> > > clang for i686 in some cases will generate a proper function call for
> > > 64-bit
> > > __atomic builtins (gcc seems to always generate cmpxchng8b for such
> cases).
> > > Does anyone consider it as a potential problem?
> > > It probably not a big deal, but would like to know broader opinion.
> > I had looked at this some time back for GCC. The function call is generated
> only if the underlying platform does not support the atomic
> > instructions for the operand size. Otherwise, gcc generates the instructions
> directly.
> > I would think the behavior would be the same for clang.
>
> From what I see not really.
> As an example:
>
> $ cat tatm11.c
> #include <stdint.h>
>
> struct x {
> uint64_t v __attribute__((aligned(8)));
> };
>
> uint64_t
> ffxadd1(struct x *x, uint32_t n, uint32_t m)
> {
> return __atomic_add_fetch(&x->v, n, __ATOMIC_RELAXED);
> }
>
> uint64_t
> ffxadd11(uint64_t *v, uint32_t n, uint32_t m)
> {
> return __atomic_add_fetch(v, n, __ATOMIC_RELAXED);
> }
>
> gcc for i686 will generate code with cmpxchng8b for both cases.
> clang will generate cmpxchng8b for ffxadd1() - when data is explicitly 8B
> aligned,
> but will emit a function call for ffxadd11().
I guess your testbed is an i386 platform. However, what I see here is
different.
Testbed i686: Ubuntu 18.04.4 LTS/GCC 8.3/ Clang 9.0.0-2
Both Clang and GCC for i686 generate code with xadd for these two cases.
Testbed i386: Ubuntu 16.04 LTS (Installed libatomic)/GCC 5.4.0/ Clang 4.0.0
GCC will generate code with cmpxchng8b for both cases.
Clang generated code emits a function call for both cases.
Thanks,
Phil
>
> >
> > >
> > > > else {
> > > > sqn = sa->sqn.outb.raw + n;
> > > > sa->sqn.outb.raw = sqn;
> > > > diff --git a/lib/librte_ipsec/sa.h b/lib/librte_ipsec/sa.h index
> > > > d22451b..cab9a2e 100644
> > > > --- a/lib/librte_ipsec/sa.h
> > > > +++ b/lib/librte_ipsec/sa.h
> > > > @@ -120,7 +120,7 @@ struct rte_ipsec_sa {
> > > > */
> > > > union {
> > > > union {
> > > > - rte_atomic64_t atom;
> > > > + uint64_t atom;
> > > > uint64_t raw;
> > > > } outb;
> > >
> > > If we don't need rte_atomic64 here anymore, then I think we can
> collapse the
> > > union to just:
> > > uint64_t outb;
> > >
> > > > struct {
> > > > --
> > > > 2.7.4
