Hi Phil,
> <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.
I suppose you meant x86_64 here (-m64), right?
>
> 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.
That's exactly what I am talking about above.
X86_64 (64 bit binary) - no function calls for both gcc and clang
i686 (32 bit binary) - no function calls with gcc, functions calls with clang
when explicit alignment is not specified.
As I said in my initial email, that's probably not a big deal -
from what I was told so far we don't officially support clang for IA-32
and I don't know does anyone uses it at all right now.
Though if someone thinks it is a potential problem here -
it is better to flag it at early stage.
So once again my questions to the community:
1/ Does anyone builds/uses DPDK with i686-clang?
2/ If there are anyone, can these persons try to evaluate
how big perf drop it would cause for them?
3/ Is there an option to switch to i686-gcc (supported one)?
Konstantin
> >
> > >
> > > >
> > > > > 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
