Re: [dpdk-dev] [PATCH v3] lib/timer: relax barrier for status update

["Carrillo, Erik G" <erik.g.carrillo@intel.com>]

> -----Original Message-----
> From: Phil Yang <Phil.Yang@arm.com>
> Sent: Sunday, April 26, 2020 2:36 AM
> To: thomas@monjalon.net
> Cc: Carrillo, Erik G <erik.g.carrillo@intel.com>; rsanford@akamai.com;
> dev@dpdk.org; david.marchand@redhat.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; Gavin Hu <Gavin.Hu@arm.com>; nd
> <nd@arm.com>; nd <nd@arm.com>
> Subject: RE: [dpdk-dev] [PATCH v3] lib/timer: relax barrier for status update
> 
> > -----Original Message-----
> > From: Thomas Monjalon <thomas@monjalon.net>
> > Sent: Sunday, April 26, 2020 1:18 AM
> > To: Phil Yang <Phil.Yang@arm.com>
> > Cc: erik.g.carrillo@intel.com; rsanford@akamai.com; dev@dpdk.org;
> > david.marchand@redhat.com; Honnappa Nagarahalli
> > <Honnappa.Nagarahalli@arm.com>; Gavin Hu <Gavin.Hu@arm.com>; nd
> > <nd@arm.com>
> > Subject: Re: [dpdk-dev] [PATCH v3] lib/timer: relax barrier for status
> > update
> >
> > 24/04/2020 09:24, Phil Yang:
> > > Volatile has no ordering semantics. The rte_timer structure defines
> > > timer status as a volatile variable and uses the rte_r/wmb barrier
> > > to guarantee inter-thread visibility.
> > >
> > > This patch optimized the volatile operation with c11 atomic
> > > operations and one-way barrier to save the performance penalty.
> > > According to the timer_perf_autotest benchmarking results, this
> > > patch can uplift 10%~16% timer appending performance, 3%~20% timer
> > > resetting performance and
> > 45%
> > > timer callbacks scheduling performance on aarch64 and no loss in
> > > performance for x86.
> > >
> > > Suggested-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > > Acked-by: Erik Gabriel Carrillo <erik.g.carrillo@intel.com>
> > [...]
> > > --- a/lib/librte_timer/rte_timer.h
> > > +++ b/lib/librte_timer/rte_timer.h
> > > @@ -101,7 +101,7 @@ struct rte_timer
> > > -	volatile union rte_timer_status status; /**< Status of timer. */
> > > +	union rte_timer_status status; /**< Status of timer. */
> >
> > Unfortunately, I cannot merge this patch because it breaks the ABI:
> >
> >   [C]'function void rte_timer_init(rte_timer*)' at rte_timer.c:214:1
> > has some indirect sub-type changes:
> >     parameter 1 of type 'rte_timer*' has sub-type changes:
> >       in pointed to type 'struct rte_timer' at rte_timer.h:100:1:
> >         type size hasn't changed
> >         1 data member changes (2 filtered):
> >          type of 'volatile rte_timer_status rte_timer::status' changed:
> >            entity changed from 'volatile rte_timer_status' to 'union
> > rte_timer_status' at rte_timer.h:67:1
> >            type size hasn't changed
> >
> 
> I think we can revert it to the original definition of rte_timer and keep the
> union rte_timer_status volatile-qualified.
> Because with or without this 'volatile' qualify, it generates the same code on
> aarch64 and x86.
> So it seems acceptable to ignore it to make the ABI compatible?
> 
> Thank,
> Phil

I was wondering about this also.  Is the performance improvement on aarch64 still the same in that case?

Thanks,
Erik