Re: [dpdk-dev] [PATCH 1/2] eal: add macro to mark variable mostly read only

[Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>]

On Wed, Apr 18, 2018 at 06:43:11PM +0100, Ferruh Yigit wrote:
> On 4/18/2018 4:30 PM, Pavan Nikhilesh wrote:
> > Add macro to mark a variable to be mostly read only and place it in a
> > separate section.
> >
> > Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>
> > ---
> >
> >  Group together mostly read only data to avoid cacheline bouncing, also
> >  useful for auditing purposes.
> >
> >  lib/librte_eal/common/include/rte_common.h | 5 +++++
> >  1 file changed, 5 insertions(+)
> >
> > diff --git a/lib/librte_eal/common/include/rte_common.h b/lib/librte_eal/common/include/rte_common.h
> > index 6c5bc5a76..f2ff2e9e6 100644
> > --- a/lib/librte_eal/common/include/rte_common.h
> > +++ b/lib/librte_eal/common/include/rte_common.h
> > @@ -114,6 +114,11 @@ static void __attribute__((constructor(prio), used)) func(void)
> >   */
> >  #define __rte_noinline  __attribute__((noinline))
> >
> > +/**
> > + * Mark a variable to be mostly read only and place it in a separate section.
> > + */
> > +#define __rte_read_mostly __attribute__((__section__(".read_mostly")))
>

Hi Ferruh,

> Hi Pavan,
>
> Is the section ".read_mostly" treated specially [1] or is this just for grouping
> symbols together (to reduce cacheline bouncing)?

The section .read_mostly is not treated specially it's just for grouping
symbols.

>
> [1]
> If this is special section, can you please point counter part in the kernel?

The kernel has something similar[1] but they have a custom linker script to
arrange symbols.

[1] https://github.com/torvalds/linux/blob/a27fc14219f2e3c4a46ba9177b04d9b52c875532/arch/x86/include/asm/cache.h#L11
kernel commit id 54cb27a71f51d304342c79e62fd7667f2171062b

>
>
> > +
> >  /*********** Macros for pointer arithmetic ********/
> >
> >  /**
> > --
> > 2.17.0
> >
>