Re: [dpdk-dev] [PATCH v6] eal: add cache-line demote support

[Ruifeng Wang <Ruifeng.Wang@arm.com>]

> -----Original Message-----
> From: Bruce Richardson <bruce.richardson@intel.com>
> Sent: Wednesday, October 14, 2020 12:20 AM
> To: Ruifeng Wang <Ruifeng.Wang@arm.com>
> Cc: Omkar Maslekar <omkar.maslekar@intel.com>; dev@dpdk.org;
> ciara.loftus@intel.com; drc@linux.vnet.ibm.com; jerinj@marvell.com;
> Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v6] eal: add cache-line demote support
> 
> On Tue, Oct 13, 2020 at 02:59:24AM +0000, Ruifeng Wang wrote:
> >
> > > -----Original Message-----
> > > From: Omkar Maslekar <omkar.maslekar@intel.com>
> > > Sent: Monday, October 12, 2020 6:20 PM
> > > To: dev@dpdk.org
> > > Cc: bruce.richardson@intel.com; ciara.loftus@intel.com;
> > > omkar.maslekar@intel.com; drc@linux.vnet.ibm.com;
> > > jerinj@marvell.com; Ruifeng Wang <Ruifeng.Wang@arm.com>;
> Honnappa
> > > Nagarahalli <Honnappa.Nagarahalli@arm.com>
> > > Subject: [PATCH v6] eal: add cache-line demote support
> > >
> > > rte_cldemote is similar to a prefetch hint - in reverse.
> > > cldemote(addr) enables software to hint to hardware that line is likely to
> be shared.
> > > Useful in core-to-core communications where cache-line is likely to
> > > be shared. ARM and PPC implementation is provided with NOP and can
> > > be added if any equivalent instructions could be used for
> > > implementation on those architectures.
> > >
> > > Signed-off-by: Omkar Maslekar <omkar.maslekar@intel.com>
> > > Acked-by: Bruce Richardson <bruce.richardson@intel.com>
> > >
> > > ---
> > > v6: marked rte_cldemote as experimental
> > >     added rte_cldemote call in existing app/test_prefetch.c
> > >
> > > v5: documentation updated
> > >     fixed formatting issue in release notes
> > >     added Acked-by: Bruce Richardson <bruce.richardson@intel.com>
> > > *
> > > v4: updated bold text for title and fixed margin in release notes
> > > *
> > > v3: fixed warning regarding whitespace
> > > *
> > > v2: documentation updated
> > > ---
> > > ---
> <snip>
> >
> > > +/**
> > > + * Demote a cache line to a more distant level of cache from the
> processor.
> > > + *
> > > + * CLDEMOTE hints to hardware to move (demote) a cache line from
> > > +the closest to
> > > + * the processor to a level more distant from the processor. It is
> > > +a hint and
> > > + * not guarantee. rte_cldemote is intended to move the cache line
> > > +to the more
> > > + * remote cache, where it expects sharing to be efficient and to
> > > +indicate that a
> > > + * line may be accessed by a different core in the future.
> > > + *
> > > + * @param p
> > > + *   Address to demote
> > > + */
> > > +static inline void
> > > +__rte_experimental
> >
> > 1. Experimental tag is only needed in this file. Tags at other places can be
> removed.
> 
> I'm not sure that is the case. The generic file is used when preparing the docs,
> so the experimental tag needs to go there for the docs, but when actually
> using the function in compiled code the "generic" version is unused.
> Therefore we need the experimental tag there to trigger a build warning
> about using the function if the appropriate ALLOW_EXPERIMENTAL_APIS flag
> is not set.
> 
+David in cc.

I learnt this from David's comment in thread:
http://patches.dpdk.org/patch/61573/
"We only need it in the function prototype"

Hi David,
Can you comment if my understanding of experimental tag usage is correct?

/Ruifeng
> /Bruce