Re: [dpdk-dev] [PATCH v4 2/4] mempool: add non-IO flag

[Olivier Matz <olivier.matz@6wind.com>]

On Fri, Oct 15, 2021 at 09:58:49AM +0000, Dmitry Kozlyuk wrote:
> 
> 
> > -----Original Message-----
> > From: Olivier Matz <olivier.matz@6wind.com>
> > Sent: 15 октября 2021 г. 12:43
> > To: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
> > Cc: Dmitry Kozlyuk <dkozlyuk@nvidia.com>; dev@dpdk.org; Matan Azrad
> > <matan@nvidia.com>
> > Subject: Re: [PATCH v4 2/4] mempool: add non-IO flag
> > 
> > External email: Use caution opening links or attachments
> > 
> > 
> > On Fri, Oct 15, 2021 at 12:33:31PM +0300, Andrew Rybchenko wrote:
> > > On 10/15/21 12:18 PM, Dmitry Kozlyuk wrote:
> > > >> -----Original Message-----
> > > >> From: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru> [...]
> > > >>> diff --git a/lib/mempool/rte_mempool.c b/lib/mempool/rte_mempool.c
> > > >>> index 51c0ba2931..2204f140b3 100644
> > > >>> --- a/lib/mempool/rte_mempool.c
> > > >>> +++ b/lib/mempool/rte_mempool.c
> > > >>> @@ -371,6 +371,8 @@ rte_mempool_populate_iova(struct rte_mempool
> > > >>> *mp, char *vaddr,
> > > >>>
> > > >>>       STAILQ_INSERT_TAIL(&mp->mem_list, memhdr, next);
> > > >>>       mp->nb_mem_chunks++;
> > > >>> +     if (iova == RTE_BAD_IOVA)
> > > >>> +             mp->flags |= MEMPOOL_F_NON_IO;
> > > >>
> > > >> As I understand rte_mempool_populate_iova() may be called few times
> > > >> for one mempool. The flag must be set if all invocations are done
> > > >> with RTE_BAD_IOVA. So, it should be set by default and just removed
> > > >> when iova != RTE_BAD_IOVA happens.
> > > >
> > > > I don't agree at all. If any object of the pool is unsuitable for
> > > > IO, the pool cannot be considered suitable for IO. So if there's a
> > > > single invocation with RTE_BAD_IOVA, the flag must be set forever.
> > >
> > > If so, some objects may be used for IO, some cannot be used.
> > > What should happen if an application allocates an object which is
> > > suitable for IO and try to use it this way?
> > 
> > If the application can predict if the allocated object is usable for IO
> > before allocating it, I would be surprised to have it used for IO. I agree
> > with Dmitry here.
> 
> The flag hints to components, PMDs before all,
> that objects from this mempool will never be used for IO,
> so that the component can save some memory mapping or DMA configuration.
> If the flag is set when even a single object may be used for IO,
> the consumer of the flag will not be ready for that.
> Whatever a corner case it is, Andrew is correct.
> There is a subtle difference between "pool is not usable"
> (as described now) and "objects from this mempool will never be used"
> (as stated above), I'll highlight it in the flag description.

OK, agreed, thanks.