[dpdk-dev] [EXT] Re: [PATCH v11 2/4] eal: add legacy kni option

[Jerin Jacob]

On Thu, Oct 24, 2019 at 11:05 PM Olivier Matz <olivier.matz at 6wind.com> wrote:
>
> Hi,
>
> On Wed, Oct 23, 2019 at 08:32:08PM +0530, Jerin Jacob wrote:
> > On Wed, Oct 23, 2019 at 8:17 PM Olivier Matz <olivier.matz at 6wind.com> wrote:
> > >
> > > Hi,
> > >
> > > On Wed, Oct 23, 2019 at 03:42:39PM +0530, Jerin Jacob wrote:
> > > > On Tue, Oct 22, 2019 at 7:01 PM Vamsi Krishna Attunuru
> > > > <vattunuru at marvell.com> wrote:
> > > > >
> > > > > Hi Ferruh,
> > > > >
> > > > > Can you please explain the problems in using kni dedicated mbuf alloc routines while enabling kni iova=va mode. Please see the below discussion with Andrew. He wanted to know the problems in having newer APIs.
> > > >
> > > >
> > > > While waiting  for the Ferruh reply, I would like to summarise the
> > > > current status
> > > >
> > > > # In order to make KNI work with IOVA as VA, We need to make sure
> > > > mempool pool _object_ should not span across two huge pages
> > > >
> > > > # This problem can be fixed by, either of:
> > > >
> > > > a) Introduce a flag in mempool to define this constraint, so that,
> > > > when only needed, this constraint enforced and this is in line
> > > > with existing semantics of addressing such problems in mempool
> > > >
> > > > b) Instead of creating a flag, Make this behavior by default in
> > > > mempool for IOVA as VA case
> > > >
> > > > Upside:
> > > > b1) There is no need for specific mempool_create for KNI.
> > > >
> > > > Downside:
> > > > b2) Not align with existing mempool API semantics
> > > > b3) There will be a trivial amount of memory waste as we can not
> > > > allocate from the edge. Considering the normal huge
> > > > page memory size is 1G or 512MB this not a real issue.
> > > >
> > > > c) Make IOVA as PA when KNI kernel module is loaded
> > > >
> > > > Upside:
> > > > c1) Doing option (a) would call for new KNI specific mempool create
> > > > API i.e existing KNI applications need a one-line change in
> > > > application to make it work with release 19.11 or later.
> > > >
> > > > Downslide:
> > > > c2) Driver which needs RTE_PCI_DRV_NEED_IOVA_AS_VA can not work with KNI
> > > > c3) Need root privilege to run KNI as IOVA as PA need root privilege
> > > >
> > > > For the next year, we expect applications to work 19.11 without any
> > > > code change. My personal opinion to make go with option (a)
> > > > and update the release notes to document the change any it simple
> > > > one-line change.
> > > >
> > > > The selection of (a) vs (b) is between KNI and Mempool maintainers.
> > > > Could we please reach a consensus? Or can we discuss this TB meeting?
> > > >
> > > > We are going back and forth on this feature on for the last 3
> > > > releases. Now that, we solved all the technical problems, please help
> > > > us
> > > > to decide (a) vs (b) to make forward progress.
> > >
> > > Thank you for the summary.
> > > What is not clear to me is if (a) or (b) may break an existing
> > > application, and if yes, in which case.
> >
> > Thanks for the reply.
> >
> > To be clear we are talking about out of tree KNI tree application.
> > Which they don't want to
> > change rte_pktmbuf_pool_create() to rte_kni_pktmbuf_pool_create() and
> > build for v19.11
> >
> > So in case (b) there is no issue as It will be using rte_pktmbuf_pool_create ().
> > But in case of (a) it will create an issue if out of tree KNI
> > application is using rte_pktmbuf_pool_create() which is not using the
> > NEW flag.
>
> Following yesterday's discussion at techboard, I looked at the mempool
> code and at my previous RFC patch. It took some time to remind me what
> was my worries.

Thanks for the review Olivier.

Just to make sure the correct one is reviewed.

1) v7 had similar issue mentioned
http://patches.dpdk.org/patch/56585/

2) v11 addressed the review comments and you have given the Acked-by
for mempool change
http://patches.dpdk.org/patch/61559/

My thought process in the TB meeting was, since
rte_mempool_populate_from_pg_sz_chunks() reviwed
replace rte_pktmbuf_pool_create's  rte_mempool_populate_default() with
rte_mempool_populate_from_pg_sz_chunks()
in IOVA == VA case to avoid a new KNI mempool_create API.

>
> Currently, in rte_mempool_populate_default(), when the mempool is
> populated, we first try to allocate one iova-contiguous block of (n *
> elt_size). On success, we use this memory to fully populate the mempool
> without taking care of crossing page boundaries.
>
> If we change the behavior to prevent objects from crossing pages, the
> assumption that allocating (n * elt_size) is always enough becomes
> wrong.  By luck, there is no real impact, because if the mempool is not
> fully populated after this first iteration, it will allocate a new
> chunk.
>
> To be rigorous, we need to better calculate the amount of memory to
> allocate, according to page size.
>
> Looking at the code, I found another problem in the same area: let's say
> we populate a mempool that requires 1.1GB (and we use 1G huge pages):
>
> 1/ mempool code will first tries to allocate an iova-contiguous zone
>    of 1.1G -> fail
> 2/ it then tries to allocate a page-aligned non iova-contiguous
>    zone of 1.1G, which is 2G. On success, a lot of memory is wasted.
> 3/ on error, we try to allocate the biggest zone, it can still return
>    a zone between 1.1G and 2G, which can also waste memory.
>
> I will rework my mempool patchset to properly address these issues,
> hopefully tomorrow.

OK.


>
> Also, I thought about another idea to solve your issue, not sure it is
> better but it would not imply to change the mempool behavior. If I
> understood the problem, when a mbuf is accross 2 pages, the copy of the
> data can fail in kni because the mbuf is not virtually contiguous in the

For KNI use case, we would need _physically_ contiguous to make sure
that using, get_user_pages_remote() we get  physically contiguous memory map,
so that both KNI kernel thread and KNI kernel context and DPDK userspace can
use the same memory in different contexts.



> kernel. So why not in this case splitting the memcpy() into several,
> each of them being on a single page (and calling phys2virt() for each
> page)? The same would have to be done when accessing the fields of the
> mbuf structure if it crosses a page boundary. Would that work? This

If the above is the requirement, Does this logic need to be in slow
path or fast path?

> could be a B plan.

OK.

>
> Olivier