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

[Olivier Matz]

Hi Jerin, Hi Vamsi,

On Fri, Oct 25, 2019 at 09:20:20AM +0000, Vamsi Krishna Attunuru wrote:
> 
> 
> > -----Original Message-----
> > From: Jerin Jacob <jerinjacobk at gmail.com>
> > Sent: Friday, October 25, 2019 1:01 AM
> > To: Olivier Matz <olivier.matz at 6wind.com>
> > Cc: Vamsi Krishna Attunuru <vattunuru at marvell.com>; Andrew Rybchenko
> > <arybchenko at solarflare.com>; Ferruh Yigit <ferruh.yigit at intel.com>;
> > thomas at monjalon.net; Jerin Jacob Kollanukkaran <jerinj at marvell.com>; Kiran
> > Kumar Kokkilagadda <kirankumark at marvell.com>; anatoly.burakov at intel.com;
> > stephen at networkplumber.org; dev at dpdk.org
> > Subject: Re: [dpdk-dev] [EXT] Re: [PATCH v11 2/4] eal: add legacy kni option
> > 
> > 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
> > https://urldefense.proofpoint.com/v2/url?u=http-
> > 3A__patches.dpdk.org_patch_56585_&d=DwIBaQ&c=nKjWec2b6R0mOyPaz7xtf
> > Q&r=WllrYaumVkxaWjgKto6E_rtDQshhIhik2jkvzFyRhW8&m=MMwAZe76YMVHe
> > 8UcHjL4IBnfX5YvtbocwICAZGBY97A&s=mfN_afnyFm65sQYzaAg_-
> > uM9o22A5j392TdBZY-bKK4&e=

The v7 has the problem I described below: the iova-contiguous allocation
may fail because the calculated size is too small, and remaining objects
will be added in another chunk. This can happen if a fully
iova-contiguous mempool is requested (it happens for octeontx).

Also, the way page size is retrieved in
rte_mempool_op_populate_default() assume that memzones are used, which
is not correct.

> > 2) v11 addressed the review comments and you have given the Acked-by for
> > mempool change https://urldefense.proofpoint.com/v2/url?u=http-
> > 3A__patches.dpdk.org_patch_61559_&d=DwIBaQ&c=nKjWec2b6R0mOyPaz7xtf
> > Q&r=WllrYaumVkxaWjgKto6E_rtDQshhIhik2jkvzFyRhW8&m=MMwAZe76YMVHe
> > 8UcHjL4IBnfX5YvtbocwICAZGBY97A&s=frFvKOHFDRhTam6jDZZc6omK2gb1RU62
> > xzAiiBMnf0I&e=

The v11 looked fine to me, because it does not impact any default
behavior, and the plan was to remove this new function when my mempool
patchset was in.


> > 
> > 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.
> 
> Hi Olivier,
> 
> Thanks for the review, I think the below mentioned problems exist with 
> current mempool_populate_default() api and will there be high chances of hitting 
> those problems when we precalculate the memory size(after preventing objs from 
> pg boundary and fit complete mempool memory in single mem chunk) and if
> mempool size goes beyond page size as below example. ?,  

Yes, the problem described below (alloc a mempool of 1.1GB resulting in
2GB reserved) exists in the current version. It will be fixed in the new
version of my "mempool: avoid objects allocations across pages"
patchset.

FYI, a reworked patchset is alsmost ready, I'll send it monday.

> 
> Regards,
> Vamsi
> 
> > >
> > > 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?

In fast path. But I don't think the performance impact would be
significative.

Vamsi, do you confirm this approach could also solve the issue without
changing the mempool?

> > 
> > > could be a B plan.
> > 
> > OK.
> > 
> > >
> > > Olivier