Re: [dpdk-dev] [PATCH v7 1/4] mempool: modify mempool populate() to skip objects from page boundaries

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

On Wed, Jul 17, 2019 at 04:36:37PM +0300, Andrew Rybchenko wrote:
> On 7/17/19 12:04 PM, vattunuru@marvell.com wrote:
> > From: Vamsi Attunuru <vattunuru@marvell.com>
> > 
> > Currently the phys address of a mempool object populated by the mempool
> > populate default() routine may not be contiguous with in that mbuf range.
> > 
> > Patch ensures that each object's phys address is contiguous by modifying
> > default behaviour of mempool populate() to prevent objects from being
> > across 2 pages, expect if the size of object is bigger than size of page.
> > 
> > Since the overhead after this modification will be very minimal considering
> > the hugepage sizes of 512M & 1G, default behaviour is modified except for
> > the object sizes bigger than the page size.
> > 
> > Signed-off-by: Vamsi Attunuru <vattunuru@marvell.com>
> > Signed-off-by: Kiran Kumar K <kirankumark@marvell.com>
> 
> NACK
> 
> Looking at MEMPOOL_F_NO_IOVA_CONTIG description I don't
> understand why the patch is necessary at all. So, I'd like to
> know more. Exact conditions, IOVA mode, hugepage sizes,
> mempool flags and how it is populated.
> 
> > ---
> >   lib/librte_mempool/rte_mempool.c             |  2 +-
> >   lib/librte_mempool/rte_mempool_ops_default.c | 33 ++++++++++++++++++++++++++--
> >   2 files changed, 32 insertions(+), 3 deletions(-)
> > 
> > diff --git a/lib/librte_mempool/rte_mempool.c b/lib/librte_mempool/rte_mempool.c
> > index 7260ce0..1c48325 100644
> > --- a/lib/librte_mempool/rte_mempool.c
> > +++ b/lib/librte_mempool/rte_mempool.c
> > @@ -339,7 +339,7 @@ rte_mempool_populate_iova(struct rte_mempool *mp, char *vaddr,
> >   	i = rte_mempool_ops_populate(mp, mp->size - mp->populated_size,
> >   		(char *)vaddr + off,
> >   		(iova == RTE_BAD_IOVA) ? RTE_BAD_IOVA : (iova + off),
> > -		len - off, mempool_add_elem, NULL);
> > +		len - off, mempool_add_elem, opaque);
> 
> The last argument is the callback opaque value. mempool_add_elem()
> does not use the opaque. But it is incorrect to use it for other purposes
> and require it to be memzone.
> 
> >   	/* not enough room to store one object */
> >   	if (i == 0) {
> > diff --git a/lib/librte_mempool/rte_mempool_ops_default.c b/lib/librte_mempool/rte_mempool_ops_default.c
> > index 4e2bfc8..85da264 100644
> > --- a/lib/librte_mempool/rte_mempool_ops_default.c
> > +++ b/lib/librte_mempool/rte_mempool_ops_default.c
> > @@ -45,19 +45,48 @@ rte_mempool_op_calc_mem_size_default(const struct rte_mempool *mp,
> >   	return mem_size;
> >   }
> > +/* Returns -1 if object falls on a page boundary, else returns 0 */
> > +static inline int
> > +mempool_check_obj_bounds(void *obj, uint64_t hugepage_sz, size_t elt_sz)
> > +{
> > +	uintptr_t page_end, elt_addr = (uintptr_t)obj;
> > +	uint32_t pg_shift = rte_bsf32(hugepage_sz);
> > +	uint64_t page_mask;
> > +
> > +	page_mask =  ~((1ull << pg_shift) - 1);
> > +	page_end = (elt_addr & page_mask) + hugepage_sz;
> > +
> > +	if (elt_addr + elt_sz > page_end)
> > +		return -1;
> > +
> > +	return 0;
> > +}
> > +
> >   int
> >   rte_mempool_op_populate_default(struct rte_mempool *mp, unsigned int max_objs,
> >   		void *vaddr, rte_iova_t iova, size_t len,
> >   		rte_mempool_populate_obj_cb_t *obj_cb, void *obj_cb_arg)
> >   {
> > -	size_t total_elt_sz;
> > -	size_t off;
> > +	struct rte_memzone *mz = obj_cb_arg;
> > +	size_t total_elt_sz, off;
> 
> Why two variables are combined into one here? It is unrelated change.
> 
> >   	unsigned int i;
> >   	void *obj;
> >   	total_elt_sz = mp->header_size + mp->elt_size + mp->trailer_size;
> >   	for (off = 0, i = 0; off + total_elt_sz <= len && i < max_objs; i++) {
> > +
> > +		/* Skip page boundary check if element is bigger than page */
> > +		if (mz->hugepage_sz >= total_elt_sz) {
> > +			if (mempool_check_obj_bounds((char *)vaddr + off,
> > +						    mz->hugepage_sz,
> > +						    total_elt_sz) < 0) {
> > +				i--; /* Decrement count & skip this obj */
> > +				off += total_elt_sz;
> > +				continue;
> > +			}
> > +		}
> > +
> 
> What I don't like here is that it makes one memory chunk insufficient
> to populate all objects. I.e. we calculated memory chunk size required,
> but skipped some object. May be it is not a problem, but breaks the
> logic existing in the code.

+1

We should not skip the object, but realign it to the next page. This
way it won't break the logic, because this is what is expected when
required space is calculated in rte_mempool_op_calc_mem_size_default().