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SubjectRe: [PATCH 4/4] RDMA/umem: batch page unpin in __ib_mem_release()
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On 2/3/21 2:00 PM, Joao Martins wrote:
> Use the newly added unpin_user_page_range_dirty_lock()
> for more quickly unpinning a consecutive range of pages
> represented as compound pages. This will also calculate
> number of pages to unpin (for the tail pages which matching
> head page) and thus batch the refcount update.
>
> Running a test program which calls mr reg/unreg on a 1G in size
> and measures cost of both operations together (in a guest using rxe)
> with THP and hugetlbfs:

In the patch subject line:

s/__ib_mem_release/__ib_umem_release/

>
> Before:
> 590 rounds in 5.003 sec: 8480.335 usec / round
> 6898 rounds in 60.001 sec: 8698.367 usec / round
>
> After:
> 2631 rounds in 5.001 sec: 1900.618 usec / round
> 31625 rounds in 60.001 sec: 1897.267 usec / round
>
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> ---
> drivers/infiniband/core/umem.c | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> index 2dde99a9ba07..ea4ebb3261d9 100644
> --- a/drivers/infiniband/core/umem.c
> +++ b/drivers/infiniband/core/umem.c
> @@ -47,17 +47,17 @@
>
> static void __ib_umem_release(struct ib_device *dev, struct ib_umem *umem, int dirty)
> {
> - struct sg_page_iter sg_iter;
> - struct page *page;
> + bool make_dirty = umem->writable && dirty;
> + struct scatterlist *sg;
> + int i;

Maybe unsigned int is better, so as to perfectly match the scatterlist.length.

>
> if (umem->nmap > 0)
> ib_dma_unmap_sg(dev, umem->sg_head.sgl, umem->sg_nents,
> DMA_BIDIRECTIONAL);
>
> - for_each_sg_page(umem->sg_head.sgl, &sg_iter, umem->sg_nents, 0) {
> - page = sg_page_iter_page(&sg_iter);
> - unpin_user_pages_dirty_lock(&page, 1, umem->writable && dirty);
> - }
> + for_each_sg(umem->sg_head.sgl, sg, umem->nmap, i)

The change from umem->sg_nents to umem->nmap looks OK, although we should get
IB people to verify that there is not some odd bug or reason to leave it as is.

> + unpin_user_page_range_dirty_lock(sg_page(sg),
> + DIV_ROUND_UP(sg->length, PAGE_SIZE), make_dirty);

Is it really OK to refer directly to sg->length? The scatterlist library goes
to some effort to avoid having callers directly access the struct member variables.

Actually, the for_each_sg() code and its behavior with sg->length and sg_page(sg)
confuses me because I'm new to it, and I don't quite understand how this works.
Especially with SG_CHAIN. I'm assuming that you've monitored /proc/vmstat for
nr_foll_pin* ?

>
> sg_free_table(&umem->sg_head);
> }
>

thanks,
--
John Hubbard
NVIDIA

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