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SubjectRe: [PATCH 4/4] RDMA/umem: batch page unpin in __ib_mem_release()
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On 2/4/21 12:15 AM, John Hubbard wrote:
> 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/
>
Ah, yes.

>>
>> 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.
>
Will fix.

>>
>> 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.
>
/me nods

fwiw this was suggested by Jason :) as the way I had done was unnecessarily allocating a
page to unpin pages.

>> + 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.

So IIUC this can be done given how ib_umem_get allocates scatterlists (i.e. see the call
to __sg_alloc_table_from_pages()). It builds a scatterlist with a segment size in device
DMA max segment size (e.g. 64K, 2G or etc depending on what the device sets it to) and
after created each scatterlist I am iterating represents a contiguous range of PFNs with a
starting page. And if you keep pinning contiguous amounts of memory, it keeps coalescing
this to the previously allocated sgl.

> Especially with SG_CHAIN. I'm assuming that you've monitored /proc/vmstat for
> nr_foll_pin* ?
>
Yeap I did. I see no pages left unpinned.

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

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