linux-kernel - Re: [PATCH 4/4] RDMA/umem: batch page unpin in __ib_mem

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Date:   Thu, 4 Feb 2021 12:29:44 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Doug Ledford <dledford@...hat.com>,
        Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org
Subject: Re: [PATCH 4/4] RDMA/umem: batch page unpin in __ib_mem_release()



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@...cle.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,
>