linux-kernel - Re: [PATCH for-next 4/6] RDMA/hns: Support flexible pagesize

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Message-ID: <20240105141849.GE50608@ziepe.ca>
Date: Fri, 5 Jan 2024 10:18:49 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Junxian Huang <huangjunxian6@...ilicon.com>
Cc: Leon Romanovsky <leon@...nel.org>, linux-rdma@...r.kernel.org,
	linuxarm@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH for-next 4/6] RDMA/hns: Support flexible pagesize

On Fri, Jan 05, 2024 at 01:55:11PM +0800, Junxian Huang wrote:
> 
> 
> On 2024/1/5 4:29, Jason Gunthorpe wrote:
> > On Tue, Dec 26, 2023 at 05:16:33PM +0800, Junxian Huang wrote:
> >>
> >>
> >> On 2023/12/26 16:52, Leon Romanovsky wrote:
> >>> On Mon, Dec 25, 2023 at 03:53:28PM +0800, Junxian Huang wrote:
> >>>> From: Chengchang Tang <tangchengchang@...wei.com>
> >>>>
> >>>> In the current implementation, a fixed page size is used to
> >>>> configure the PBL, which is not flexible enough and is not
> >>>> conducive to the performance of the HW.
> >>>>
> >>>> Signed-off-by: Chengchang Tang <tangchengchang@...wei.com>
> >>>> Signed-off-by: Junxian Huang <huangjunxian6@...ilicon.com>
> >>>> ---
> >>>>  drivers/infiniband/hw/hns/hns_roce_alloc.c  |   6 -
> >>>>  drivers/infiniband/hw/hns/hns_roce_device.h |   9 ++
> >>>>  drivers/infiniband/hw/hns/hns_roce_mr.c     | 168 +++++++++++++++-----
> >>>>  3 files changed, 139 insertions(+), 44 deletions(-)
> >>>
> >>> I'm wonder if the ib_umem_find_best_pgsz() API should be used instead.
> >>> What is missing there?
> >>>
> >>> Thanks
> >>
> >> Actually this API is used for umem.
> >> For kmem, we add hns_roce_find_buf_best_pgsz() to do a similar job.
> > 
> > But why do you need to do something like this for kmem? It looked to
> > me like kmem knows its allocation size when it was allocated, how come
> > you need to iterate over all of it again?
> > 
> > Jason
> > 
> 
> kmem was split into multiple small pages for allocation to prevent allocation
> failure due to memory fragmentation.
> 
> And now we add this function to confirm whether these small pages have contiguous
> address. If so, they can be combined into one huge page for use, which is more
> likely when iommu/smmu is enabled.

That seems unncessary. The chances you get contiguous pages from
a lot of random allocations is really slim.

If you care about this optimization then you should have the allocator
explicitly request contiguous pages with high order allocations in a
manner that quickly fails and falls back to PAGE_SIZE.

Then you just use the size that the allocator was able to get, not try
to figure it out after the fact.

Jason