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

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Message-ID: <4894db95-817e-ed88-c374-5f4dc118e5b7@hisilicon.com>
Date: Tue, 9 Jan 2024 09:47:34 +0800
From: Junxian Huang <huangjunxian6@...ilicon.com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 2024/1/5 22:18, Jason Gunthorpe wrote:
> 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

Thanks for the suggestion! We'll consider a new implementation.
For this patch, I'll drop the kmem modification and re-send a v2.

Junxian