Date:   Tue, 15 May 2018 12:19:09 +0300
From:   Tariq Toukan <tariqt@...lanox.com>
To:     Qing Huang <qing.huang@...cle.com>, tariqt@...lanox.com,
        davem@...emloft.net, haakon.bugge@...cle.com, yanjun.zhu@...cle.com
Cc:     netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] mlx4_core: allocate ICM memory in page size chunks



On 14/05/2018 7:41 PM, Qing Huang wrote:
> 
> 
> On 5/13/2018 2:00 AM, Tariq Toukan wrote:
>>
>>
>> On 11/05/2018 10:23 PM, Qing Huang wrote:
>>> When a system is under memory presure (high usage with fragments),
>>> the original 256KB ICM chunk allocations will likely trigger kernel
>>> memory management to enter slow path doing memory compact/migration
>>> ops in order to complete high order memory allocations.
>>>
>>> When that happens, user processes calling uverb APIs may get stuck
>>> for more than 120s easily even though there are a lot of free pages
>>> in smaller chunks available in the system.
>>>
>>> Syslog:
>>> ...
>>> Dec 10 09:04:51 slcc03db02 kernel: [397078.572732] INFO: task
>>> oracle_205573_e:205573 blocked for more than 120 seconds.
>>> ...
>>>
>>> With 4KB ICM chunk size on x86_64 arch, the above issue is fixed.
>>>
>>> However in order to support smaller ICM chunk size, we need to fix
>>> another issue in large size kcalloc allocations.
>>>
>>> E.g.
>>> Setting log_num_mtt=30 requires 1G mtt entries. With the 4KB ICM chunk
>>> size, each ICM chunk can only hold 512 mtt entries (8 bytes for each mtt
>>> entry). So we need a 16MB allocation for a table->icm pointer array to
>>> hold 2M pointers which can easily cause kcalloc to fail.
>>>
>>> The solution is to use vzalloc to replace kcalloc. There is no need
>>> for contiguous memory pages for a driver meta data structure (no need
>>> of DMA ops).
>>>
>>> Signed-off-by: Qing Huang <qing.huang@...cle.com>
>>> Acked-by: Daniel Jurgens <danielj@...lanox.com>
>>> Reviewed-by: Zhu Yanjun <yanjun.zhu@...cle.com>
>>> ---
>>> v2 -> v1: adjusted chunk size to reflect different architectures.
>>>
>>>   drivers/net/ethernet/mellanox/mlx4/icm.c | 14 +++++++-------
>>>   1 file changed, 7 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/mellanox/mlx4/icm.c 
>>> b/drivers/net/ethernet/mellanox/mlx4/icm.c
>>> index a822f7a..ccb62b8 100644
>>> --- a/drivers/net/ethernet/mellanox/mlx4/icm.c
>>> +++ b/drivers/net/ethernet/mellanox/mlx4/icm.c
>>> @@ -43,12 +43,12 @@
>>>   #include "fw.h"
>>>     /*
>>> - * We allocate in as big chunks as we can, up to a maximum of 256 KB
>>> - * per chunk.
>>> + * We allocate in page size (default 4KB on many archs) chunks to 
>>> avoid high
>>> + * order memory allocations in fragmented/high usage memory situation.
>>>    */
>>>   enum {
>>> -    MLX4_ICM_ALLOC_SIZE    = 1 << 18,
>>> -    MLX4_TABLE_CHUNK_SIZE    = 1 << 18
>>> +    MLX4_ICM_ALLOC_SIZE    = 1 << PAGE_SHIFT,
>>> +    MLX4_TABLE_CHUNK_SIZE    = 1 << PAGE_SHIFT
>>
>> Which is actually PAGE_SIZE.
> 
> Yes, we wanted to avoid high order memory allocations.
> 

Then please use PAGE_SIZE instead.

>> Also, please add a comma at the end of the last entry.
> 
> Hmm..., followed the existing code style and checkpatch.pl didn't 
> complain about the comma.
> 

I am in favor of having a comma also after the last element, so that 
when another enum element is added we do not modify this line again, 
which would falsely affect git blame.

I know it didn't exist before your patch, but once we're here, let's do it.

>>
>>>   };
>>>     static void mlx4_free_icm_pages(struct mlx4_dev *dev, struct 
>>> mlx4_icm_chunk *chunk)
>>> @@ -400,7 +400,7 @@ int mlx4_init_icm_table(struct mlx4_dev *dev, 
>>> struct mlx4_icm_table *table,
>>>       obj_per_chunk = MLX4_TABLE_CHUNK_SIZE / obj_size;
>>>       num_icm = (nobj + obj_per_chunk - 1) / obj_per_chunk;
>>>   -    table->icm      = kcalloc(num_icm, sizeof(*table->icm), 
>>> GFP_KERNEL);
>>> +    table->icm      = vzalloc(num_icm * sizeof(*table->icm));
>>
>> Why not kvzalloc ?
> 
> I think table->icm pointer array doesn't really need physically 
> contiguous memory. Sometimes high order
> memory allocation by kmalloc variants may trigger slow path and cause 
> tasks to be blocked.
> 

This is control path so it is less latency-sensitive.
Let's not produce unnecessary degradation here, please call kvzalloc so 
we maintain a similar behavior when contiguous memory is available, and 
a fallback for resiliency.

> Thanks,
> Qing
> 
>>
>>>       if (!table->icm)
>>>           return -ENOMEM;
>>>       table->virt     = virt;
>>> @@ -446,7 +446,7 @@ int mlx4_init_icm_table(struct mlx4_dev *dev, 
>>> struct mlx4_icm_table *table,
>>>               mlx4_free_icm(dev, table->icm[i], use_coherent);
>>>           }
>>>   -    kfree(table->icm);
>>> +    vfree(table->icm);
>>>         return -ENOMEM;
>>>   }
>>> @@ -462,5 +462,5 @@ void mlx4_cleanup_icm_table(struct mlx4_dev *dev, 
>>> struct mlx4_icm_table *table)
>>>               mlx4_free_icm(dev, table->icm[i], table->coherent);
>>>           }
>>>   -    kfree(table->icm);
>>> +    vfree(table->icm);
>>>   }
>>>
>>
>> Thanks for your patch.
>>
>> I need to verify there is no dramatic performance degradation here.
>> You can prepare and send a v3 in the meanwhile.
>>
>> Thanks,
>> Tariq
>> -- 
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> 

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