Re: [PATCH v2 1/1] net/mlx5: Added cond_resched() to crdump collection

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Mohamed Khalfella <mkhalfella@purestorage.com>
Date: Fri, 30 Aug 2024 11:01:19 -0700

> On 2024-08-30 15:07:45 +0200, Alexander Lobakin wrote:
>> From: Mohamed Khalfella <mkhalfella@purestorage.com>
>> Date: Thu, 29 Aug 2024 15:38:56 -0600
>>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
>>> index 6b774e0c2766..bc6c38a68702 100644
>>> --- a/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
>>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
>>> @@ -269,6 +269,7 @@ int mlx5_vsc_gw_read_block_fast(struct mlx5_core_dev *dev, u32 *data,
>>>  {
>>>  	unsigned int next_read_addr = 0;
>>>  	unsigned int read_addr = 0;
>>> +	unsigned int count = 0;
>>>  
>>>  	while (read_addr < length) {
>>>  		if (mlx5_vsc_gw_read_fast(dev, read_addr, &next_read_addr,
>>> @@ -276,6 +277,9 @@ int mlx5_vsc_gw_read_block_fast(struct mlx5_core_dev *dev, u32 *data,
>>>  			return read_addr;
>>>  
>>>  		read_addr = next_read_addr;
>>> +		/* Yield the cpu every 128 register read */
>>> +		if ((++count & 0x7f) == 0)
>>> +			cond_resched();
>>
>> Why & 0x7f, could it be written more clearly?
>>
>> 		if (++count == 128) {
>> 			cond_resched();
>> 			count = 0;
>> 		}
>>
>> Also, I'd make this open-coded value a #define somewhere at the
>> beginning of the file with a comment with a short explanation.

This is still valid.

> 
> What you are suggesting should work also. I copied the style from
> mlx5_vsc_wait_on_flag() to keep the code consistent. The comment above
> the line should make it clear.

I just don't see a reason to make the code less readable.

> 
>>
>> BTW, why 128? Not 64, not 256 etc? You just picked it, I don't see any
>> explanation in the commitmsg or here in the code why exactly 128. Have
>> you tried different values?
> 
> This mostly subjective. For the numbers I saw in the lab, this will
> release the cpu after ~4.51ms. If crdump takes ~5s, the code should
> release the cpu after ~18.0ms. These numbers look reasonable to me.

So just mention in the commit message that you tried different values
and 128 gave you the best results.

Thanks,
Olek