Re: [PATCH v3 1/1] net/mlx5: Added cond_resched() to crdump collection

[Moshe Shemesh <moshe@nvidia.com>]

On 9/5/2024 7:02 AM, Mohamed Khalfella wrote:
> Collecting crdump involves reading vsc registers from pci config space
> of mlx device, which can take long time to complete. This might result
> in starving other threads waiting to run on the cpu.
> 
> Numbers I got from testing ConnectX-5 Ex MCX516A-CDAT in the lab:
> 
> - mlx5_vsc_gw_read_block_fast() was called with length = 1310716.
> - mlx5_vsc_gw_read_fast() reads 4 bytes at a time. It was not used to
>    read the entire 1310716 bytes. It was called 53813 times because
>    there are jumps in read_addr.
> - On average mlx5_vsc_gw_read_fast() took 35284.4ns.
> - In total mlx5_vsc_wait_on_flag() called vsc_read() 54707 times.
>    The average time for each call was 17548.3ns. In some instances
>    vsc_read() was called more than one time when the flag was not set.
>    As expected the thread released the cpu after 16 iterations in
>    mlx5_vsc_wait_on_flag().
> - Total time to read crdump was 35284.4ns * 53813 ~= 1.898s.
> 
> It was seen in the field that crdump can take more than 5 seconds to
> complete. During that time mlx5_vsc_wait_on_flag() did not release the
> cpu because it did not complete 16 iterations. It is believed that pci
> config reads were slow. Adding cond_resched() every 128 register read
> improves the situation. In the common case the, crdump takes ~1.8989s,
> the thread yields the cpu every ~4.51ms. If crdump takes ~5s, the thread
> yields the cpu every ~18.0ms.
> 
> Fixes: 8b9d8baae1de ("net/mlx5: Add Crdump support")
> Reviewed-by: Yuanyuan Zhong<yzhong@purestorage.com>
> Signed-off-by: Mohamed Khalfella<mkhalfella@purestorage.com>
> ---

Reviewed-by: Moshe Shemesh <moshe@nvidia.com>