linux-kernel - Re: [PATCH] net/mlx5: Added cond

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Message-ID: <ea1c88ea-7583-4cfe-b0ef-a224806c96b1@intel.com>
Date: Tue, 20 Aug 2024 12:09:37 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Mohamed Khalfella <mkhalfella@...estorage.com>
CC: Yuanyuan Zhong <yzhong@...estorage.com>, Saeed Mahameed
	<saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>, Tariq Toukan
	<tariqt@...dia.com>, "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Shay Drori <shayd@...dia.com>, Moshe Shemesh
	<moshe@...dia.com>, <netdev@...r.kernel.org>, <linux-rdma@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net/mlx5: Added cond_resched() to crdump collection

On 8/19/24 23:42, Mohamed Khalfella wrote:
> Collecting crdump involves dumping vsc registers from pci config space
> of mlx device. The code can run for long time starving other threads
> want to run on the cpu. Added conditional reschedule between register
> reads and while waiting for register value to release the cpu more
> often.
> 
> Reviewed-by: Yuanyuan Zhong <yzhong@...estorage.com>
> Signed-off-by: Mohamed Khalfella <mkhalfella@...estorage.com>
> ---
>   drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> 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 d0b595ba6110..377cc39643b4 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/pci_vsc.c
> @@ -191,6 +191,7 @@ static int mlx5_vsc_wait_on_flag(struct mlx5_core_dev *dev, u8 expected_val)
>   		if ((retries & 0xf) == 0)
>   			usleep_range(1000, 2000);
>   
> +		cond_resched();

the sleeping logic above (including what is out of git diff context) is
a bit weird (tight loop with a sleep after each 16 attempts, with an
upper bound of 2k attempts!)

My understanding of usleep_range() is that it puts process to sleep
(and even leads to sched() call).
So cond_resched() looks redundant here.

>   	} while (flag != expected_val);
>   
>   	return 0;
> @@ -280,6 +281,7 @@ int mlx5_vsc_gw_read_block_fast(struct mlx5_core_dev *dev, u32 *data,
>   			return read_addr;
>   
>   		read_addr = next_read_addr;
> +		cond_resched();

Would be great to see how many registers there are/how long it takes to
dump them in commit message.
My guess is that a single mlx5_vsc_gw_read_fast() call is very short and
there are many. With that cond_resched() should be rather under some
if (iterator % XXX == 0) condition.

>   	}
>   	return length;
>   }