linux-kernel - Re: [PATCH v2 1/1] net/mlx5: Added cond

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Message-ID: <ZtII7_XLo2i1aZcj@ceto>
Date: Fri, 30 Aug 2024 11:01:19 -0700
From: Mohamed Khalfella <mkhalfella@...estorage.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Saeed Mahameed <saeedm@...dia.com>, Leon Romanovsky <leon@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	yzhong@...estorage.com, Tariq Toukan <tariqt@...dia.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 v2 1/1] net/mlx5: Added cond_resched() to crdump
 collection

On 2024-08-30 15:07:45 +0200, Alexander Lobakin wrote:
> From: Mohamed Khalfella <mkhalfella@...estorage.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.

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.

> 
> 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.