Subject: Re: [RFC][Patch v1 2/3] i40e: limit msix vectors based on housekeeping CPUs

From: Marcelo Tosatti <[email-masked]>
Date: Fri, 11 Sep 2020 12:23:44 -0300

On Wed, Sep 09, 2020 at 11:08:17AM -0400, Nitesh Narayan Lal wrote:
> In a realtime environment, it is essential to isolate unwanted IRQs from
> isolated CPUs to prevent latency overheads. Creating MSIX vectors only
> based on the online CPUs could lead to a potential issue on an RT setup
> that has several isolated CPUs but a very few housekeeping CPUs. This is
> because in these kinds of setups an attempt to move the IRQs to the
> limited housekeeping CPUs from isolated CPUs might fail due to the per
> CPU vector limit. This could eventually result in latency spikes because
> of the IRQ threads that we fail to move from isolated CPUs.
> 
> This patch prevents i40e to add vectors only based on available
> housekeeping CPUs by using num_housekeeping_cpus().
> 
> Signed-off-by: Nitesh Narayan Lal <[email-masked]>
> ---
>  drivers/net/ethernet/intel/i40e/i40e_main.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index 2e433fdbf2c3..3b4cd4b3de85 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
> @@ -5,6 +5,7 @@
>  #include <linux/of_net.h>
>  #include <linux/pci.h>
>  #include <linux/bpf.h>
> +#include <linux/sched/isolation.h>
>  #include <generated/utsrelease.h>
>  
>  /* Local includes */
> @@ -11002,7 +11003,7 @@ static int i40e_init_msix(struct i40e_pf *pf)
>  	 * will use any remaining vectors to reach as close as we can to the
>  	 * number of online CPUs.
>  	 */
> -	cpus = num_online_cpus();
> +	cpus = num_housekeeping_cpus();
>  	pf->num_lan_msix = min_t(int, cpus, vectors_left / 2);
>  	vectors_left -= pf->num_lan_msix;
>  
> -- 
> 2.27.0

For patches 1 and 2:

Reviewed-by: Marcelo Tosatti <[email-masked]>