Subject: Re: [PATCH v4] net/mlx5: Reclaim max 50K pages at once

From: Leon Romanovsky <[email-masked]>
Date: Mon, 24 Jun 2024 12:57:57 +0300

On Wed, Jun 19, 2024 at 06:58:27PM +0530, Anand Khoje wrote:
> In non FLR context, at times CX-5 requests release of ~8 million FW pages.
> This needs humongous number of cmd mailboxes, which to be released once
> the pages are reclaimed. Release of humongous number of cmd mailboxes is
> consuming cpu time running into many seconds. Which with non preemptible
> kernels is leading to critical process starving on that cpu’s RQ.
> To alleviate this, this change restricts the total number of pages
> a worker will try to reclaim maximum 50K pages in one go.
> The limit 50K is aligned with the current firmware capacity/limit of
> releasing 50K pages at once per MLX5_CMD_OP_MANAGE_PAGES + MLX5_PAGES_TAKE
> device command.
> 
> Our tests have shown significant benefit of this change in terms of
> time consumed by dma_pool_free().
> During a test where an event was raised by HCA
> to release 1.3 Million pages, following observations were made:
> 
> - Without this change:
> Number of mailbox messages allocated was around 20K, to accommodate
> the DMA addresses of 1.3 million pages.
> The average time spent by dma_pool_free() to free the DMA pool is between
> 16 usec to 32 usec.
>            value  ------------- Distribution ------------- count
>              256 |                                         0
>              512 |@                                        287
>             1024 |@@@                                      1332
>             2048 |@                                        656
>             4096 |@@@@@                                    2599
>             8192 |@@@@@@@@@@                               4755
>            16384 |@@@@@@@@@@@@@@@                          7545
>            32768 |@@@@@                                    2501
>            65536 |                                         0
> 
> - With this change:
> Number of mailbox messages allocated was around 800; this was to
> accommodate DMA addresses of only 50K pages.
> The average time spent by dma_pool_free() to free the DMA pool in this case
> lies between 1 usec to 2 usec.
>            value  ------------- Distribution ------------- count
>              256 |                                         0
>              512 |@@@@@@@@@@@@@@@@@@                       346
>             1024 |@@@@@@@@@@@@@@@@@@@@@@                   435
>             2048 |                                         0
>             4096 |                                         0
>             8192 |                                         1
>            16384 |                                         0
> 
> Signed-off-by: Anand Khoje <[email-masked]>
> Reviewed-by: Leon Romanovsky <[email-masked]>
> ---
> Changes in v4:
>   - Fixed a nit in patch subject.
> ---
>  drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c b/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
> index dcf58ef..06eee3a 100644
> --- a/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
> +++ b/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
> @@ -608,6 +608,7 @@ enum {
>  	RELEASE_ALL_PAGES_MASK = 0x4000,
>  };
>  
> +#define MAX_RECLAIM_NPAGES -50000
>  static int req_pages_handler(struct notifier_block *nb,
>  			     unsigned long type, void *data)
>  {
> @@ -639,9 +640,13 @@ static int req_pages_handler(struct notifier_block *nb,
>  
>  	req->dev = dev;
>  	req->func_id = func_id;
> -	req->npages = npages;
>  	req->ec_function = ec_function;
>  	req->release_all = release_all;
> +	if (npages < MAX_RECLAIM_NPAGES)
> +		req->npages = MAX_RECLAIM_NPAGES;
> +	else
> +		req->npages = npages;
> +

BTW, this can be written as:
	req->npages = max_t(s32, npages, MAX_RECLAIM_NPAGES);

Thanks

>  	INIT_WORK(&req->work, pages_work_handler);
>  	queue_work(dev->priv.pg_wq, &req->work);
>  	return NOTIFY_OK;
> -- 
> 1.8.3.1
>