linux-kernel - Re: [PATCH 3/4] scsi: core: Cap shost max_sectors according to DMA optimum mapping limits

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Date:   Sat, 21 May 2022 08:30:30 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, jejb@...ux.ibm.com, martin.petersen@...cle.com,
        hch@....de, m.szyprowski@...sung.com, robin.murphy@....com
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-ide@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-scsi@...r.kernel.org, liyihang6@...ilicon.com,
        chenxiang66@...ilicon.com, thunder.leizhen@...wei.com
Subject: Re: [PATCH 3/4] scsi: core: Cap shost max_sectors according to DMA
 optimum mapping limits

On 5/20/22 17:23, John Garry wrote:
> Streaming DMA mappings may be considerably slower when mappings go through
> an IOMMU and the total mapping length is somewhat long. This is because the
> IOMMU IOVA code allocates and free an IOVA for each mapping, which may
> affect performance.
> 
> For performance reasons set the request_queue max_sectors from
> dma_opt_mapping_size(), which knows this mapping limit.
> 
> In addition, the shost->max_sectors is repeatedly set for each sdev in
> __scsi_init_queue(). This is unnecessary, so set once when adding the
> host.
> 
> Signed-off-by: John Garry <john.garry@...wei.com>
> ---
>  drivers/scsi/hosts.c    | 5 +++++
>  drivers/scsi/scsi_lib.c | 4 ----
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> index f69b77cbf538..a3ae6345473b 100644
> --- a/drivers/scsi/hosts.c
> +++ b/drivers/scsi/hosts.c
> @@ -225,6 +225,11 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
>  	shost->cmd_per_lun = min_t(int, shost->cmd_per_lun,
>  				   shost->can_queue);
>  
> +	if (dma_dev->dma_mask) {
> +		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
> +				dma_opt_mapping_size(dma_dev) >> SECTOR_SHIFT);
> +	}

Nit: you could drop the curly brackets here.

> +
>  	error = scsi_init_sense_cache(shost);
>  	if (error)
>  		goto fail;
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 8d18cc7e510e..2d43bb8799bd 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1884,10 +1884,6 @@ void __scsi_init_queue(struct Scsi_Host *shost, struct request_queue *q)
>  		blk_queue_max_integrity_segments(q, shost->sg_prot_tablesize);
>  	}
>  
> -	if (dev->dma_mask) {
> -		shost->max_sectors = min_t(unsigned int, shost->max_sectors,
> -				dma_max_mapping_size(dev) >> SECTOR_SHIFT);
> -	}
>  	blk_queue_max_hw_sectors(q, shost->max_sectors);
>  	blk_queue_segment_boundary(q, shost->dma_boundary);
>  	dma_set_seg_boundary(dev, shost->dma_boundary);

Reviewed-by: Damien Le Moal <damien.lemoal@...nsource.wdc.com>

-- 
Damien Le Moal
Western Digital Research