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

Damien Le Moal
2022-05-20T23:30:41Z

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@huawei.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@opensource.wdc.com>

--
Damien Le Moal
Western Digital Research