Messages in this thread Patch in this message | | | Date | Mon, 11 Jul 2022 15:49:46 +0100 | Subject | Re: [PATCH v5 0/5] DMA mapping changes for SCSI core | From | John Garry <> |
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On 11/07/2022 11:40, Damien Le Moal wrote: > On 7/11/22 16:36, John Garry wrote: >> On 11/07/2022 00:08, Damien Le Moal wrote: >>>> Ah, I think that I misunderstood Damien's question. I thought he was >>>> asking why not keep shost max_sectors at dma_max_mapping_size() and then >>>> init each sdev request queue max hw sectors at dma_opt_mapping_size(). >>> I was suggesting the reverse:) Keep the device hard limit >>> (max_hw_sectors) to the max dma mapping and set the soft limit >>> (max_sectors) to the optimal dma mapping size. >> >> Sure, but as I mentioned below, I only see a small % of requests whose >> mapping size exceeds max_sectors but that still causes a big performance >> hit. So that is why I want to set the hard limit as the optimal dma >> mapping size. > > How can you possibly end-up with requests larger than max_sectors ? BIO > split is done using this limit, right ? Or is it that request merging is > allowed up to max_hw_sectors even if the resulting request size exceeds > max_sectors ? >
Ah, I see how I thought that I was seeing requests whose size exceeded max_sectors. Somebody must have changed a single disk in my system and this odd disk has a higher default max_sectors_kb -- 512 vs 128 for the rest.
So ignoring my nonesence that I was seeing oversize requests, as for the idea to set default max_sectors at dma_opt_mapping_size(), I see some issues: - for SAS disks I have no common point to impose this limit. Maybe in the slave configure callback, but each SAS driver has its own implementation generally - Even if we do config in slave_configure callback the max_sectors value is overwritten later in sd_revalidate_disk().
This following change could sort the issue though:
---8<---- diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c index 895b56c8f25e..bb49bea3d161 100644 --- a/drivers/scsi/sd.c +++ b/drivers/scsi/sd.c @@ -3214,6 +3214,8 @@ static int sd_revalidate_disk(struct gendisk *disk) sector_t old_capacity = sdkp->capacity; unsigned char *buffer; unsigned int dev_max, rw_max; + struct Scsi_Host *host = sdp->host; + struct device *dev = host->dma_dev;
SCSI_LOG_HLQUEUE(3, sd_printk(KERN_INFO, sdkp, "sd_revalidate_disk\n")); @@ -3296,8 +3298,13 @@ static int sd_revalidate_disk(struct gendisk *disk) (sector_t)BLK_DEF_MAX_SECTORS); }
- /* Do not exceed controller limit */ - rw_max = min(rw_max, queue_max_hw_sectors(q)); + if (dev->dma_mask) { + /* Do not exceed dma optimal limit */ + rw_max = min_t(unsigned int, rw_max, + dma_opt_mapping_size(dev) >> SECTOR_SHIFT); + } else { + rw_max = min(rw_max, queue_max_hw_sectors(q)); + }
/* * Only update max_sectors if previously unset or if the current value --->8--- Or I could go with the method in this series, which is not preferred. Let me know what you think.
Thanks, John
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