On Mon, Jun 03, 2019 at 01:44:22PM -0700, Guenter Roeck wrote: > On Sun, Apr 28, 2019 at 03:39:32PM +0800, Ming Lei wrote: > > Now scsi_mq_setup_tags() pre-allocates a big buffer for IO sg list, > > and the buffer size is scsi_mq_sgl_size() which depends on smaller > > value between shost->sg_tablesize and SG_CHUNK_SIZE. > > > > Modern HBA's DMA is often capable of deadling with very big segment > > number, so scsi_mq_sgl_size() is often big. Suppose the max sg number > > of SG_CHUNK_SIZE is taken, scsi_mq_sgl_size() will be 4KB. > > > > Then if one HBA has lots of queues, and each hw queue's depth is > > high, pre-allocation for sg list can consume huge memory. > > For example of lpfc, nr_hw_queues can be 70, each queue's depth > > can be 3781, so the pre-allocation for data sg list is 70*3781*2k > > =517MB for single HBA. > > > > There is Red Hat internal report that scsi_debug based tests can't > > be run any more since legacy io path is killed because too big > > pre-allocation. > > > > So switch to runtime allocation for sg list, meantime pre-allocate 2 > > inline sg entries. This way has been applied to NVMe PCI for a while, > > so it should be fine for SCSI too. Also runtime sg entries allocation > > has verified and run always in the original legacy io path. > > > > Not see performance effect in my big BS test on scsi_debug. > > > > This patch causes a variety of boot failures in -next. Typical failure > pattern is scsi hangs or failure to find a root file system. For example, > on alpha, trying to boot from usb: I guess it is because alpha doesn't support sg chaining, and CONFIG_ARCH_NO_SG_CHAIN is enabled. ARCHs not supporting sg chaining can only be arm, alpha and parisc. Please test the following patch and see if it makes a difference: diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c index 6e81258471fa..9ef632963740 100644 --- a/drivers/scsi/scsi_lib.c +++ b/drivers/scsi/scsi_lib.c @@ -44,9 +44,13 @@ * Size of integrity metadata is usually small, 1 inline sg should * cover normal cases. */ +#ifndef CONFIG_ARCH_NO_SG_CHAIN #define SCSI_INLINE_PROT_SG_CNT 1 - #define SCSI_INLINE_SG_CNT 2 +#else +#define SCSI_INLINE_PROT_SG_CNT 0 +#define SCSI_INLINE_SG_CNT 0 +#endif static struct kmem_cache *scsi_sdb_cache; static struct kmem_cache *scsi_sense_cache; Thanks, Ming
