Re: [PATCH V4 3/3] scsi: core: avoid to pre-allocate big chunk for sg list

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Jun 04, 2019 at 12:10:00PM +0800, Ming Lei wrote:
> On Mon, Jun 03, 2019 at 08:49:10PM -0700, Guenter Roeck wrote:
> > On 6/3/19 6:00 PM, Ming Lei wrote:
> > > 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.
> > > 
> > 
> > I don't think it is that simple. I do see the problem on x86 (32 and 64 bit)
> > sparc, ppc, and m68k as well, and possibly others (I didn't check all because
> > -next is in terrible shape right now). Error log is always a bit different
> > but similar.
> > 
> > On sparc:
> > 
> > scsi host0: Data transfer overflow.
> > scsi host0: cur_residue[0] tot_residue[-181604017] len[8192]
> > scsi host0: DMA length is zero!
> > scsi host0: cur adr[f000f000] len[00000000]
> > scsi host0: Data transfer overflow.
> > scsi host0: cur_residue[0] tot_residue[-181604017] len[8192]
> > scsi host0: DMA length is zero!
> > 
> > On ppc:
> > 
> > scsi host0: DMA length is zero!
> > scsi host0: cur adr[0fd21000] len[00000000]
> > scsi host0: Aborting command [(ptrval):28]
> > scsi host0: Current command [(ptrval):28]
> > scsi host0:  Active command [(ptrval):28]
> > 
> > On x86, x86_64 (after reverting a different crash-causing patch):
> > 
> > [   20.226809] scsi host0: DMA length is zero!
> > [   20.227459] scsi host0: cur adr[00000000] len[00000000]
> > [   50.588814] scsi host0: Aborting command [(____ptrval____):28]
> > [   50.589210] scsi host0: Current command [(____ptrval____):28]
> > [   50.589447] scsi host0:  Active command [(____ptrval____):28]
> > [   50.589674] scsi host0: Dumping command log
> 
> OK, I did see one boot crash issue on x86_64 with -next, so could
> you share us that patch which needs to be reverted? Meantime, please
> provide me your steps for reproducing this issue? (rootfs image, kernel
> config, qemu command)
> 
The patch to be reverted is this one. I'll prepare the rest of the
information later today.

> BTW, the patch has been tested in RH QE lab, so far not see such reports
> yet.
> 
FWIW, I don't think the RE QE lab tests any of the affected configurations.

Guenter