On Wed, 2011-08-31 at 13:19 -0500, Rob Herring wrote:
> On 08/31/2011 12:51 PM, Will Deacon wrote:
> > On Wed, Aug 31, 2011 at 06:46:50PM +0100, Nicolas Pitre wrote:
> >> On Wed, 31 Aug 2011, Will Deacon wrote:
> >>
> >>> On Wed, Aug 31, 2011 at 02:43:33PM +0100, Mark Salter wrote:
> >>>> On Wed, 2011-08-31 at 09:49 +0100, Will Deacon wrote:
> >>>>> On Wed, Aug 31, 2011 at 01:23:47AM +0100, Chen Peter-B29397 wrote:
> >>>>>> One question: why this write buffer issue did not happen at UP ARM V7 platform, whose dma buffer
> >>>>>> also uncache, but bufferable?
> >>>>>
> >>>>> Which CPU was on this platform?
> >>>>
> >>>> Using a 3.1.0-rc4+ kernel on a Pandaboard, and running 'hdparm -t' on a
> >>>> usb disk drive, I see ~5.8MB/s read speed. Same kernel, but passing
> >>>> nosmp on the commandline, I see 20.3MB/s.
> >>>>
> >>>> Can someone explain why nosmp would make such a difference?
> >>>
> >>> Oh gawd, that's horrible. I have a feeling it's probably a separate issue
> >>> though, caused by:
> >>>
> >>> omap_modify_auxcoreboot0(0x200, 0xfffffdff);
> >>>
> >>> in boot_secondary for OMAP. Unfortunately I have no idea what that line is
> >>> doing because it ends up talking to the secure monitor.
> >>
> >> Well, this issue is apparently affecting other ARMv9 implementations
> >> too. In which case this code in arch/arm/mm/mmu.c could be responsible:
> >>
> >> if (is_smp()) {
> >> /*
> >> * Mark memory with the "shared" attribute
> >> * for SMP systems
> >> */
> >> user_pgprot |= L_PTE_SHARED;
> >> kern_pgprot |= L_PTE_SHARED;
> >> vecs_pgprot |= L_PTE_SHARED;
> >> mem_types[MT_DEVICE_WC].prot_sect |= PMD_SECT_S;
> >> mem_types[MT_DEVICE_WC].prot_pte |= L_PTE_SHARED;
> >> mem_types[MT_DEVICE_CACHED].prot_sect |= PMD_SECT_S;
> >> mem_types[MT_DEVICE_CACHED].prot_pte |= L_PTE_SHARED;
> >> mem_types[MT_MEMORY].prot_sect |= PMD_SECT_S;
> >> mem_types[MT_MEMORY].prot_pte |= L_PTE_SHARED;
> >> mem_types[MT_MEMORY_NONCACHED].prot_sect |= PMD_SECT_S;
> >> mem_types[MT_MEMORY_NONCACHED].prot_pte |= L_PTE_SHARED;
> >> }
> >>
> >> However I don't see the nosmp kernel argument having any effect on the
> >> result from is_smp().
> >
> > Yes, the first thing that sprung to mind was the shared attribute, but like
> > you say, that doesn't seem to be affected by the nosmp command line
> > argument.
> >
> > Another thing that Marc and I tried on OMAP4 was not bringing up the secondary
> > CPU during boot (by commenting out most of smp_init). In this case, I/O
> > performance was good until we tried to online the secondary CPU. The online
> > failed but after that the I/O performance was certainly degraded.
> >
>
> Was the SCU enabled at that point? One diff between nosmp boot and
> offlining the 2nd core would be that the SCU remains enabled in the
> latter case. I think the SCU does not get enabled for nosmp.
>
> Do we really know which write buffer the data is sitting? Some
> experiments to only flush the L1 write buffer would be interesting.
> Perhaps something executed on the 2nd core has a mb which doesn't help
> for SMP because the other core's L1 write buffer is not flushed, but it
> helps for nosmp because everything runs on 1 core and any occurrence of
> a mb will flush all data out. I wouldn't expect the behavior to be so
> consistent though. Could it be something is not visible to the other
> core rather than not visible to the EHCI controller?
One experiment I did a few days ago was to pin processes and interrupts
to core#0 (except IPI and local timer). This didn't make any noticeable
difference.
My current understanding is that the writes are getting hung up in a
cache and not a write buffer. I am seeing delays of 10-15ms between
queuing the urb and getting an interrupt for urb completion. That
drops to a few hundred microseconds with the explicit flushing added
to the ehci driver. I don't see how any write buffer could hold data
that long without draining out on its own. What I see seems to suggest
that the memory is only coherent among the cores and not coherent for
CPU writes/device reads. Adding just a dsb() for the ehci flush does
not help. An outer_sync() is also necessary.
--Mark
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