[Crash-utility] [PATCH] Speed up "kmem -[sS]" by optimizing is_page_ptr()
Dave Anderson
anderson at redhat.com
Thu Feb 15 16:45:58 UTC 2018
----- Original Message -----
> Hi Dave,
>
> Thank you for your comment!
>
> On 2/13/2018 2:33 PM, Dave Anderson wrote:
> >
> > Hi Kasuhito,
> >
> > I am planning on releasing crash-7.2.1 today, so this will have to be deferred
> > to 7.2.2.
> >
> > Because of your questions about ppc64, possible backwards-compatibility issues,
> > or potential future changes to page.flags usage, this permanent change to the
> > is_page_ptr() function solely for the purposes of SLUB's count_partial() function
> > makes me nervous.
>
> Sorry, my explanation was not enough. Please let me supplement a little.
> As far as I know, the count_partial() is the function which could receive the
> effect of this patch most. But is_page_ptr() could be called many times also
> through the other functions, so this will improve them, too. Moreover, the
> is_page_ptr()'s loop could loop NR_MEM_SECTIONS() times, which is 33554432
> on x86_64(5level) if I understand correctly, in the worst case.
>
> So I thought that ideally we should improve the is_page_ptr() itself if possible,
> rather than find the callers which could call it many times and change them.
> Also, I am willing to drop the definition of VMEMMAP_VADDR for ppc64 this
> time.
OK, I understand your point. But what concerns me is that the function's
purpose is to absolutely identify whether the incoming page structure address
is a correct page structure address. But if an invalid address gets passed
into is_page_ptr(), your patch would take the invalid address, calculate an
invalid "nr", and continue from there, right?
Dave
>
> >
> > There is really no compelling reason that count_partial() absolutely *must* use
> > is_page_ptr(), and so I'm thinking that perhaps you could come up with a less
> > heavy-handed method for simply testing whether a page.lru entry points to another
> > vmemmap'd page. Something along the lines of adding this for
> > vmemmap-enabled kernels:
> >
> > #define IN_VMEMMAP_RANGE(page) ((page >= VMEMMAP_VADDR) && (page <=
> > VMEMMAP_END))
> >
> > and then have count_partial() replace the is_page_ptr() call with another
> > slub function that does something like this for vmemmap-enabled kernels:
> >
> > (IN_VMMEMAP_RANGE(next) && accessible(next))
> >
> > Or instead of accessible(), it could read "next" as a list_head with RETURN_ON_ERROR,
> > and verify that next->prev points back to the current list_head.
> >
> > Non-vmemmap-enabled kernels could still use is_page_ptr().
> >
> > What do you think of doing something like that?
>
> Given possible compatibility issues you said, I think that the way you suggested
> might well be enough for now. I'll try a method like the above.
>
> Thanks,
> Kazuhito Hagio
>
> >
> > Dave
> >
> >
> >
> >
> > ----- Original Message -----
> >> Hi,
> >>
> >> The "kmem -[sS]" commands can take several minutes to complete with
> >> the following conditions:
> >> * The system has a lot of memory sections with CONFIG_SPARSEMEM.
> >> * The kernel uses SLUB and it has a very long partial slab list.
> >>
> >> crash> kmem -s dentry | awk '{print strftime("%T"), $0}'
> >> 10:18:34 CACHE NAME OBJSIZE ALLOCATED
> >> TOTAL
> >> SLABS SSIZE
> >> 10:19:41 ffff88017fc78a00 dentry 192 9038949
> >> 10045728
> >> 239184 8k
> >> crash> kmem -S dentry | bash -c 'cat >/dev/null ; echo $SECONDS'
> >> 334
> >>
> >> One of the causes is that is_page_ptr() in count_partial() checks if
> >> a given slub page address is a page struct by searching all memory
> >> sections linearly for the one which includes it.
> >>
> >> nr_mem_sections = NR_MEM_SECTIONS();
> >> for (nr = 0; nr < nr_mem_sections ; nr++) {
> >> if ((sec_addr = valid_section_nr(nr))) {
> >> ...
> >>
> >> With CONFIG_SPARSEMEM{_VMEMMAP}, we can calculate the memory section
> >> which includes a page struct with its page.flags, or its address and
> >> VMEMMAP_VADDR. With this patch doing so, the computation amount can be
> >> significantly reduced in that case.
> >>
> >> crash> kmem -s dentry | awk '{print strftime("%T"), $0}'
> >> 10:34:55 CACHE NAME OBJSIZE ALLOCATED
> >> TOTAL
> >> SLABS SSIZE
> >> 10:34:55 ffff88017fc78a00 dentry 192 9038949
> >> 10045728
> >> 239184 8k
> >> crash> kmem -S dentry | bash -c 'cat >/dev/null ; echo $SECONDS'
> >> 2
> >>
> >> This patch uses VMEMMAP_VADDR. It is not defined on PPC64, but it looks
> >> like PPC64 supports VMEMMAP flag and machdep->machspec->vmemmap_base is
> >> it, so this patch also defines it for PPC64. This might need some help
> >> from PPC folks.
> >>
> >> Signed-off-by: Kazuhito Hagio <k-hagio at ab.jp.nec.com>
> >> ---
> >> defs.h | 2 ++
> >> memory.c | 15 +++++++++++++++
> >> 2 files changed, 17 insertions(+)
> >>
> >> diff --git a/defs.h b/defs.h
> >> index aa17792..84e68ca 100644
> >> --- a/defs.h
> >> +++ b/defs.h
> >> @@ -3861,6 +3861,8 @@ struct efi_memory_desc_t {
> >> #define IS_VMALLOC_ADDR(X) machdep->machspec->is_vmaddr(X)
> >> #define KERNELBASE machdep->pageoffset
> >>
> >> +#define VMEMMAP_VADDR (machdep->machspec->vmemmap_base)
> >> +
> >> #define PGDIR_SHIFT (machdep->pageshift + (machdep->pageshift -3) +
> >> (machdep->pageshift - 2))
> >> #define PMD_SHIFT (machdep->pageshift + (machdep->pageshift - 3))
> >>
> >> diff --git a/memory.c b/memory.c
> >> index 0df8ecc..0696763 100644
> >> --- a/memory.c
> >> +++ b/memory.c
> >> @@ -13348,10 +13348,25 @@ is_page_ptr(ulong addr, physaddr_t *phys)
> >> ulong nr_mem_sections;
> >> ulong coded_mem_map, mem_map, end_mem_map;
> >> physaddr_t section_paddr;
> >> +#ifdef VMEMMAP_VADDR
> >> + ulong flags;
> >> +#endif
> >>
> >> if (IS_SPARSEMEM()) {
> >> nr_mem_sections = NR_MEM_SECTIONS();
> >> +#ifdef VMEMMAP_VADDR
> >> + nr = nr_mem_sections;
> >> + if (machdep->flags & VMEMMAP)
> >> + nr = pfn_to_section_nr((addr - VMEMMAP_VADDR) / SIZE(page));
> >> + else if (readmem(addr + OFFSET(page_flags), KVADDR, &flags,
> >> + sizeof(ulong), "page.flags", RETURN_ON_ERROR|QUIET))
> >> + nr = (flags >> (SIZE(page_flags)*8 - SECTIONS_SHIFT())
> >> + & ((1UL << SECTIONS_SHIFT()) - 1));
> >> +
> >> + if (nr < nr_mem_sections) {
> >> +#else
> >> for (nr = 0; nr < nr_mem_sections ; nr++) {
> >> +#endif
> >> if ((sec_addr = valid_section_nr(nr))) {
> >> coded_mem_map = section_mem_map_addr(sec_addr);
> >> mem_map = sparse_decode_mem_map(coded_mem_map,
> >> nr);
> >> --
> >> 1.8.3.1
> >>
> >> --
> >> Crash-utility mailing list
> >> Crash-utility at redhat.com
> >> https://www.redhat.com/mailman/listinfo/crash-utility
> >>
> >
> > --
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>
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