[Crash-utility] [PATCH] Speed up "kmem -[sS]" by optimizing is_page_ptr()

[Dave Anderson]

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.

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?

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
> 
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