Re: [PATCH 4/4] x86: Use the page tables to look up kernel addresses in backtrace

[Chuck Ebbert]

On Fri, 10 Oct 2014 16:25:17 -0700
Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:

> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> 
> On my workstation which has a lot of modules loaded:
> 
> $ lsmod | wc -l
> 80
> 
> backtrace from the NMI for perf record -g can take a quite long time.
> 
> This leads to frequent messages like:
> perf interrupt took too long (7852 > 7812), lowering kernel.perf_event_max_sample_rate to 16000
> 
> One larger part of the PMI cost is each text address check during
> the backtrace taking upto to 3us, like this:
> 
>   1)               |          print_context_stack_bp() {
>   1)               |            __kernel_text_address() {
>   1)               |              is_module_text_address() {
>   1)               |                __module_text_address() {
>   1)   1.611 us    |                  __module_address();
>   1)   1.939 us    |                }
>   1)   2.296 us    |              }
>   1)   2.659 us    |            }
>   1)               |            __kernel_text_address() {
>   1)               |              is_module_text_address() {
>   1)               |                __module_text_address() {
>   1)   0.724 us    |                  __module_address();
>   1)   1.064 us    |                }
>   1)   1.430 us    |              }
>   1)   1.798 us    |            }
>   1)               |            __kernel_text_address() {
>   1)               |              is_module_text_address() {
>   1)               |                __module_text_address() {
>   1)   0.656 us    |                  __module_address();
>   1)   1.012 us    |                }
>   1)   1.356 us    |              }
>   1)   1.761 us    |            }
> 
> So just with a reasonably sized backtrace easily 10-20us can be spent
> on just checking the frame pointer IPs.
> 
> So essentially currently the module lookup is N-MODULES*M-length of backtrace
> 
> This patch uses the NX bits in the page tables to check for
> valid kernel addresses instead. This can be done in any context
> because kernel page tables are not removed (if they were it could
> be handled by RCU like the user page tables)
> 
> The lookup here is 2-4 memory accesses bounded.
> 
> Anything with no NX bit set and is in kernel space is a valid
> kernel executable. Unlike the previous scheme this will also
> handle cases like the profiler hitting BIOS code or similar
> (e.g. the PCI BIOS on 32bit)
> 
> On systems without NX we fall back to the previous scheme.
> 
> Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/dumpstack.c | 38 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 37 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> index b74ebc7..9279549 100644
> --- a/arch/x86/kernel/dumpstack.c
> +++ b/arch/x86/kernel/dumpstack.c
> @@ -90,6 +90,42 @@ static inline int valid_stack_ptr(struct thread_info *tinfo,
>  	return p > t && p < t + THREAD_SIZE - size;
>  }
>  
> +/*
> + * Check if the address is in a executable page.
> + * This can be much faster than looking it up in the module
> + * table list when many modules are loaded.
> + *
> + * This is safe in any context because kernel page tables
> + * are never removed.
> + */
> +static bool addr_is_executable(unsigned long addr)

That name is confusing. Maybe call it x86_kernel_text_address()?

> +{
> +	pgd_t *pgd;
> +	pud_t *pud;
> +	pmd_t *pmd;
> +	pte_t *pte;
> +
> +	if (!(__supported_pte_mask & _PAGE_NX))
> +		return __kernel_text_address(addr);
> +	if (addr < __PAGE_OFFSET)
> +		return false;

Can't you check __PAGE_OFFSET first? That would speed up the non-NX
case...

> +	pgd = pgd_offset_k(addr);
> +	if (!pgd_present(*pgd))
> +		return false;
> +	pud = pud_offset(pgd, addr);
> +	if (!pud_present(*pud))
> +		return false;
> +	if (pud_large(*pud))
> +		return pte_exec(*(pte_t *)pud);
> +	pmd = pmd_offset(pud, addr);
> +	if (!pmd_present(*pmd))
> +		return false;
> +	if (pmd_large(*pmd))
> +		return pte_exec(*(pte_t *)pmd);
> +	pte = pte_offset_kernel(pmd, addr);
> +	return pte_present(*pte) && pte_exec(*pte);
> +}
> +
>  unsigned long
>  print_context_stack(struct thread_info *tinfo,
>  		unsigned long *stack, unsigned long bp,
> @@ -102,7 +138,7 @@ print_context_stack(struct thread_info *tinfo,
>  		unsigned long addr;
>  
>  		addr = *stack;
> -		if (__kernel_text_address(addr)) {
> +		if (addr_is_executable(addr)) {
>  			if ((unsigned long) stack == bp + sizeof(long)) {
>  				ops->address(data, addr, 1);
>  				frame = frame->next_frame;

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