dynamically calculating NKPT [was: Re: huge ktr buffer]

[Alan Cox]

On 02/05/2013 09:45, mdf at FreeBSD.org wrote:
> On Tue, Feb 5, 2013 at 7:14 AM, Konstantin Belousov <kostikbel at gmail.com> wrote:
>> On Mon, Feb 04, 2013 at 03:05:15PM -0800, Neel Natu wrote:
>>> Hi,
>>>
>>> I have a patch to dynamically calculate NKPT for amd64 kernels. This
>>> should fix the various issues that people pointed out in the email
>>> thread.
>>>
>>> Please review and let me know if there are any objections to committing this.
>>>
>>> Also, thanks to Alan (alc@) for reviewing and providing feedback on
>>> the initial version of the patch.
>>>
>>> Patch (also available at http://people.freebsd.org/~neel/patches/nkpt_diff.txt):
>>>
>>> Index: sys/amd64/include/pmap.h
>>> ===================================================================
>>> --- sys/amd64/include/pmap.h  (revision 246277)
>>> +++ sys/amd64/include/pmap.h  (working copy)
>>> @@ -113,13 +113,7 @@
>>>       ((unsigned long)(l2) << PDRSHIFT) | \
>>>       ((unsigned long)(l1) << PAGE_SHIFT))
>>>
>>> -/* Initial number of kernel page tables. */
>>> -#ifndef NKPT
>>> -#define      NKPT            32
>>> -#endif
>>> -
>>>  #define NKPML4E              1               /* number of kernel PML4 slots */
>>> -#define NKPDPE               howmany(NKPT, NPDEPG)/* number of kernel PDP slots */
>>>
>>>  #define      NUPML4E         (NPML4EPG/2)    /* number of userland PML4 pages */
>>>  #define      NUPDPE          (NUPML4E*NPDPEPG)/* number of userland PDP pages */
>>> @@ -181,6 +175,7 @@
>>>  #define      PML4map         ((pd_entry_t *)(addr_PML4map))
>>>  #define      PML4pml4e       ((pd_entry_t *)(addr_PML4pml4e))
>>>
>>> +extern int nkpt;             /* Initial number of kernel page tables */
>>>  extern u_int64_t KPDPphys;   /* physical address of kernel level 3 */
>>>  extern u_int64_t KPML4phys;  /* physical address of kernel level 4 */
>>>
>>> Index: sys/amd64/amd64/minidump_machdep.c
>>> ===================================================================
>>> --- sys/amd64/amd64/minidump_machdep.c        (revision 246277)
>>> +++ sys/amd64/amd64/minidump_machdep.c        (working copy)
>>> @@ -232,7 +232,7 @@
>>>       /* Walk page table pages, set bits in vm_page_dump */
>>>       pmapsize = 0;
>>>       pdp = (uint64_t *)PHYS_TO_DMAP(KPDPphys);
>>> -     for (va = VM_MIN_KERNEL_ADDRESS; va < MAX(KERNBASE + NKPT * NBPDR,
>>> +     for (va = VM_MIN_KERNEL_ADDRESS; va < MAX(KERNBASE + nkpt * NBPDR,
>>>           kernel_vm_end); ) {
>>>               /*
>>>                * We always write a page, even if it is zero. Each
>>> @@ -364,7 +364,7 @@
>>>       /* Dump kernel page directory pages */
>>>       bzero(fakepd, sizeof(fakepd));
>>>       pdp = (uint64_t *)PHYS_TO_DMAP(KPDPphys);
>>> -     for (va = VM_MIN_KERNEL_ADDRESS; va < MAX(KERNBASE + NKPT * NBPDR,
>>> +     for (va = VM_MIN_KERNEL_ADDRESS; va < MAX(KERNBASE + nkpt * NBPDR,
>>>           kernel_vm_end); va += NBPDP) {
>>>               i = (va >> PDPSHIFT) & ((1ul << NPDPEPGSHIFT) - 1);
>>>
>>> Index: sys/amd64/amd64/pmap.c
>>> ===================================================================
>>> --- sys/amd64/amd64/pmap.c    (revision 246277)
>>> +++ sys/amd64/amd64/pmap.c    (working copy)
>>> @@ -202,6 +202,10 @@
>>>  vm_offset_t virtual_avail;   /* VA of first avail page (after kernel bss) */
>>>  vm_offset_t virtual_end;     /* VA of last avail page (end of kernel AS) */
>>>
>>> +int nkpt;
>>> +SYSCTL_INT(_machdep, OID_AUTO, nkpt, CTLFLAG_RD, &nkpt, 0,
>>> +    "Number of kernel page table pages allocated on bootup");
>>> +
>>>  static int ndmpdp;
>>>  static vm_paddr_t dmaplimit;
>>>  vm_offset_t kernel_vm_end = VM_MIN_KERNEL_ADDRESS;
>>> @@ -495,17 +499,42 @@
>>>
>>>  CTASSERT(powerof2(NDMPML4E));
>>>
>>> +/* number of kernel PDP slots */
>>> +#define      NKPDPE(ptpgs)           howmany((ptpgs), NPDEPG)
>>> +
>>>  static void
>>> +nkpt_init(vm_paddr_t addr)
>>> +{
>>> +     int pt_pages;
>>> +
>>> +#ifdef NKPT
>>> +     pt_pages = NKPT;
>>> +#else
>>> +     pt_pages = howmany(addr, 1 << PDRSHIFT);
>>> +     pt_pages += NKPDPE(pt_pages);
>>> +
>>> +     /*
>>> +      * Add some slop beyond the bare minimum required for bootstrapping
>>> +      * the kernel.
>>> +      *
>>> +      * This is quite important when allocating KVA for kernel modules.
>>> +      * The modules are required to be linked in the negative 2GB of
>>> +      * the address space.  If we run out of KVA in this region then
>>> +      * pmap_growkernel() will need to allocate page table pages to map
>>> +      * the entire 512GB of KVA space which is an unnecessary tax on
>>> +      * physical memory.
>>> +      */
>>> +     pt_pages += 4;          /* 8MB additional slop for kernel modules */
>> 8MB might be to low. I just checked one of my machines with fully
>> modularized kernel, it takes slightly more than 6 MB to load 50 modules.
>> I think that 16MB would be safer, but it probably needs to be scaled
>> down based on the available phys memory. amd64 kernel could be booted
>> on 128MB machine still.
> Is there no way to not map the entire 512GB?  Otherwise this patch
> could really hose some vendors.  E.g. the kernel module for the OneFS
> file system is around 8MB all by itself.

Mapping the entire 512 GB from the start would require the preallocation
of 1 GB of memory for page table pages.

> I found when we moved from FreeBSD 6 to 7 that the NKPT of 32 was
> insufficient for our system to even boot so I put it back to 240 (I
> didn't want to spend a lot of time playing).  At that time our module
> was loaded by the boot loader; now we do it during init to save some
> seconds on boot.  But we're probably not the only ones with a large
> kernel module.

This patch should make life easier for people who are loading modules
through the boot loader.  It will account for the size of these modules
in sizing NKPT (or now nkpt).