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SubjectPer-Processor Data Page
Date
On Wed, 8 Dec 1999, Linus wrote:
> On Wed, 8 Dec 1999, Richard Gooch wrote:
> >
> > I don't see why the kernel can't map this magic page to the same
> > virtual address for each process. I assumed you'd want to do that for
> > code anyway.
>
> So what is your argument with my approach then?
>
> That's EXACTLY what I've been arguing the whole time. No MAP_INHERIT
> anywhere, just a magic mapping that is always present.
>
> Linus

I've been following a couple of threads lately on the kernel mailing list
and the discussions have revolved around system calls, and access to kernel
data from user mode. I'd like to propose the following solution for
the ia32 architecture, which kind of melds together some ideas that have been
tossing around in my head for a while.

I did read Linus 'sucks eggs through a straw' command, but I don't believe
that I agree at this time.

Note the solution as presented below is not novel and is used in various forms
by at least two other ia32 unix operating systems.

Thanks,

scott lurndal
sgi

(I'd also like to point out that one perhaps significant issue revolving around
the sysenter instruction, introduced in the PII, is that it is fixed to
ring 0 - the use of this instruction in place of int (linux) or lcall
(unixware et. al.), does make writing a virtual machine to execute linux
somewhat more difficult (where the virtual machine runs in ring 0, and
linux runs in ring 1 or 2).

-----
The Problems:

1) Obtaining a pointer to the current task_struct on ia32 architectures
currently operates by masking the stack pointer (assuming it is
correct) to obtain the base address of an 8-kilobyte area which
provides both the task structure and the kernel stack for the
process or clone thread.

Issues:

a) Kernel stack overflow (e.g. due to a module or driver ported from
another architecture which uses automatic storage rather than the
kernel memory allocator for medium size structures, or due to the use
of recursion in kernel code) can result in the wrong task structure
being used, and corresponding kernel data corruption or other
panic situations.

b) Kernel stack overflow can result in corruption of the task structure,
often resulting in a situation that cannot be recovered from by
killing only the involved task.

2) Obtaining the processor number of the currently executing processor is
either extremely expensive (asking the local APIC) or potentially
error-prone (see issues with masking the stack pointer, above).

3) Collecting performance statistics for SMP machines currently causes
unnecessary cache line contention due to the use of arrays of integers
indexed by processor number. Currently, statistics such as context
switches are collected on a per-system basis; but such statistics are
much more useful for performance evaluation if collected on a per-cpu
basis, and they could be easily collected in a per-processor data page.
Likewise, a statistic such as system-calls should be collected on a per-cpu
basis.

4) For pthreads, it is required to have a notion of thread private storage.
In liu of using a fixed register (which many risc architectures can
afford to do), using a user-readable per-processor data page at a
fixed virtual address can make it a trivial and efficient operation for
a thread to access a pointer to its private data. (User level threading
schemes, and M-N schemes can also use this mechanism, albeit with
a system call to establish a new value for the private data pointer)

5) Certain system calls such as the getpid family and gettimeofday are often
executed frequently by certain applications (for example, oracle issues
a copious number of gettimeofday calls to timestamp redo log records and
various transaction related operations), and the data could be trivally
and efficiently accessed through a user-readable per-processor data page.

6) Kernel stacks must be fixed in size and aligned on 2-page boundaries.

A Solution:

A solution to the above set of problems is to provide a fixed virtual mapping
to a pair of physical pages which are associated with each processor, and to
ensure that this mapping is used by a thread of execution when executing on
said processor.

For example, a physical page per processor could be associated with the
virtual address 0xc0000000 with read/write access for ring 0 and no access
for ring 1-3, while virtual address 0xc0001000 which has read/write access
for ring 0 and read-only access for rings 1-3 can map to an additional
physical page.
Thus, CURRENT on ia32 architectures would be a macro which simply accessed
the word at some fixed offset from 0xc0000000 to get a pointer to the current
task structure (established at __switch_to time). Likewise, to obtain the
current processor number, accessing a word at some fixed offset from
0xc0000000 is all that is required.
This proposal would separate kernel stack from kernel data structures and
allow both variable sized kernel stacks, as well as allowing kernel stacks
to grow into a guard page, if such a feature is desired.

From the standpoint of a pthreads library, when it is required to obtain the
thread private data pointer, a simple access to some fixed offset from
0xc0001000 would suffice - likewise for the current pid, as well as the
gettimeofday[1] value.


[1] gettimeofday being implemented in this fashion would require that when the
time value is updated by the kernel, it be updated on the user private page
for each CPU. An additional alternative would be to make a third
virtually mapped page, e.g. 0xc0002000 be a system-wide shared page
with user readability for such things as struct timeval which aren't
processor dependent.


How would this solution be implemented:

First, each potential thread of execution (kernel thread, process thread
and clone thread) must have its own set of page tables.

Second, the sets of page tables that describe completely shared address
spaces (e.g. for clones) must be related by some datastructure, e.g. an
additional linked list which links all related mm's together.

Third, at context switch time (in __switch_to), the two physical pages
assigned to the current processor are plugged into the page table entries
for virtual addresses 0xc0000000 and 0xc00001000 just before cr3 is
loaded.
Fourth, prior to dispatching to the new thread, some fields in the per-cpu
data pages are updated:

(code added to __switch_to)

new_task->mm->kpdaptr = mycpu_kpda_pte;
new_task->mm->updaptr = mycpu_upda_pte;

movl mm->cr3, cr3

kpda->current = new_task;
upda->threadprivate = new_task->threadprivate;
upda->pid = new_task->pid;
upda->ppid = new_task->ppid;


typedef struct _kernel_data_area {
struct task_struct *current; /* Task_struct running in this cpu */
int cpu; /* Cpu # of this cpu */
int filler[100];
unsigned long long context_switches; /* Times cpu has ctx switched */
unsigned long long system_calls; /* # syscalls on this cpu */
/* etc */
} kpda_t;
#define KPDA (*((kpda_t *)(0xc0000000)))
#define CURRENT KDPA->current


typedef struct _user_data_area {
void *threadprivate; /* Thread private data */
int filler[100];
struct timeval tv; /* Timeval for gettimeofday */
pid_t pid;
pid_t ppid;
/* etc */
} upda_t;
#define UPDA (*((upda_t *)(0xc0001000)))

----
Cons:

1) during dispatching, linux currently will not reload the page
tables (cr3) if the newly scheduled thread shares an address
space. This optimization will no longer be possible.

2) During address space updates of shared address spaces, care must
be taken to update all page tables for all threads/clones sharing
the page tables and to invalidate any extant mappings for the
involved pages.

This can be implemented in a number of ways, including bringing all
cpus to a barrier while this address space is updated (expensive),
or bringing all cpus executing components of the shared address
space to a barrier (less expensive) or invalidating the entries
in all related page tables and forcing a tlb flush for the related
tlb entries which will cause further accesses to the new pages
to fault into the kernel (least expensive, but most complicated).

However, it is useful to note that each process will get its
own page directory, but they will share most page tables (except
for the page table containing the virtual address range
0xc0000000 - 0xc0001000).
Pros:
1) Decoupling the task_struct from the kernel stack makes many
enhancements of the kernel stack architecture possible including
but not limited to:

1) Larger kernel stacks
2) Variable sized kernel stacks
3) Automatically growing kernel stacks.

2) The CPU can be determined efficiently and has no reliance on
correctness of the current task structure.

3) Thread-private data can be implemented efficiently.

4) Some system calls, especially those that return static data,
can be implemented very efficiently - not requiring a kernel
boundary crossing.

----
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