strange AMD x Intel Behaviour in 2.4.26
From: Fabiano Ramos
Date: Thu Oct 07 2004 - 13:30:35 EST
Hi All.
I wrote a syscall, similar to ptrace. Its purpose is to
count the number of user mode instructions and the number of
syscalls invoked by a program. The reason why I did not use ptrace,
is that I do not want the overhead of child -> kernel-> tracer on every
instruction, since I just want to count them.
The code is producing correct results (same as ptrace, I mean)
but is RUNNING FASTER on a 500Mhz AMD K6-2 than on a 2.6Ghz HT
Pentium 4 !!!! The monitored code runs faster on P4 if not being
monitored, as expected.
Both machines are using a stock Slackware 10 distribution, but
the kernel modification. I append the patch AND the program using the
syscall to monitor its child.
Any clues?
TIA
Fabiano
diff -ruN linux-2.4.26/Makefile linux-2.4.26-syscounter/Makefile
--- linux-2.4.26/Makefile 2004-04-14 10:05:41.000000000 -0300
+++ linux-2.4.26-syscounter/Makefile 2004-10-07 10:33:43.000000000 -0300
@@ -1,7 +1,7 @@
VERSION = 2
PATCHLEVEL = 4
SUBLEVEL = 26
-EXTRAVERSION =
+EXTRAVERSION =-syscounter
KERNELRELEASE=$(VERSION).$(PATCHLEVEL).$(SUBLEVEL)$(EXTRAVERSION)
diff -ruN linux-2.4.26/arch/i386/kernel/Makefile linux-2.4.26-
syscounter/arch/i386/kernel/Makefile
--- linux-2.4.26/arch/i386/kernel/Makefile 2003-11-28 16:26:19.000000000
-0200
+++ linux-2.4.26-syscounter/arch/i386/kernel/Makefile 2004-10-07
10:33:32.000000000 -0300
@@ -18,7 +18,7 @@
obj-y := process.o semaphore.o signal.o entry.o traps.o irq.o vm86.o \
ptrace.o i8259.o ioport.o ldt.o setup.o time.o sys_i386.o \
- pci-dma.o i386_ksyms.o i387.o bluesmoke.o dmi_scan.o
+ pci-dma.o i386_ksyms.o i387.o bluesmoke.o dmi_scan.o syscounter.o
ifdef CONFIG_PCI
diff -ruN linux-2.4.26/arch/i386/kernel/entry.S linux-2.4.26-
syscounter/arch/i386/kernel/entry.S
--- linux-2.4.26/arch/i386/kernel/entry.S 2003-06-13 11:51:29.000000000
-0300
+++ linux-2.4.26-syscounter/arch/i386/kernel/entry.S 2004-10-07
11:25:37.000000000 -0300
@@ -69,6 +69,9 @@
NT_MASK = 0x00004000
VM_MASK = 0x00020000
+PAGE_OFFSET = 0xC0000000
+PID_OFFSET = 0x7C # pid offset into task structure
+
/*
* these are offsets into the task-struct.
*/
@@ -199,9 +202,20 @@
* less clear than it otherwise should be.
*/
-ENTRY(system_call)
- pushl %eax # save orig_eax
+ENTRY(system_call)
+ pushl %eax # save orig_eax
SAVE_ALL
+ cmp $-1, tracedpid # is someone being traced?
+ je original_syscall
+ GET_CURRENT(%ebx)
+ movl %ebx, taskstruct # verifying pid
+ movl taskstruct, %ebx
+ movl PID_OFFSET(%ebx), %ebx
+ cmp %ebx, tracedpid # is this process being traced ?
+ jne original_syscall # if not, go on!
+ addl $1, syscalls # one more monitored syscall
+ adcl $0, syscalls+4
+original_syscall:
GET_CURRENT(%ebx)
testb $0x02,tsk_ptrace(%ebx) # PT_TRACESYS
jne tracesys
@@ -325,6 +339,30 @@
jmp ret_from_exception
ENTRY(debug)
+ pushl %ebx
+ # the stack layout is now
+ # eflags
+ # cs <-- 0x08 (%esp)
+ # eip <-- 0x04 (%esp)
+ # ebx <-- %esp
+ GET_CURRENT(%ebx)
+ movl %ebx, taskstruct
+ movl taskstruct, %ebx
+ movl PID_OFFSET(%ebx), %ebx
+ cmp %ebx, tracedpid # is this process being traced ?
+ jne original_debug
+ movl 0x04(%esp), %ebx # ebx = eip
+ cmp %ebx, PAGE_OFFSET # was the trap generated in
userland ? (includes libc code)
+ jl count_user
+ popl %ebx
+ iret # not calling original if monitoring this process
+count_user:
+ addl $1, userinstrexec
+ adcl $0, userinstrexec+4
+ popl %ebx
+ iret
+original_debug:
+ popl %ebx
pushl $0
pushl $ SYMBOL_NAME(do_debug)
jmp error_code
@@ -663,6 +701,7 @@
.long SYMBOL_NAME(sys_ni_syscall) /* sys_epoll_wait */
.long SYMBOL_NAME(sys_ni_syscall) /* sys_remap_file_pages */
.long SYMBOL_NAME(sys_ni_syscall) /* sys_set_tid_address */
+ .long SYMBOL_NAME(sys_counter) /* 259 */
.rept NR_syscalls-(.-sys_call_table)/4
.long SYMBOL_NAME(sys_ni_syscall)
diff -ruN linux-2.4.26/arch/i386/kernel/syscounter.c linux-2.4.26-
syscounter/arch/i386/kernel/syscounter.c
--- linux-2.4.26/arch/i386/kernel/syscounter.c 1969-12-31
21:00:00.000000000 -0300
+++ linux-2.4.26-syscounter/arch/i386/kernel/syscounter.c 2004-10-07
11:34:30.000000000 -0300
@@ -0,0 +1,114 @@
+#include <linux/syscounter.h>
+
+volatile long long userinstrexec = 0;
+volatile long long syscalls = 0;
+struct task_struct *taskstruct = NULL;
+pid_t tracedpid = -1;
+
+
+static inline int get_stack_long(struct task_struct *task, int offset)
{
+
+ unsigned char *stack;
+
+ stack = (unsigned char *)task->thread.esp0;
+ stack += offset;
+ return (*((int *)stack));
+}
+
+static inline int put_stack_long(struct task_struct *task, int offset,
unsigned long data) {
+
+ unsigned char * stack;
+
+ stack = (unsigned char *) task->thread.esp0;
+ stack += offset;
+ *(unsigned long *) stack = data;
+ return 0;
+
+}
+
+/* syscall that will be used to count instructions */
+asmlinkage int sys_counter(int pid, int action, void* retval) {
+
+ struct task_struct *tsk = NULL;
+ int ret;
+
+ lock_kernel();
+ ret = 0;
+
+ if (action == GET_USER) {
+ if (retval == NULL)
+ ret = INVALIDPOINTER;
+ else {
+ if (copy_to_user(retval, (void *) &userinstrexec,
sizeof(long long)) != 0)
+ ret = BADUSERLANDCOPY;
+ }
+ goto out;
+ }
+ else if (action == GET_SYSCALLS) {
+ if (retval == NULL)
+ ret = INVALIDPOINTER;
+ else {
+ if (copy_to_user(retval, (void *) &syscalls, sizeof(long long)) !=
0)
+ ret = BADUSERLANDCOPY;
+ }
+ goto out;
+ }
+ else if (action == STOP) {
+ tracedpid = -1;
+ goto out;
+ }
+ else if (action == RESET) {
+ tracedpid = -1;
+ userinstrexec=0;
+ syscalls = 0;
+ goto out;
+ }
+ else if (action == START) {
+
+ if (tracedpid != -1) {
+ ret = TRACEINPROGRESS;
+ }
+ else {
+ long tmp;
+
+ read_lock(&tasklist_lock);
+ tsk = find_task_by_pid(pid);
+ read_unlock(&tasklist_lock);
+
+ if (tsk) {
+
+ get_task_struct(tsk);
+
+ tracedpid = pid;
+ userinstrexec = 0;
+ syscalls = 0;
+
+ // ok, now we set up the trap bit in child's stack
+ tmp = get_stack_long(tsk, EFL_OFFSET) | TF_FLAG;
+ put_stack_long(tsk, EFL_OFFSET, tmp);
+
+ // and make it runnable
+ wake_up_process(tsk);
+
+ free_task_struct(tsk);
+
+
+ }
+ else {
+ ret = NOSUCHPROCESS;
+ }
+
+ }
+ goto out;
+
+ }
+ else {
+ ret = UNKNOWNOPERATION;
+ goto out;
+ }
+
+out:
+ unlock_kernel();
+ return ret;
+}
+
diff -ruN linux-2.4.26/include/asm-i386/unistd.h linux-2.4.26-
syscounter/include/asm-i386/unistd.h
--- linux-2.4.26/include/asm-i386/unistd.h 2002-11-28 21:53:15.000000000
-0200
+++ linux-2.4.26-syscounter/include/asm-i386/unistd.h 2004-10-07
10:34:22.000000000 -0300
@@ -257,6 +257,7 @@
#define __NR_alloc_hugepages 250
#define __NR_free_hugepages 251
#define __NR_exit_group 252
+#define __NR_counter 259
/* user-visible error numbers are in the range -1 - -124: see <asm-
i386/errno.h> */
diff -ruN linux-2.4.26/include/linux/syscounter.h linux-2.4.26-
syscounter/include/linux/syscounter.h
--- linux-2.4.26/include/linux/syscounter.h 1969-12-31
21:00:00.000000000 -0300
+++ linux-2.4.26-syscounter/include/linux/syscounter.h 2004-10-07
11:32:15.000000000 -0300
@@ -0,0 +1,42 @@
+#ifndef __SYSCOUNTER_H
+#define __SYSCOUNTER_H
+
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/smp_lock.h>
+#include <asm/uaccess.h>
+
+/* debug (trap) bit in eflags */
+#define TF_FLAG 0x00000100
+/* from ptrace.c */
+#define EFL 14
+#define EFL_OFFSET ((EFL-2)*4-sizeof(struct pt_regs))
+
+
+
+/* operations */
+#define START 0
+#define STOP 1
+#define GET_SYS 2
+#define GET_USER 3
+#define RESET 4
+#define GET_SYSCALLS 5
+
+
+/* error codes of new syscalls */
+//#define NOTRACEDPROCESS -1
+#define NOSUCHPROCESS -2
+#define TRACEINPROGRESS -3
+#define UNKNOWNOPERATION -4
+#define INVALIDACTION -5
+#define PROCESSTRACED -6 //process already traced
+#define PROCESSNOTTRACED -7
+#define INVALIDPOINTER -8
+#define BADUSERLANDCOPY -9
+
+
+
+
+#endif
Now the tracer:
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sched.h>
#include <sys/ptrace.h>
#include <sys/ipc.h>
#include <sys/types.h>
#include <sys/sem.h>
#include <sys/wait.h>
#include <assert.h>
#include "counter.h"
int main(int argc, char **argv){
long long c=0;
int retval;
struct sched_param sched_p;
int child;
child = fork();
if (child == 0) {
// setting realtime priority
sched_p.sched_priority = sched_get_priority_max(SCHED_FIFO);
retval = sched_setscheduler (0, SCHED_RR, &sched_p);
if (retval < 0){
printf ("Error setting realtime policy\n");
}
// so that we gain control back after execv()
ptrace(PTRACE_TRACEME,0,0,0);
execv(argv[1],&argv[1]);
}
else {
waitpid(child,&retval,0);
//ok, execve returned
c = 0;
// start our count engine
counter(child,START,NULL);
// calls ptrace so that we are notified again when the child finishes
ptrace(PTRACE_CONT,child,0,0);
waitpid(child,&retval,0);
// ok, child finished. We stop the engine
counter(0,STOP, NULL);
counter(0,GET_USER, (void *)&c);
printf("\n[PARENT] User instructions = %llu\n",c);
counter(0,GET_SYSCALLS, (void *)&c);
printf("\n[PARENT] Syscalls: %llu\n",c);
counter(0,RESET,NULL);
}
return 0;
}
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