Re: [uml-devel] [PATCH] host context switch reduction

[Laurent Vivier <LaurentVivier@wanadoo.fr>]

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Hi,

as it is a plebiscite ;-), find attached the patch for UML 2.6.6 and
measurements I made on my poor netserver.

Laurent

Le mar 18/05/2004 à 19:59, Jeff Dike a écrit :
> On Tue, May 18, 2004 at 12:22:45AM +0200, roland wrote:
> > the question is:
> > is the "real world" performance benefit, uml get`s from this patch worth taking your time and is it implemented in a way, so that
> > uml maintainers are happy with that ?
> 
> The patch seems reasonable to me.  I'd be happiest with Laurent pushing this
> into mainline himself.
> 
> > i cannot really estimate, what performance benefits your patch brings to uml in detail, but what i have seen so far, this seems
> > quite worth doing the work.  reducing context switches by 1/3 is a LOT, imho - and reducing the execution time of the getpid-loop to
> > nearly the half is quite impressive.
> 
> Yeah, but no real workloads do while(1) getpid();
> 
> The kernel build improvement is obviously smaller, but still worth having.
> 
> > are there any "con`s" that argue for that work _NOT_ being done ?
> 
> No.  But testing, and happy reports to the appropriate mailing lists would
> help.
> 
> 				Jeff

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HP Netserver LH Pro bi-pro 200 MHz / 256 MB RAM

NATIF:

netserver:/usr/src# uname -a
Linux netserver 2.4.24-sysemu #4 SMP Tue Feb 17 16:43:11 CET 2004 i686 GNU/Linux

netserver:/usr/src# time ./getpid 1000000

real    0m1.763s
user    0m0.910s
sys     0m0.860s

real    0m1.759s
user    0m0.900s
sys     0m0.870s

real    0m1.759s
user    0m0.950s
sys     0m0.800s

UML with sysemu:

(none):~# uname -a
Linux (none) 2.6.6-1um #10 Wed May 19 12:54:37 CEST 2004 i686 unknown

(none):/mnt/usr/src# time ./getpid 1000000 

real    1m1.508s
user    0m7.960s
sys     0m53.450s

real    1m1.881s
user    0m6.630s
sys     0m55.240s

real    1m1.920s
user    0m6.580s
sys     0m55.340s

UML w/o sysemu

real    1m32.833s
user    0m7.610s
sys     1m25.130s

real    1m32.921s
user    0m7.890s
sys     1m24.980s

real    1m33.493s
user    0m8.010s
sys     1m24.960s

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diff -rNc linux-2.6.6/arch/um/kernel/process.c uml-linux-2.6.6/arch/um/kernel/process.c
*** linux-2.6.6/arch/um/kernel/process.c	Wed May 19 14:27:13 2004
--- uml-linux-2.6.6/arch/um/kernel/process.c	Wed May 19 14:25:40 2004
***************
*** 19,24 ****
--- 19,25 ----
  #include <asm/sigcontext.h>
  #include <asm/unistd.h>
  #include <asm/page.h>
+ #include <asm/user.h>
  #include "user_util.h"
  #include "kern_util.h"
  #include "user.h"
***************
*** 227,232 ****
--- 228,267 ----
  	}
  	stop_ptraced_child(pid, stack, 0);
  	printk("OK\n");
+ 
+ #ifdef PTRACE_SYSEMU
+ 	printk("Checking syscall emulation patch for ptrace...");
+ 	use_sysemu = 0;
+ 	pid = start_ptraced_child(&stack);
+ 	if(ptrace(PTRACE_SYSEMU, pid, 0, 0) >= 0) {
+ 		struct user_regs_struct regs;
+ 
+ 		if (waitpid(pid, &status, WUNTRACED) < 0)
+ 			panic("check_ptrace : wait failed, errno = %d", errno);
+ 		if(!WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
+ 			panic("check_ptrace : expected SIGTRAP, "
+ 			      "got status = %d", status);
+ 
+ 		if (ptrace(PTRACE_GETREGS, pid, 0, &regs) < 0)
+ 			panic("check_ptrace : failed to read child "
+ 			      "registers, errno = %d", errno);
+ 		regs.orig_eax = pid;
+ 		if (ptrace(PTRACE_SETREGS, pid, 0, &regs) < 0)
+ 			panic("check_ptrace : failed to modify child "
+ 			      "registers, errno = %d", errno);
+ 
+ 		stop_ptraced_child(pid, stack, 0);
+ 
+ 		printk("OK\n");
+ 		use_sysemu = 1;
+ 	}
+ 	else
+ 	{
+ 		printk("missing\n");
+ 		stop_ptraced_child(pid, stack, 1);
+ 	}
+ 
+ # endif /* PTRACE_SYSEMU */
  }
  
  int run_kernel_thread(int (*fn)(void *), void *arg, void **jmp_ptr)
diff -rNc linux-2.6.6/arch/um/kernel/skas/include/ptrace-skas.h uml-linux-2.6.6/arch/um/kernel/skas/include/ptrace-skas.h
*** linux-2.6.6/arch/um/kernel/skas/include/ptrace-skas.h	Wed May 19 14:27:13 2004
--- uml-linux-2.6.6/arch/um/kernel/skas/include/ptrace-skas.h	Wed May 19 11:22:04 2004
***************
*** 10,15 ****
--- 10,22 ----
  
  #ifdef UML_CONFIG_MODE_SKAS
  
+ /* syscall emulation path in ptrace */
+ 
+ #ifndef PTRACE_SYSEMU
+ #define PTRACE_SYSEMU 25
+ #endif
+ extern int use_sysemu;
+ 
  #include "skas_ptregs.h"
  
  #define HOST_FRAME_SIZE 17
diff -rNc linux-2.6.6/arch/um/kernel/skas/process.c uml-linux-2.6.6/arch/um/kernel/skas/process.c
*** linux-2.6.6/arch/um/kernel/skas/process.c	Wed May 19 14:27:13 2004
--- uml-linux-2.6.6/arch/um/kernel/skas/process.c	Wed May 19 11:22:04 2004
***************
*** 28,33 ****
--- 28,37 ----
  #include "chan_user.h"
  #include "signal_user.h"
  
+ #ifdef PTRACE_SYSEMU
+ int use_sysemu = 0;
+ #endif
+ 
  int is_skas_winch(int pid, int fd, void *data)
  {
  	if(pid != getpid())
***************
*** 68,73 ****
--- 72,81 ----
  		return;
  	}
  
+ #ifdef PTRACE_SYSEMU
+ 	if (!use_sysemu)
+ 	{
+ #endif
  	err = ptrace(PTRACE_POKEUSER, pid, PT_SYSCALL_NR_OFFSET, __NR_getpid);
  	if(err < 0)
  	        panic("handle_trap - nullifying syscall failed errno = %d\n", 
***************
*** 82,87 ****
--- 90,98 ----
  	if((err < 0) || !WIFSTOPPED(status) || (WSTOPSIG(status) != SIGTRAP))
  		panic("handle_trap - failed to wait at end of syscall, "
  		      "errno = %d, status = %d\n", errno, status);
+ #ifdef PTRACE_SYSEMU
+ 	}
+ #endif
  
  	handle_syscall(regs);
  }
***************
*** 139,144 ****
--- 150,160 ----
  
  	restore_registers(regs);
  		
+ #ifdef PTRACE_SYSEMU
+ 	if (use_sysemu)
+ 		err = ptrace(PTRACE_SYSEMU, pid, 0, 0);
+ 	else
+ #endif
  	err = ptrace(PTRACE_SYSCALL, pid, 0, 0);
  	if(err)
  		panic("userspace - PTRACE_SYSCALL failed, errno = %d\n", 
***************
*** 177,182 ****
--- 193,204 ----
  
  		restore_registers(regs);
  
+ #ifdef PTRACE_SYSEMU
+ 		if (use_sysemu)
+ 			op = singlestepping_skas() ? PTRACE_SINGLESTEP : 
+ 				PTRACE_SYSEMU;
+ 		else
+ #endif
  		op = singlestepping_skas() ? PTRACE_SINGLESTEP : 
  			PTRACE_SYSCALL;
  		err = ptrace(op, pid, 0, 0);

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