Re: Small speedup for 'system_call'

Alan Cox (
Mon, 4 Nov 1996 21:52:57 +0000 (GMT)

> I was reading the system call entry code in arch/i386/kernel/entry.S.
> This is from 2.1.7:

That reminds me. Here is another one

> movl SYMBOL_NAME(sys_call_table)(,%eax,4),%eax
> testl %eax,%eax
> je ret_from_sys_call


Bletch. Just load empty syscall table entries with the function vector
of a routine that returns -ENOSYS. That saves another 2 or 3 clocks on
every single system call. Even if you dont expand the table.

> testb $0x20,flags(%ebx) # PF_TRACESYS
> jne tracesys
> call *%eax
> movl %eax,EAX(%esp) # save the return value
> .globl ret_from_sys_call
> ret_from_sys_call:

ret_from_sys_call could be a lot shorter if we had 'fast_ret_from_syscall'
and signal handlers and other stuff called


which swapped the return address or set a single flag that jumped around
all the reschedule, signal and other tests.