Re: [Bug 350] New: i386 context switch very slow compared to 2.4 due to wrmsr (performance)

Dave Jones
2003-02-12T02:53:23Z

On Tue, Feb 11, 2003 at 05:35:43PM -0800, Martin J. Bligh wrote:

> The reason it rewrites SYSENTER_CS is non obviously vm86 which
> doesn't guarantee the MSR stays constant (sigh). I think this would
> be better handled by having a global flag or process flag when any process
> uses vm86 and not do it when this flag is not set (as in 99% of all
> normal use cases)

I feel I'm missing something obvious here, but is this part the
low-hanging fruit that it seems ?

Dave

--- bk-linus/arch/i386/kernel/sysenter.c 2003-02-12 00:10:15.000000000 -0100
+++ linux-2.5/arch/i386/kernel/sysenter.c 2003-02-12 01:53:58.000000000 -0100
@@ -20,6 +20,8 @@

extern asmlinkage void sysenter_entry(void);

+int trashed_sysenter_cs;
+
/*
* Create a per-cpu fake "SEP thread" stack, so that we can
* enter the kernel without having to worry about things like
--- bk-linus/include/asm-i386/processor.h 2003-02-12 00:15:23.000000000 -0100
+++ linux-2.5/include/asm-i386/processor.h 2003-02-12 01:53:43.000000000 -0100
@@ -408,19 +408,26 @@ struct thread_struct {
.io_bitmap = { [ 0 ... IO_BITMAP_SIZE ] = ~0 }, \
}

+extern int trashed_sysenter_cs;
+
static inline void load_esp0(struct tss_struct *tss, unsigned long esp0)
{
tss->esp0 = esp0;
if (cpu_has_sep) {
- wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
+ if (trashed_sysenter_cs==1) {
+ wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
+ trashed_sysenter_cs = 0;
+ }
wrmsr(MSR_IA32_SYSENTER_ESP, esp0, 0);
}
}

static inline void disable_sysenter(void)
{
- if (cpu_has_sep)
+ if (cpu_has_sep) {
wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);
+ trashed_sysenter_cs = 1;
+ }
}

#define start_thread(regs, new_eip, new_esp) do { \

--
| Dave Jones. http://www.codemonkey.org.uk
| SuSE Labs