Re: [PATCH] x86/smp: Add irq_enter/exit() in smp_reschedule_interrupt()

From: Peter Zijlstra
Date: Mon Oct 17 2016 - 04:23:34 EST


On Mon, Oct 17, 2016 at 12:19:43PM +0800, Wanpeng Li wrote:
> 2016-10-16 21:39 GMT+08:00 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:

> >> [<ffffffff9d492b95>] do_trace_write_msr+0x135/0x140
> >> [<ffffffff9d06f860>] native_write_msr+0x20/0x30
> >> [<ffffffff9d065fad>] native_apic_msr_eoi_write+0x1d/0x30
> >> [<ffffffff9d05bd1d>] smp_reschedule_interrupt+0x1d/0x30
> >> [<ffffffff9d8daec6>] reschedule_interrupt+0x96/0xa0

> >> __visible void smp_reschedule_interrupt(struct pt_regs *regs)
> >> {
> >> + irq_enter();
> >> ack_APIC_irq();
> >> __smp_reschedule_interrupt();
> >> + irq_exit();
> >
> > Urgh, I really hate this...
> >
> > So now we're making a very frequent interrupt slower because of debug
> > code :/
>
> Do you have a better idea? :)

Something like the below avoids all that. Paravirt will still need fixing.

The thing is, many many smp_reschedule_interrupt() invocations don't
actually execute anything much at all and are only send to tickle the
return to user path (which does the actual preemption).

Having to do the whole irq_enter/irq_exit dance just for this unlikely
debug case totally blows.

---
arch/x86/include/asm/apic.h | 2 +-
arch/x86/include/asm/msr.h | 15 +++++++++++++++
2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index f5aaf6c83222..b97bfeed6456 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -196,7 +196,7 @@ static inline void native_apic_msr_write(u32 reg, u32 v)

static inline void native_apic_msr_eoi_write(u32 reg, u32 v)
{
- wrmsr(APIC_BASE_MSR + (APIC_EOI >> 4), APIC_EOI_ACK, 0);
+ wrmsr_notrace(APIC_BASE_MSR + (APIC_EOI >> 4), APIC_EOI_ACK, 0);
}

static inline u32 native_apic_msr_read(u32 reg)
diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index b5fee97813cd..45c080449d5b 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -127,6 +127,16 @@ notrace static inline void native_write_msr(unsigned int msr,
}

/* Can be uninlined because referenced by paravirt */
+notrace static inline void native_write_msr_notrace(unsigned int msr,
+ unsigned low, unsigned high)
+{
+ asm volatile("1: wrmsr\n"
+ "2:\n"
+ _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wrmsr_unsafe)
+ : : "c" (msr), "a"(low), "d" (high) : "memory");
+}
+
+/* Can be uninlined because referenced by paravirt */
notrace static inline int native_write_msr_safe(unsigned int msr,
unsigned low, unsigned high)
{
@@ -228,6 +238,11 @@ static inline void wrmsr(unsigned msr, unsigned low, unsigned high)
native_write_msr(msr, low, high);
}

+static inline void wrmsr_notrace(unsigned msr, unsigned low, unsigned high)
+{
+ native_write_msr_notrace(msr, low, high);
+}
+
#define rdmsrl(msr, val) \
((val) = native_read_msr((msr)))