Re: [PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support

[Waiman Long <waiman.long@hp.com>]

On 10/29/2014 03:05 PM, Waiman Long wrote:
> On 10/27/2014 05:22 PM, Waiman Long wrote:
>> On 10/27/2014 02:04 PM, Peter Zijlstra wrote:
>>> On Mon, Oct 27, 2014 at 01:38:20PM -0400, Waiman Long wrote:
>>>> On 10/24/2014 04:54 AM, Peter Zijlstra wrote:
>>>>> On Thu, Oct 16, 2014 at 02:10:38PM -0400, Waiman Long wrote:
>>>>>
>>>>>> Since enabling paravirt spinlock will disable unlock function 
>>>>>> inlining,
>>>>>> a jump label can be added to the unlock function without adding 
>>>>>> patch
>>>>>> sites all over the kernel.
>>>>> But you don't have to. My patches allowed for the inline to remain,
>>>>> again reducing the overhead of enabling PV spinlocks while running 
>>>>> on a
>>>>> real machine.
>>>>>
>>>>> Look at:
>>>>>
>>>>>    http://lkml.kernel.org/r/20140615130154.213923590@chello.nl
>>>>>
>>>>> In particular this hunk:
>>>>>
>>>>> Index: linux-2.6/arch/x86/kernel/paravirt_patch_64.c
>>>>> ===================================================================
>>>>> --- linux-2.6.orig/arch/x86/kernel/paravirt_patch_64.c
>>>>> +++ linux-2.6/arch/x86/kernel/paravirt_patch_64.c
>>>>> @@ -22,6 +22,10 @@ DEF_NATIVE(pv_cpu_ops, swapgs, "swapgs")
>>>>>   DEF_NATIVE(, mov32, "mov %edi, %eax");
>>>>>   DEF_NATIVE(, mov64, "mov %rdi, %rax");
>>>>>
>>>>> +#if defined(CONFIG_PARAVIRT_SPINLOCKS)&&   
>>>>> defined(CONFIG_QUEUE_SPINLOCK)
>>>>> +DEF_NATIVE(pv_lock_ops, queue_unlock, "movb $0, (%rdi)");
>>>>> +#endif
>>>>> +
>>>>>   unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
>>>>>   {
>>>>>          return paravirt_patch_insns(insnbuf, len,
>>>>> @@ -61,6 +65,9 @@ unsigned native_patch(u8 type, u16 clobb
>>>>>                  PATCH_SITE(pv_cpu_ops, clts);
>>>>>                  PATCH_SITE(pv_mmu_ops, flush_tlb_single);
>>>>>                  PATCH_SITE(pv_cpu_ops, wbinvd);
>>>>> +#if defined(CONFIG_PARAVIRT_SPINLOCKS)&&   
>>>>> defined(CONFIG_QUEUE_SPINLOCK)
>>>>> +               PATCH_SITE(pv_lock_ops, queue_unlock);
>>>>> +#endif
>>>>>
>>>>>          patch_site:
>>>>>                  ret = paravirt_patch_insns(ibuf, len, start, end);
>>>>>
>>>>>
>>>>> That makes sure to overwrite the callee-saved call to the
>>>>> pv_lock_ops::queue_unlock with the immediate asm "movb $0, (%rdi)".
>>>>>
>>>>>
>>>>> Therefore you can retain the inlined unlock with hardly (there 
>>>>> might be
>>>>> some NOP padding) any overhead at all. On PV it reverts to a callee
>>>>> saved function call.
>>>> My concern is that spin_unlock() can be called in many places, 
>>>> including
>>>> loadable kernel modules. Can the paravirt_patch_ident_32() function 
>>>> able to
>>>> patch all of them in reasonable time? How about a kernel module 
>>>> loaded later
>>>> at run time?
>>> modules should be fine, see arch/x86/kernel/module.c:module_finalize()
>>> ->  apply_paravirt().
>>>
>>> Also note that the 'default' text is an indirect call into the paravirt
>>> ops table which routes to the 'right' function, so even if the text
>>> patching would be 'late' calls would 'work' as expected, just slower.
>>
>> Thanks for letting me know about that. I have this concern because 
>> your patch didn't change the current configuration of disabling 
>> unlock inlining when paravirt_spinlock is enabled. With that, I think 
>> it is worthwhile to reduce the performance delta between the PV and 
>> non-PV kernel on bare metal.
>
> I am sorry that the unlock call sites patching code doesn't work in a 
> virtual guest. Your pvqspinlock patch did an unconditional patching 
> even in a virtual guest. I added check for the 
> paravirt_spinlocks_enabled, but it turned out that some spin_unlock() 
> seemed to be called before paravirt_spinlocks_enabled is set. As a 
> result, some call sites were still patched resulting in missed wake 
> up's and system hang.
>
> At this point, I am going to leave out that change from my patch set 
> until we can figure out a better way of doing that.
>

Below was a partial kernel log with the unlock call site patch code in a 
KVM guest:

[    0.438006] native_patch: patch out pv_queue_unlock!
[    0.438565] native_patch: patch out pv_queue_unlock!
[    0.439006] native_patch: patch out pv_queue_unlock!
[    0.439638] native_patch: patch out pv_queue_unlock!
[    0.440052] native_patch: patch out pv_queue_unlock!
[    0.441006] native_patch: patch out pv_queue_unlock!
[    0.441566] native_patch: patch out pv_queue_unlock!
[    0.442035] ftrace: allocating 24168 entries in 95 pages
[    0.451208] Switched APIC routing to physical flat.
[    0.453202] ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1
[    0.454002] smpboot: CPU0: Intel QEMU Virtual CPU version 1.5.3 (fam: 
06, model: 06, stepping: 03)
[    0.456000] Performance Events: Broken PMU hardware detected, using 
software events only.
[    0.456003] Failed to access perfctr msr (MSR c1 is 0)
[    0.457151] KVM setup paravirtual spinlock
[    0.460039] NMI watchdog: disabled (cpu0): hardware events not enabled

It could be seen that some unlock call sites were patched before the KVM 
setup code set the paravirt_spinlocks_enabled flag.

-Longman