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

[Waiman Long <waiman.long@xxxxxx>]

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@xxxxxxxxx
> > > >
> > > > 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.
-Longman

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