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

[Waiman Long <waiman.long@xxxxxx>]

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.
-Longman

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