Re: [PATCH] x86/paravirt: Disable virt spinlock when CONFIG_PARAVIRT_SPINLOCKS disabled

[Chen Yu]

On 2024-05-23 at 09:30:59 -0700, Dave Hansen wrote:
> On 5/16/24 06:02, Chen Yu wrote:
> > Performance drop is reported when running encode/decode workload and
> > BenchSEE cache sub-workload.
> > Bisect points to commit ce0a1b608bfc ("x86/paravirt: Silence unused
> > native_pv_lock_init() function warning"). When CONFIG_PARAVIRT_SPINLOCKS
> > is disabled the virt_spin_lock_key is set to true on bare-metal.
> > The qspinlock degenerates to test-and-set spinlock, which decrease the
> > performance on bare-metal.
> > 
> > Fix this by disabling virt_spin_lock_key if CONFIG_PARAVIRT_SPINLOCKS
> > is not set, or it is on bare-metal.
> 
> This is missing some background:
> 
> The kernel can change spinlock behavior when running as a guest.  But
> this guest-friendly behavior causes performance problems on bare metal.
> So there's a 'virt_spin_lock_key' static key to switch between the two
> modes.
> 
> The static key is always enabled by default (run in guest mode) and
> should be disabled for bare metal (and in some guests that want native
> behavior).
> 
> ... then describe the regression and the fix
>
Thanks Juergen for your review.

And thanks Dave for the write up, I'll refine the log according to your suggestion. 

> > diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> > index 5358d43886ad..ee51c0949ed8 100644
> > --- a/arch/x86/kernel/paravirt.c
> > +++ b/arch/x86/kernel/paravirt.c
> > @@ -55,7 +55,7 @@ DEFINE_STATIC_KEY_TRUE(virt_spin_lock_key);
> >  
> >  void __init native_pv_lock_init(void)
> >  {
> > -	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) &&
> > +	if (!IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS) ||
> >  	    !boot_cpu_has(X86_FEATURE_HYPERVISOR))
> >  		static_branch_disable(&virt_spin_lock_key);
> >  }
> This gets used at a single site:
> 
>         if (pv_enabled())
>                 goto pv_queue;
> 
>         if (virt_spin_lock(lock))
>                 return;
> 
> which is logically:
> 
> 	if (IS_ENABLED(CONFIG_PARAVIRT_SPINLOCKS))
> 		goto ...; // don't look at virt_spin_lock_key
> 
> 	if (virt_spin_lock_key)
> 		return; // On virt, but non-paravirt.  Did Test-and-Set
> 			// spinlock.
>

Thanks for the description in detail, my original change might break the
"X86_FEATURE_HYPERVISOR + NO_CONFIG_PARAVIRT_SPINLOCKS " case that, the guest
can not fall into test-and-set.
 
> So I _think_ Arnd was trying to optimize native_pv_lock_init() away when
> it's going to get skipped over anyway by the 'goto'.
> 
> But this took me at least 30 minutes of scratching my head and trying to
> untangle the whole thing.  It's all far too subtle for my taste, and all
> of that to save a few bytes of init text in a configuration that's
> probably not even used very often (PARAVIRT=y, but PARAVIRT_SPINLOCKS=n).
> 
> Let's just keep it simple.  How about the attached patch?

Yes, this one works, I'll refine it.

thanks,
Chenyu