Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall

From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <longman@redhat.com>
Cc: Juergen Gross <jgross@suse.com>,
	Yi Sun <yi.y.sun@linux.intel.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
	chao.p.peng@intel.com, chao.gao@intel.com,
	isaku.yamahata@intel.com, michael.h.kelley@microsoft.com,
	tianyu.lan@microsoft.com, "K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	Stephen Hemminger <sthemmin@microsoft.com>,
	"mingo@redhat.com" <mingo@redhat.com>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall
Date: Mon, 22 Oct 2018 19:15:16 +0200	[thread overview]
Message-ID: <20181022171516.GH3117@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <ba75b994-c1a6-6e91-6499-b9af90636fcf@redhat.com>

Firstly, who come a patch that is grubbing around in kernel/locking/ has
an x86/hyperv subject and isn't Cc'ed to the locking maintainers?

On Mon, Oct 22, 2018 at 12:31:45PM -0400, Waiman Long wrote:
> On 10/22/2018 03:32 AM, Juergen Gross wrote:
> > On 22/10/2018 03:53, Yi Sun wrote:
> >> On 18-10-19 16:20:52, Juergen Gross wrote:
> >>> On 19/10/2018 15:13, Yi Sun wrote:
> >> [...]
> >>
> >>>> diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> >>>> index 0130e48..9e88c7e 100644
> >>>> --- a/kernel/locking/qspinlock_paravirt.h
> >>>> +++ b/kernel/locking/qspinlock_paravirt.h
> >>>> @@ -7,6 +7,8 @@
> >>>>  #include <linux/bootmem.h>
> >>>>  #include <linux/debug_locks.h>
> >>>>  
> >>>> +#include <asm/mshyperv.h>
> >>>> +
> >>>>  /*
> >>>>   * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
> >>>>   * of spinning them.
> >>>> @@ -305,6 +307,10 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
> >>>>  				wait_early = true;
> >>>>  				break;
> >>>>  			}
> >>>> +#if defined(CONFIG_X86_64) && defined(CONFIG_PARAVIRT_SPINLOCKS) && IS_ENABLED(CONFIG_HYPERV)
> >>>> +			if (!hv_notify_long_spin_wait(SPIN_THRESHOLD - loop))
> >>>> +				break;
> >>>> +#endif

Secondly; how come you thought that was acceptable in any way shape or
form?

> > vcpu_is_preempted() is already part of this loop. And this is a paravirt
> > hook. Can't you make use of that? This might require adding another
> > parameter to this hook, but I'd prefer that over another pv-spinlock
> > hook.

> I agree with Juergen on that. I would suggest rename the
> vcpu_is_preempted hook into a more generic vcpu_stop_spinning, perhaps,
> so different hypervisors can act on the information accordingly. Adding
> an extra parameter is fine.

No; no extra parameters. vcpu_is_preempted() is a simple and intuitive
interface. Why would we want to make it complicated?