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

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Date:   Mon, 22 Oct 2018 19:15:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Juergen Gross <jgross@...e.com>, Yi Sun <yi.y.sun@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        chao.p.peng@...el.com, chao.gao@...el.com,
        isaku.yamahata@...el.com, michael.h.kelley@...rosoft.com,
        tianyu.lan@...rosoft.com, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1 2/2] x86/hyperv: make HvNotifyLongSpinWait hypercall


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?