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Date: Thu, 10 Nov 2022 21:38:02 +1000
Message-Id: <CO8L3YUSJ1HW.236EC6T4G0X5D@bobo>
Subject: Re: [PATCH 16/17] powerpc/qspinlock: allow indefinite spinning on a
 preempted owner
From: "Nicholas Piggin" <npiggin@gmail.com>
To: "Jordan Niethe" <jniethe5@gmail.com>, <linuxppc-dev@lists.ozlabs.org>
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References: <20220728063120.2867508-1-npiggin@gmail.com>
 <20220728063120.2867508-18-npiggin@gmail.com>
 <6307bef2943054255d5241d41efd218a4f487194.camel@gmail.com>
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On Thu Nov 10, 2022 at 10:44 AM AEST, Jordan Niethe wrote:
> On Thu, 2022-07-28 at 16:31 +1000, Nicholas Piggin wrote:
> [resend as utf-8, not utf-7]
> > Provide an option that holds off queueing indefinitely while the lock
> > owner is preempted. This could reduce queueing latencies for very
> > overcommitted vcpu situations.
> >=20
> > This is disabled by default.
> > ---
> >  arch/powerpc/lib/qspinlock.c | 91 +++++++++++++++++++++++++++++++-----
> >  1 file changed, 79 insertions(+), 12 deletions(-)
> >=20
> > diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.=
c
> > index 24f68bd71e2b..5cfd69931e31 100644
> > --- a/arch/powerpc/lib/qspinlock.c
> > +++ b/arch/powerpc/lib/qspinlock.c
> > @@ -35,6 +35,7 @@ static int HEAD_SPINS __read_mostly =3D (1<<8);
> > =20
> >  static bool pv_yield_owner __read_mostly =3D true;
> >  static bool pv_yield_allow_steal __read_mostly =3D false;
> > +static bool pv_spin_on_preempted_owner __read_mostly =3D false;
> >  static bool pv_yield_prev __read_mostly =3D true;
> >  static bool pv_yield_propagate_owner __read_mostly =3D true;
> >  static bool pv_prod_head __read_mostly =3D false;
> > @@ -220,13 +221,15 @@ static struct qnode *get_tail_qnode(struct qspinl=
ock *lock, u32 val)
> >  	BUG();
> >  }
> > =20
> > -static __always_inline void __yield_to_locked_owner(struct qspinlock *=
lock, u32 val, bool paravirt, bool clear_mustq)
> > +static __always_inline void __yield_to_locked_owner(struct qspinlock *=
lock, u32 val, bool paravirt, bool clear_mustq, bool *preempted)
> >  {
> >  	int owner;
> >  	u32 yield_count;
> > =20
> >  	BUG_ON(!(val & _Q_LOCKED_VAL));
> > =20
> > +	*preempted =3D false;
> > +
> >  	if (!paravirt)
> >  		goto relax;
> > =20
> > @@ -241,6 +244,8 @@ static __always_inline void __yield_to_locked_owner=
(struct qspinlock *lock, u32
> > =20
> >  	spin_end();
> > =20
> > +	*preempted =3D true;
> > +
> >  	/*
> >  	 * Read the lock word after sampling the yield count. On the other si=
de
> >  	 * there may a wmb because the yield count update is done by the
> > @@ -265,14 +270,14 @@ static __always_inline void __yield_to_locked_own=
er(struct qspinlock *lock, u32
> >  	spin_cpu_relax();
> >  }
> > =20
> > -static __always_inline void yield_to_locked_owner(struct qspinlock *lo=
ck, u32 val, bool paravirt)
> > +static __always_inline void yield_to_locked_owner(struct qspinlock *lo=
ck, u32 val, bool paravirt, bool *preempted)
>
> It seems like preempted parameter could be the return value of
> yield_to_locked_owner(). Then callers that don't use the value returned i=
n
> preempted don't need to create an unnecessary variable to pass in.

That works.

Thanks,
Nick