linux-kernel - Re: [PATCH v2 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Date:   Mon, 1 Apr 2019 11:33:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alex Kogan <alex.kogan@...cle.com>
Cc:     linux@...linux.org.uk, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, longman@...hat.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com, rahul.x.yadav@...cle.com
Subject: Re: [PATCH v2 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On Mon, Apr 01, 2019 at 11:06:53AM +0200, Peter Zijlstra wrote:
> On Fri, Mar 29, 2019 at 11:20:04AM -0400, Alex Kogan wrote:
> > diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
> > index bc6d3244e1af..71ee4b64c5d4 100644
> > --- a/kernel/locking/mcs_spinlock.h
> > +++ b/kernel/locking/mcs_spinlock.h
> > @@ -17,8 +17,18 @@
> >  
> >  struct mcs_spinlock {
> >  	struct mcs_spinlock *next;
> > +#ifndef CONFIG_NUMA_AWARE_SPINLOCKS
> >  	int locked; /* 1 if lock acquired */
> >  	int count;  /* nesting count, see qspinlock.c */
> > +#else /* CONFIG_NUMA_AWARE_SPINLOCKS */
> > +	uintptr_t locked; /* 1 if lock acquired, 0 if not, other values */
> > +			  /* represent a pointer to the secondary queue head */
> > +	u32 node_and_count;	/* node id on which this thread is running */
> > +				/* with two lower bits reserved for nesting */
> > +				/* count, see qspinlock.c */
> > +	u32 encoded_tail; /* encoding of this node as the main queue tail */
> > +	struct mcs_spinlock *tail;    /* points to the secondary queue tail */
> > +#endif /* CONFIG_NUMA_AWARE_SPINLOCKS */
> >  };
> 
> Please, have another look at the paravirt code, in particular at struct
> pv_node and its usage. This is horrible.

Thing is, this turns into a right mess when you also have PV spinlocks
on.

One thing we could maybe do is change locked and count to u8, then your
overlay structure could be something like:

struct mcs_spinlock {
	struct mcs_spinlock *next;
	u8 locked;
	u8 count;
};

struct cna_node {
	/* struct mcs_spinlock overlay */
	struct mcs_spinlock *next;
	u8 locked;
	u8 count;

	/* our CNA bits, consuming the slack and PV space */
	u16 node;
	u32 encoded_tail;
	struct mcs_spinlock *head;
	struct mcs_spinlock *tail;
};

Then you also don't need the first two patches.