linux-kernel - Re: [PATCH RFC v2 tip/core/rcu] Maintain special bits at bottom of ->dynticks counter

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Date:   Mon, 13 Feb 2017 18:57:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, luto@...nel.org,
        tglx@...utronix.de, fweisbec@...il.com, cmetcalf@...lanox.com,
        mingo@...nel.org
Subject: Re: [PATCH RFC v2 tip/core/rcu] Maintain special bits at bottom of
 ->dynticks counter

On Mon, Feb 13, 2017 at 09:01:04AM -0800, Paul E. McKenney wrote:
> > I think I've asked this before, but why does this live in the guts of
> > RCU?
> > 
> > Should we lift this state tracking stuff out and make RCU and
> > NOHZ(_FULL) users of it, or doesn't that make sense (reason)?
> 
> The dyntick-idle stuff is pretty specific to RCU.  And what precisely
> would be helped by moving it?

Maybe untangle the inter-dependencies somewhat. It just seems a wee bit
odd to have arch TLB invalidate depend on RCU implementation details
like this.

> But that was an excellent question, as it reminded me of RCU's
> dyntick-idle's NMI handling, and I never did ask Andy if it was OK for
> rcu_eqs_special_exit() to be invoked when exiting NMI handler, which would
> currently happen.  It would be easy for me to pass in a flag indicating
> whether or not the call is in NMI context, if that is needed.
> 
> It is of course not possible to detect this at rcu_eqs_special_set()
> time, because rcu_eqs_special_set() has no way of knowing that the next
> event that pulls the remote CPU out of idle will be an NMI.
> 
> > In any case, small nit below:
> > 
> > 
> > > +	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks);
> > > +	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > > +		     !(seq & RCU_DYNTICK_CTRL_CTR));
> > > +	if (seq & RCU_DYNTICK_CTRL_MASK) {
> > > +		atomic_and(~RCU_DYNTICK_CTRL_MASK, &rdtp->dynticks);
> > > +		smp_mb__after_atomic(); /* _exit after clearing mask. */
> > > +		/* Prefer duplicate flushes to losing a flush. */
> > > +		rcu_eqs_special_exit();
> > > +	}
> > 
> > we have atomic_andnot() for just these occasions :-)
> 
> I suppose that that could generate more efficient code on some
> architectures.  I have changed this.

Right, saves 1 instruction on a number of archs. Not the end of the
world of course, but since we have the thing might as well use it.

> > > +/*
> > > + * Set the special (bottom) bit of the specified CPU so that it
> > > + * will take special action (such as flushing its TLB) on the
> > > + * next exit from an extended quiescent state.  Returns true if
> > > + * the bit was successfully set, or false if the CPU was not in
> > > + * an extended quiescent state.
> > > + */
> > > +bool rcu_eqs_special_set(int cpu)
> > > +{
> > > +	int old;
> > > +	int new;
> > > +	struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
> > > +
> > > +	do {
> > > +		old = atomic_read(&rdtp->dynticks);
> > > +		if (old & RCU_DYNTICK_CTRL_CTR)
> > > +			return false;
> > > +		new = old | RCU_DYNTICK_CTRL_MASK;
> > > +	} while (atomic_cmpxchg(&rdtp->dynticks, old, new) != old);
> > > +	return true;
> > 
> > Is that what we call atomic_fetch_or() ?
> 
> I don't think so.  The above code takes an early exit if the next bit
> up is set, which atomic_fetch_or() does not.  If the CPU is not in
> an extended quiescent state (old & RCU_DYNTICK_CTRL_CTR), then this
> code returns false to indicate that TLB shootdown cannot wait.

Oh duh yes, reading be hard.

> So it is more like a very specific form of atomic_fetch_or_unless().

Right, I actually have a similar construct in set_nr_if_polling().