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 13:21:15 +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 Thu, Feb 09, 2017 at 03:51:03PM -0800, Paul E. McKenney wrote:
>     Currently, IPIs are used to force other CPUs to invalidate their TLBs
>     in response to a kernel virtual-memory mapping change.  This works, but
>     degrades both battery lifetime (for idle CPUs) and real-time response
>     (for nohz_full CPUs), and in addition results in unnecessary IPIs due to
>     the fact that CPUs executing in usermode are unaffected by stale kernel
>     mappings.  It would be better to cause a CPU executing in usermode to
>     wait until it is entering kernel mode to do the flush, first to avoid
>     interrupting usemode tasks and second to handle multiple flush requests
>     with a single flush in the case of a long-running user task.
>     
>     This commit therefore reserves a bit at the bottom of the ->dynticks
>     counter, which is checked upon exit from extended quiescent states.
>     If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is
>     invoked, which, if not supplied, is an empty single-pass do-while loop.
>     If this bottom bit is set on -entry- to an extended quiescent state,
>     then a WARN_ON_ONCE() triggers.
>     
>     This bottom bit may be set using a new rcu_eqs_special_set() function,
>     which returns true if the bit was set, or false if the CPU turned
>     out to not be in an extended quiescent state.  Please note that this
>     function refuses to set the bit for a non-nohz_full CPU when that CPU
>     is executing in usermode because usermode execution is tracked by RCU
>     as a dyntick-idle extended quiescent state only for nohz_full CPUs.
> 
>     Changes since v1:  Fix ordering of atomic_and() and the call to
>     rcu_eqs_special_exit() in rcu_dynticks_eqs_exit().
>  
>     Reported-by: Andy Lutomirski <luto@...capital.net>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

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)?

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 :-)

> +/*
> + * 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() ?