Subject: Re: [PATCH net-next v3 2/3] udp: implement memory accounting helpers

From: Paolo Abeni <[email-masked]>
Date: Thu, 29 Sep 2016 16:01:14 +0200

On Thu, 2016-09-29 at 06:24 -0700, Eric Dumazet wrote:
> On Thu, 2016-09-29 at 11:31 +0200, Paolo Abeni wrote:
> > On Wed, 2016-09-28 at 18:42 -0700, Eric Dumazet wrote:
> > > On Wed, 2016-09-28 at 12:52 +0200, Paolo Abeni wrote:
> > > 
> > > > +static void udp_rmem_release(struct sock *sk, int partial)
> > > > +{
> > > > +	struct udp_sock *up = udp_sk(sk);
> > > > +	int fwd, amt;
> > > > +
> > > > +	if (partial && !udp_under_memory_pressure(sk))
> > > > +		return;
> > > > +
> > > > +	/* we can have concurrent release; if we catch any conflict
> > > > +	 * we let only one of them do the work
> > > > +	 */
> > > > +	if (atomic_dec_if_positive(&up->can_reclaim) < 0)
> > > > +		return;
> > > > +
> > > > +	fwd = __udp_forward(up, atomic_read(&sk->sk_rmem_alloc));
> > > > +	if (fwd < SK_MEM_QUANTUM + partial) {
> > > > +		atomic_inc(&up->can_reclaim);
> > > > +		return;
> > > > +	}
> > > > +
> > > > +	amt = (fwd - partial) & ~(SK_MEM_QUANTUM - 1);
> > > > +	atomic_sub(amt, &up->mem_allocated);
> > > > +	atomic_inc(&up->can_reclaim);
> > > > +
> > > > +	__sk_mem_reduce_allocated(sk, amt >> SK_MEM_QUANTUM_SHIFT);
> > > > +	sk->sk_forward_alloc = fwd - amt;
> > > > +}
> > > 
> > > 
> > > This is racy... all these atomics make me nervous...
> > 
> > Ah, perhaps I got it: if we have a concurrent memory scheduling, we
> > could end up with a value of mem_allocated below the real need. 
> > 
> > That mismatch will not drift: at worst we can end up with mem_allocated
> > being single SK_MEM_QUANTUM below what is strictly needed.
> > 
> > A possible alternative could be:
> > 
> > static void udp_rmem_release(struct sock *sk, int partial)
> > {
> > 	struct udp_sock *up = udp_sk(sk);
> > 	int fwd, amt, alloc_old, alloc;
> > 
> > 	if (partial && !udp_under_memory_pressure(sk))
> > 		return;
> > 
> > 	alloc = atomic_read(&up->mem_allocated);
> > 	fwd = alloc - atomic_read(&sk->sk_rmem_alloc);
> > 	if (fwd < SK_MEM_QUANTUM + partial)
> > 		return;
> > 
> > 	amt = (fwd - partial) & ~(SK_MEM_QUANTUM - 1);
> > 	alloc_old = atomic_cmpxchg(&up->mem_allocated, alloc, alloc - amt);
> > 	/* if a concurrent update is detected, just do nothing; if said update
> > 	 * is due to another memory release, that release take care of
> > 	 * reclaiming the memory for us, too.
> > 	 * Otherwise we will be able to release on later dequeue, since
> > 	 * we will eventually stop colliding with the writer when it will
> > 	 * consume all the fwd allocated memory
> > 	 */
> > 	if (alloc_old != alloc)
> > 		return;
> > 
> > 	__sk_mem_reduce_allocated(sk, amt >> SK_MEM_QUANTUM_SHIFT);
> > 	sk->sk_forward_alloc = fwd - amt;
> 
> Can still be done from multiple cpus.
> 
> Add some ndelay() or udelay() before to simulate fact that current cpu
> could be interrupted by an NMI handler (perf for example)... or hard IRQ
> handler...
> 
> Then make sure your tests involve 16 concurrent cpus dealing with one
> udp socket...

Thank you again reviewing this.

I'm working to this sort of tests right now.

> 
> > }
> > 
> > which is even more lazy in reclaiming but should never underestimate the
> > needed forward allocation, and under pressure should eventually free the
> > needed memory.
> 
> 
> If this code is rarely used, why don't you simply use a real spinlock,
> so that we do not have to worry about all this ?
> 
> A spinlock  acquisition/release is a _single_ locked operation.
> Faster than the 3 atomic you got in last version.
> spinlock code (ticket or MCS) avoids starvation.

I'd like to avoid adding a lock, if possible, to avoid any possible
source of contention.

> Then, you can safely update multiple fields in the socket.
> 
> And you get nice lockdep support as a bonus.
> 
> cmpxchg() is fine when a single field need an exclusion. But there you
> have multiple fields to update at once :
> 
> sk_memory_allocated_add() and sk_memory_allocated_sub() can work using 
> atomic_long_add_return() and atomic_long_sub() because their caller owns
> the socket lock and can safely update sk->sk_forward_alloc without
> additional locking, but UDP wont have this luxury after your patches.

When we reach __sk_mem_reduce_allocated() we are sure we can free the
specified amount of memory, so we only need to ensure consistent
sk_prot->memory_allocated updates. The current atomic operation suffices
to this.

Paolo

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