Re: [PATCH 1/3] ptr_ring: batch ring zeroing

Michael S. Tsirkin
2017-05-10T12:20:36Z

On Wed, May 10, 2017 at 11:18:13AM +0200, Jesper Dangaard Brouer wrote:
> On Tue, 9 May 2017 16:33:14 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> > On Sat, Apr 08, 2017 at 02:14:08PM +0200, Jesper Dangaard Brouer wrote:
> > > On Fri, 7 Apr 2017 08:49:57 +0300
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > >
> > > > A known weakness in ptr_ring design is that it does not handle well the
> > > > situation when ring is almost full: as entries are consumed they are
> > > > immediately used again by the producer, so consumer and producer are
> > > > writing to a shared cache line.
> > > >
> > > > To fix this, add batching to consume calls: as entries are
> > > > consumed do not write NULL into the ring until we get
> > > > a multiple (in current implementation 2x) of cache lines
> > > > away from the producer. At that point, write them all out.
> > > >
> > > > We do the write out in the reverse order to keep
> > > > producer from sharing cache with consumer for as long
> > > > as possible.
> > > >
> > > > Writeout also triggers when ring wraps around - there's
> > > > no special reason to do this but it helps keep the code
> > > > a bit simpler.
> > > >
> > > > What should we do if getting away from producer by 2 cache lines
> > > > would mean we are keeping the ring moe than half empty?
> > > > Maybe we should reduce the batching in this case,
> > > > current patch simply reduces the batching.
> > > >
> > > > Notes:
> > > > - it is no longer true that a call to consume guarantees
> > > > that the following call to produce will succeed.
> > > > No users seem to assume that.
> > > > - batching can also in theory reduce the signalling rate:
> > > > users that would previously send interrups to the producer
> > > > to wake it up after consuming each entry would now only
> > > > need to do this once in a batch.
> > > > Doing this would be easy by returning a flag to the caller.
> > > > No users seem to do signalling on consume yet so this was not
> > > > implemented yet.
> > > >
> > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > ---
> > > >
> > > > Jason, I am curious whether the following gives you some of
> > > > the performance boost that you see with vhost batching
> > > > patches. Is vhost batching on top still helpful?
> > > >
> > > > include/linux/ptr_ring.h | 63 +++++++++++++++++++++++++++++++++++++++++-------
> > > > 1 file changed, 54 insertions(+), 9 deletions(-)
> > > >
> > > > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> > > > index 6c70444..6b2e0dd 100644
> > > > --- a/include/linux/ptr_ring.h
> > > > +++ b/include/linux/ptr_ring.h
> > > > @@ -34,11 +34,13 @@
> > > > struct ptr_ring {
> > > > int producer ____cacheline_aligned_in_smp;
> > > > spinlock_t producer_lock;
> > > > - int consumer ____cacheline_aligned_in_smp;
> > > > + int consumer_head ____cacheline_aligned_in_smp; /* next valid entry */
> > > > + int consumer_tail; /* next entry to invalidate */
> > > > spinlock_t consumer_lock;
> > > > /* Shared consumer/producer data */
> > > > /* Read-only by both the producer and the consumer */
> > > > int size ____cacheline_aligned_in_smp; /* max entries in queue */
> > > > + int batch; /* number of entries to consume in a batch */
> > > > void **queue;
> > > > };
> > > >
> > > > @@ -170,7 +172,7 @@ static inline int ptr_ring_produce_bh(struct ptr_ring *r, void *ptr)
> > > > static inline void *__ptr_ring_peek(struct ptr_ring *r)
> > > > {
> > > > if (likely(r->size))
> > > > - return r->queue[r->consumer];
> > > > + return r->queue[r->consumer_head];
> > > > return NULL;
> > > > }
> > > >
> > > > @@ -231,9 +233,38 @@ static inline bool ptr_ring_empty_bh(struct ptr_ring *r)
> > > > /* Must only be called after __ptr_ring_peek returned !NULL */
> > > > static inline void __ptr_ring_discard_one(struct ptr_ring *r)
> > > > {
> > > > - r->queue[r->consumer++] = NULL;
> > > > - if (unlikely(r->consumer >= r->size))
> > > > - r->consumer = 0;
> > > > + /* Fundamentally, what we want to do is update consumer
> > > > + * index and zero out the entry so producer can reuse it.
> > > > + * Doing it naively at each consume would be as simple as:
> > > > + * r->queue[r->consumer++] = NULL;
> > > > + * if (unlikely(r->consumer >= r->size))
> > > > + * r->consumer = 0;
> > > > + * but that is suboptimal when the ring is full as producer is writing
> > > > + * out new entries in the same cache line. Defer these updates until a
> > > > + * batch of entries has been consumed.
> > > > + */
> > > > + int head = r->consumer_head++;
> > > > +
> > > > + /* Once we have processed enough entries invalidate them in
> > > > + * the ring all at once so producer can reuse their space in the ring.
> > > > + * We also do this when we reach end of the ring - not mandatory
> > > > + * but helps keep the implementation simple.
> > > > + */
> > > > + if (unlikely(r->consumer_head - r->consumer_tail >= r->batch ||
> > > > + r->consumer_head >= r->size)) {
> > > > + /* Zero out entries in the reverse order: this way we touch the
> > > > + * cache line that producer might currently be reading the last;
> > > > + * producer won't make progress and touch other cache lines
> > > > + * besides the first one until we write out all entries.
> > > > + */
> > > > + while (likely(head >= r->consumer_tail))
> > > > + r->queue[head--] = NULL;
> > > > + r->consumer_tail = r->consumer_head;
> > > > + }
> > > > + if (unlikely(r->consumer_head >= r->size)) {
> > > > + r->consumer_head = 0;
> > > > + r->consumer_tail = 0;
> > > > + }
> > > > }
> > >
> > > I love this idea. Reviewed and discussed the idea in-person with MST
> > > during netdevconf[1] at this laptop. I promised I will also run it
> > > through my micro-benchmarking[2] once I return home (hint ptr_ring gets
> > > used in network stack as skb_array).
> >
> > I'm merging this through my tree. Any objections?
>
> I just did the micro-benchmark evaluation I promised and everything
> looks good, so no objections from me.
>
> John Fastabend recently posted a RFC patchset for removing the qdisc
> lock. The main purpose is to separate enqueue'ers and dequeue'ers from
> serializing on the same lock. But we need a new queue implementation
> that avoids the false-sharing between enq+deq.
>
> This is why John choose to use ptr_ring, changed (pfifo_fast) qdisc to
> use this ptr_ring. I think this patch might help his overload testing,
> as my theory is that he is hitting false-sharing on the queue, due to
> the queue always being full.
>
>
> > > Reviewed-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > >
> > > [1] http://netdevconf.org/2.1/
> > > [2] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/skb_array_bench01.c
>
> If you like can also add my:
>
> Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>

I pushed it out already unfortunately so I can't attach that.
Sorry. Thanks a lot for the testing!

> --
> Best regards,
> Jesper Dangaard Brouer
> MSc.CS, Principal Kernel Engineer at Red Hat
> LinkedIn: http://www.linkedin.com/in/brouer
>
> $ modprobe skb_array_test01
> $ dmesg
> [73228.381497] skb_array_test01: Loaded
> [73228.381498] skb_array_test01: PASSED - basic_init_and_cleanup()
> [73228.381498] skb_array_test01: PASSED - basic_add_and_remove_object()
> [73228.381505] skb_array_test01: PASSED - test_queue_full_condition()
> [73228.381505] skb_array_test01: PASSED - test_queue_empty_condition()
> [73228.381510] skb_array_test01: PASSED - test_queue_resize()