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lguest: avoid sending interrupts to Guest when no activity occurs.
If we track how many buffers we've used, we can tell whether we really need to interrupt the Guest. This happens as a side effect of spurious notifications. Spurious notifications happen because it can take a while before the Host thread wakes up and sets the VRING_USED_F_NO_NOTIFY flag, and meanwhile the Guest can more notifications. A real fix would be to use wake counts, rather than a suppression flag, but the practical difference is generally in the noise: the interrupt is usually coalesced into a pending one anyway so we just save a system call which isn't clearly measurable. Secs Spurious IRQS 1G TCP Guest->Host: 3.93 58 1M normal pings: 100 72 1M 1k pings (-l 120): 57 492904 Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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1 changed files with 9 additions and 0 deletions
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@ -151,6 +151,9 @@ struct virtqueue
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/* Last available index we saw. */
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u16 last_avail_idx;
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/* How many are used since we sent last irq? */
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unsigned int pending_used;
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/* Eventfd where Guest notifications arrive. */
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int eventfd;
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@ -556,6 +559,11 @@ static void trigger_irq(struct virtqueue *vq)
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{
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unsigned long buf[] = { LHREQ_IRQ, vq->config.irq };
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/* Don't inform them if nothing used. */
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if (!vq->pending_used)
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return;
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vq->pending_used = 0;
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/* If they don't want an interrupt, don't send one, unless empty. */
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if ((vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)
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&& lg_last_avail(vq) != vq->vring.avail->idx)
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@ -647,6 +655,7 @@ static void add_used(struct virtqueue *vq, unsigned int head, int len)
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/* Make sure buffer is written before we update index. */
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wmb();
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vq->vring.used->idx++;
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vq->pending_used++;
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}
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/* And here's the combo meal deal. Supersize me! */
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