netdev - Re: [PATCH vhost] vhost_net: Fix too many vring kick on busypoll

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Date:   Mon, 2 Jul 2018 10:41:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH vhost] vhost_net: Fix too many vring kick on busypoll



On 2018年06月30日 00:38, Michael S. Tsirkin wrote:
> On Fri, Jun 29, 2018 at 05:09:50PM +0900, Toshiaki Makita wrote:
>> Under heavy load vhost busypoll may run without suppressing
>> notification. For example tx zerocopy callback can push tx work while
>> handle_tx() is running, then busyloop exits due to vhost_has_work()
>> condition and enables notification but immediately reenters handle_tx()
>> because the pushed work was tx. In this case handle_tx() tries to
>> disable notification again, but when using event_idx it by design
>> cannot. Then busyloop will run without suppressing notification.
>> Another example is the case where handle_tx() tries to enable
>> notification but avail idx is advanced so disables it again. This case
>> also lead to the same situation with event_idx.
>>
>> The problem is that once we enter this situation busyloop does not work
>> under heavy load for considerable amount of time, because notification
>> is likely to happen during busyloop and handle_tx() immediately enables
>> notification after notification happens. Specifically busyloop detects
>> notification by vhost_has_work() and then handle_tx() calls
>> vhost_enable_notify().
> I'd like to understand the problem a bit better.
> Why does this happen?
> Doesn't this only happen if ring is empty?
>

My understanding is:

vhost_zerocopy_callback() try to poll vhost virtqueue. This will cause 
the busy loop in vhost_net_tx_get_vq_desc() to exit because of 
vhost_has_work() return true. Then handle_tx() tends to enable 
notification. Then guest may kick us even if handle_tx() call 
vhost_disable_notify() which in fact did nothing for even index.

Maybe we can try to call vhost_zerocopy_signal_used() if we found 
there's pending used from zerocopy instead.

Thanks