linux-kernel - Re: [PATCH V3 net-next 1/1] hv_netvsc: Use the xmit_more skb flag to optimize signaling the host

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Date:	Wed, 06 May 2015 12:17:39 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	KY Srinivasan <kys@...rosoft.com>
Cc:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>
Subject: Re: [PATCH V3 net-next 1/1] hv_netvsc: Use the xmit_more skb flag
 to optimize signaling the host

On Wed, 2015-05-06 at 18:28 +0000, KY Srinivasan wrote:

> Ah! I too was wondering how we could get into this situation. The condition you mention
> is already handled in the lower level - if the attempt to put the last packet on vmbus were to
> fail because the ring is full, we will notify the host independent of kick_q parameter - look
> at the function vmbus_sendpacket_ctl() in drivers/hv/channel.c
> 
> I will resend the patch by reverting this to version 2.

I am not really convinced by what you are saying ;)

problem is not when you fail to add an additional packet.

Problem is :

You queue a packet but not kick because skb->xmit_more = 1, and _then_
you call netif_tx_stop_queue().

No further packet will be delivered to your driver, until
netif_tx_wake_queue() is called again from netvsc_send_completion(),
possibly milli seconds later.

If qdisc was replaced by another one, no packet will kick again, unless
other traffic comes.

Given complexity of your driver, I have no easy way to check you made
this xmit_more conversion correctly, and one packet can not sit in the
equivalent of TX ring buffer for unbound time.

Better add a full explanation into your changelog.

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