linux-kernel - Re: [RFC PATCH 0/2] net: threadable napi poll loop

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Message-ID: <ff0dc5d3-03c8-0316-71ed-e70426840773@stressinduktion.org>
Date:	Thu, 12 May 2016 00:47:29 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Jiri Pirko <jiri@...lanox.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Alexander Duyck <aduyck@...antis.com>,
	Tom Herbert <tom@...bertland.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 0/2] net: threadable napi poll loop

On 11.05.2016 16:45, Eric Dumazet wrote:
> On Wed, May 11, 2016 at 7:38 AM, Paolo Abeni <pabeni@...hat.com> wrote:
> 
>> Uh, we have likely the same issue in the net_rx_action() function, which
>> also execute with bh disabled and check for jiffies changes even on
>> single core hosts ?!?
> 
> That is why we have a loop break after netdev_budget=300 packets.
> And a sysctl to eventually tune this.
> 
> Same issue for softirq handler, look at commit
> 34376a50fb1fa095b9d0636fa41ed2e73125f214
> 
> Your questions about this central piece of networking code are worrying.
> 
>>
>> Aren't jiffies updated by the timer interrupt ? and thous even with
>> bh_disabled ?!?
> 
> Exactly my point : jiffie wont be updated in your code, since you block BH.

To be fair, jiffies get updated in hardirq and not softirq context. The
cond_resched_softirq not looking for pending softirqs is indeed a problem.

Thanks,
Hannes