netdev - Re: [PATCH v4 11/13] task_isolation: net: don't flush backlog on CPUs running isolated tasks

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Message-ID: <4c9229e0e2d7ffabba1a8372d5335ddb28486b6e.camel@redhat.com>
Date:   Fri, 22 Jan 2021 17:13:44 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>
Cc:     Alex Belits <abelits@...vell.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        Prasun Kapoor <pkapoor@...vell.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "will@...nel.org" <will@...nel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 11/13] task_isolation: net: don't flush backlog on
 CPUs running isolated tasks

On Fri, 2021-01-22 at 11:13 -0300, Marcelo Tosatti wrote:
> On Thu, Oct 01, 2020 at 04:47:31PM +0200, Frederic Weisbecker wrote:
> > On Wed, Jul 22, 2020 at 02:58:24PM +0000, Alex Belits wrote:
> > > From: Yuri Norov <ynorov@...vell.com>
> > > 
> > > so we don't need to flush it.
> > 
> > What guarantees that we have no backlog on it?
> 
> From Paolo's work to use lockless reading of 
> per-CPU skb lists
> 
> https://www.spinics.net/lists/netdev/msg682693.html
> 
> It also exposed skb queue length to userspace
> 
> https://www.spinics.net/lists/netdev/msg684939.html
> 
> But if i remember correctly waiting for a RCU grace
> period was also necessary to ensure no backlog !?! 
> 
> Paolo would you please remind us what was the sequence of steps?
> (and then also, for the userspace isolation interface, where 
> the application informs the kernel that its entering isolated
> mode, is just confirming the queues have zero length is
> sufficient?).

After commit 2de79ee27fdb52626ac4ac48ec6d8d52ba6f9047, for CONFIG_RPS
enabled build, with no RFS in place to ensure backlog will be empty on
CPU X, the user must:
- configure the RPS map on each device before the device goes up to
explicitly exclude CPU X.

If CPU X is isolated after some network device already went up, to
ensure that the backlog will be empty on CPU X the user must:
- configure RPS on all the network device to exclude CPU X (as in the
previous scenario)
- wait a RCU grace period
- wait untill the backlog len on CPU X reported by procfs is 0

Cheers,

Paolo