netdev - Re: [PATCH RFC] r8152: Ensure that napi

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Date:   Fri, 14 May 2021 13:46:55 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-usb@...r.kernel.org, netdev@...r.kernel.org,
        Michal Svec <msvec@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Hayes Wang <hayeswang@...ltek.com>,
        Thierry Reding <treding@...dia.com>,
        Lee Jones <lee.jones@...aro.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH RFC] r8152: Ensure that napi_schedule() is handled

On Fri, 14 May 2021 22:25:50 +0200 Thomas Gleixner wrote:
> On Fri, May 14 2021 at 12:38, Jakub Kicinski wrote:
> 
> > On Fri, 14 May 2021 12:17:19 +0200 Thomas Gleixner wrote:  
> >> The driver invokes napi_schedule() in several places from task
> >> context. napi_schedule() raises the NET_RX softirq bit and relies on the
> >> calling context to ensure that the softirq is handled. That's usually on
> >> return from interrupt or on the outermost local_bh_enable().
> >> 
> >> But that's not the case here which causes the soft interrupt handling to be
> >> delayed to the next interrupt or local_bh_enable(). If the task in which
> >> context this is invoked is the last runnable task on a CPU and the CPU goes
> >> idle before an interrupt arrives or a local_bh_disable/enable() pair
> >> handles the pending soft interrupt then the NOHZ idle code emits the
> >> following warning.
> >> 
> >>   NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #08!!!
> >> 
> >> Prevent this by wrapping the napi_schedule() invocation from task context
> >> into a local_bh_disable/enable() pair.  
> >
> > I should have read through my inbox before replying :)
> >
> > I'd go for switching to raise_softirq_irqoff() in ____napi_schedule()...
> > why not?  
> 
> Except that some instruction cycle beancounters might complain about
> the extra conditional for the sane cases.
> 
> But yes, I'm fine with that as well. That's why this patch is marked RFC :)

When we're in the right context (irq/bh disabled etc.) the cost is just
read of preempt_count() and jump, right? And presumably preempt_count()
is in the cache already, because those sections aren't very long. Let me
make this change locally and see if it is in any way perceivable.

Obviously if anyone sees a way to solve the problem without much
ifdefinery and force_irqthreads checks that'd be great - I don't.
I'd rather avoid pushing this kind of stuff out to the drivers.