netdev - Re: [PATCH net-next] r8169: set IRQF_NO

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Date:   Mon, 2 Nov 2020 16:18:07 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        David Miller <davem@...emloft.net>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] r8169: set IRQF_NO_THREAD if MSI(X) is enabled

On 02.11.2020 13:41, Vladimir Oltean wrote:
> On Mon, Nov 02, 2020 at 09:01:00AM +0100, Heiner Kallweit wrote:
>> As mentioned by Eric it doesn't make sense to make the minimal hard irq
>> handlers used with NAPI a thread. This more contributes to the problem
>> than to the solution. The change here reflects this.
> 
> When you say that "it doesn't make sense", is there something that is
> actually measurably worse when the hardirq handler gets force-threaded?
> Rephrased, is it something that doesn't make sense in principle, or in
> practice?
> 
> My understanding is that this is not where the bulk of the NAPI poll
> processing is done anyway, so it should not have a severe negative
> impact on performance in any case.
> 
> On the other hand, moving as much code as possible outside interrupt
> context (be it hardirq or softirq) is beneficial to some use cases,
> because the scheduler is not in control of that code's runtime unless it
> is in a thread.
> 

According to my understanding the point is that executing the simple
hard irq handler for NAPI drivers doesn't cost significantly more than
executing the default hard irq handler (irq_default_primary_handler).
Therefore threadifying it means more or less just overhead.

forced threading:
1. irq_default_primary_handler, wakes irq thread
2. threadified driver hard irq handler (basically just calling napi_schedule)
3. NAPI processing

IRQF_NO_THREAD:
1. driver hard irq handler, scheduling NAPI
2. NAPI processing

>> The actual discussion would be how to make the NAPI processing a
>> thread (instead softirq).
> 
> I don't get it, so you prefer the hardirq handler to consume CPU time
> which is not accounted for by the scheduler, but for the NAPI poll, you
> do want the scheduler to account for it? So why one but not the other?
> 

The CPU time for scheduling NAPI is neglectable, but doing all the
rx and tx work in NAPI processing is significant effort.

>> For using napi_schedule_irqoff we most likely need something like
>> if (pci_dev_msi_enabled(pdev))
>> 	napi_schedule_irqoff(napi);
>> else
>> 	napi_schedule(napi);
>> and I doubt that's worth it.
> 
> Yes, probably not, hence my question.
>