netdev - Re: [PATCH] rtw88: pci: Move a mass of jobs in hw IRQ to soft IRQ

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Date:   Fri, 16 Aug 2019 17:27:51 +0800
From:   Jian-Hong Pan <jian-hong@...lessm.com>
To:     Tony Chuang <yhchuang@...ltek.com>
Cc:     Kalle Valo <kvalo@...eaurora.org>,
        "David S . Miller" <davem@...emloft.net>,
        "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux@...lessm.com" <linux@...lessm.com>
Subject: Re: [PATCH] rtw88: pci: Move a mass of jobs in hw IRQ to soft IRQ

Tony Chuang <yhchuang@...ltek.com> 於 2019年8月16日 週五 下午4:07寫道：
>
> Hi,
>
> A few more questions below
>
> > > From: Jian-Hong Pan [mailto:jian-hong@...lessm.com]
> > >
> > > There is a mass of jobs between spin lock and unlock in the hardware
> > > IRQ which will occupy much time originally. To make system work more
> > > efficiently, this patch moves the jobs to the soft IRQ (bottom half) to
> > > reduce the time in hardware IRQ.
> > >
> > > Signed-off-by: Jian-Hong Pan <jian-hong@...lessm.com>
> > > ---
> > >  drivers/net/wireless/realtek/rtw88/pci.c | 36 +++++++++++++++++++-----
> > >  1 file changed, 29 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/drivers/net/wireless/realtek/rtw88/pci.c
> > > b/drivers/net/wireless/realtek/rtw88/pci.c
> > > index 00ef229552d5..355606b167c6 100644
> > > --- a/drivers/net/wireless/realtek/rtw88/pci.c
> > > +++ b/drivers/net/wireless/realtek/rtw88/pci.c
> > > @@ -866,12 +866,29 @@ static irqreturn_t rtw_pci_interrupt_handler(int
> > irq,
> > > void *dev)
> > >  {
> > >     struct rtw_dev *rtwdev = dev;
> > >     struct rtw_pci *rtwpci = (struct rtw_pci *)rtwdev->priv;
> > > -   u32 irq_status[4];
> > > +   unsigned long flags;
> > >
> > > -   spin_lock(&rtwpci->irq_lock);
> > > +   spin_lock_irqsave(&rtwpci->irq_lock, flags);
>
> I think you can use 'spin_lock()' here as it's in IRQ context?

Ah!  You are right!  The interrupts are already disabled in the
interrupt handler.  So, there is no need to disable more once.  I can
tweak it.

> > >     if (!rtwpci->irq_enabled)
> > >             goto out;
> > >
> > > +   /* disable RTW PCI interrupt to avoid more interrupts before the end of
> > > +    * thread function
> > > +    */
> > > +   rtw_pci_disable_interrupt(rtwdev, rtwpci);
>
>
> Why do we need rtw_pci_disable_interrupt() here.
> Have you done any experiment and decided to add this.
> If you have can you share your results to me?

I got this idea "Avoid back to back interrupt" from Intel WiFi's driver.
https://elixir.bootlin.com/linux/v5.3-rc4/source/drivers/net/wireless/intel/iwlwifi/pcie/rx.c#L2071

So, I disable rtw_pci interrupt here in first half IRQ.  (Re-enable in
bottom half)

>
> > > +out:
> > > +   spin_unlock_irqrestore(&rtwpci->irq_lock, flags);
>
> spin_unlock()
>
> > > +
> > > +   return IRQ_WAKE_THREAD;
> > > +}
> > > +
> > > +static irqreturn_t rtw_pci_interrupt_threadfn(int irq, void *dev)
> > > +{
> > > +   struct rtw_dev *rtwdev = dev;
> > > +   struct rtw_pci *rtwpci = (struct rtw_pci *)rtwdev->priv;
> > > +   unsigned long flags;
> > > +   u32 irq_status[4];
> > > +
> > >     rtw_pci_irq_recognized(rtwdev, rtwpci, irq_status);
> > >
> > >     if (irq_status[0] & IMR_MGNTDOK)
> > > @@ -891,8 +908,11 @@ static irqreturn_t rtw_pci_interrupt_handler(int
> > irq,
> > > void *dev)
> > >     if (irq_status[0] & IMR_ROK)
> > >             rtw_pci_rx_isr(rtwdev, rtwpci, RTW_RX_QUEUE_MPDU);
> > >
> > > -out:
> > > -   spin_unlock(&rtwpci->irq_lock);
> > > +   /* all of the jobs for this interrupt have been done */
> > > +   spin_lock_irqsave(&rtwpci->irq_lock, flags);
> >
> > Shouldn't we protect the ISRs above?
> >
> > This patch could actually reduce the time of IRQ.
> > But I think I need to further test it with PCI MSI interrupt.
> > https://patchwork.kernel.org/patch/11081539/
> >
> > Maybe we could drop the "rtw_pci_[enable/disable]_interrupt" when MSI
> > Is enabled with this patch.
> >
> > > +   if (rtw_flag_check(rtwdev, RTW_FLAG_RUNNING))
> > > +           rtw_pci_enable_interrupt(rtwdev, rtwpci);

Then, re-enable rtw_pci interrupt here in bottom half of the IRQ.
Here is the place where Intel WiFi re-enable interrupts.
https://elixir.bootlin.com/linux/v5.3-rc4/source/drivers/net/wireless/intel/iwlwifi/pcie/rx.c#L1959

Now, we can go back to the question "Shouldn't we protect the ISRs above?"

1. What does the lock: rtwpci->irq_lock protect for?
According to the code, seems only rtw_pci interrupt's state which is
enabled or not.

2. How about the ISRs you mentioned?
This part will only be executed if there is a fresh rtw_pci interrupt.
The first half already disabled rtw_pci interrupt, so there is no more
fresh rtw_pci interrupt until rtw_pci interrupt is enabled again.
Therefor, the rtwpci->irq_lock only wraps the rtw_pci interrupt
enablement.

If there is any more entry that I missed and will interfere, please let me know.

Thank you
Jian-Hong Pan