linux-kernel - RE: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage optimization for SCSI drivers

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Date:   Wed, 10 Feb 2021 11:28:38 +1100 (AEDT)
From:   Finn Thain <fthain@...egraphics.com.au>
To:     "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>
cc:     tanxiaofei <tanxiaofei@...wei.com>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...neuler.org" <linuxarm@...neuler.org>,
        "linux-m68k@...r.kernel.org" <linux-m68k@...r.kernel.org>
Subject: RE: [Linuxarm]  Re: [PATCH for-next 00/32] spin lock usage optimization
 for SCSI drivers

On Tue, 9 Feb 2021, Song Bao Hua (Barry Song) wrote:

> > On Tue, 9 Feb 2021, Song Bao Hua (Barry Song) wrote:
> > 
> > > > On Sun, 7 Feb 2021, Xiaofei Tan wrote:
> > > >
> > > > > Replace spin_lock_irqsave with spin_lock in hard IRQ of SCSI 
> > > > > drivers. There are no function changes, but may speed up if 
> > > > > interrupt happen too often.
> > > >
> > > > This change doesn't necessarily work on platforms that support 
> > > > nested interrupts.
> > > >
> > > > Were you able to measure any benefit from this change on some 
> > > > other platform?
> > >
> > > I think the code disabling irq in hardIRQ is simply wrong. Since 
> > > this commit
> > >
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e58aa3d2d0cc 
> > > genirq: Run irq handlers with interrupts disabled
> > >
> > > interrupt handlers are definitely running in a irq-disabled context 
> > > unless irq handlers enable them explicitly in the handler to permit 
> > > other interrupts.
> > >
> > 
> > Repeating the same claim does not somehow make it true. 
> 
> Sorry for I didn't realize xiaofei had replied.
> 

I was referring to the claim in patch 00/32, i.e. that interrupt handlers 
only run when irqs are disabled.

> > If you put your claim to the test, you'll see that that interrupts are 
> > not disabled on m68k when interrupt handlers execute.
> 
> Sounds like an implementation issue of m68k since IRQF_DISABLED has been 
> totally removed.
> 

It's true that IRQF_DISABLED could be used to avoid the need for irq locks 
in interrupt handlers. So, if you want to remove irq locks from interrupt 
handlers, today you can't use IRQF_DISABLED to help you. So what?

> > 
> > The Interrupt Priority Level (IPL) can prevent any given irq handler 
> > from being re-entered, but an irq with a higher priority level may be 
> > handled during execution of a lower priority irq handler.
> > 
> 
> We used to have IRQF_DISABLED to support so-called "fast interrupt" to 
> avoid this. 
> 
> But the concept has been totally removed. That is interesting if m68k 
> still has this issue.
> 

Prioritized interrupts are beneficial. Why would you want to avoid them?

Moreover, there's no reason to believe that m68k is the only platform that 
supports nested interrupts.

> > sonic_interrupt() uses an irq lock within an interrupt handler to 
> > avoid issues relating to this. This kind of locking may be needed in 
> > the drivers you are trying to patch. Or it might not. Apparently, 
> > no-one has looked.
> 
> Thanks
> Barry
>