> -----Original Message----- > From: Finn Thain [mailto:fthain@xxxxxxxxxxxxxxxxxxx] > Sent: Wednesday, February 10, 2021 1:29 PM > To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx> > Cc: tanxiaofei <tanxiaofei@xxxxxxxxxx>; jejb@xxxxxxxxxxxxx; > martin.petersen@xxxxxxxxxx; linux-scsi@xxxxxxxxxxxxxxx; > linux-kernel@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx; > linux-m68k@xxxxxxxxxxxxxxx > 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? > I doubt this is true as it has been already thought as unnecessary in Linux: https://lwn.net/Articles/380931/ > Moreover, there's no reason to believe that m68k is the only platform that > supports nested interrupts. I doubt that is true as genirq is running understand the consumption that hardIRQ is running in irq-disabled context: "We run all handlers with interrupts disabled and expect them not to enable them. Warn when we catch one who does." https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b738a50a If it is, m68k is against the assumption of genirq. > > > > 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
