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

[Xiaofei Tan]

Hi Geert,

On 2021/2/24 17:41, Geert Uytterhoeven wrote:
> Hi Xiaofei,
>
> On Sun, Feb 7, 2021 at 12:46 PM Xiaofei Tan <tanxiaofei@xxxxxxxxxx> 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.
>
> I'll bite: how much does this speed up interrupt processing?
> What's the typical cost of saving/disabling, and restoring interrupt
> state?

It could only take a few CPU cycles. So there is little benefit for 
speeding up interrupt processing.You could take them as cleanup.

Is removing this cost worth the risk of introducing subtle
> regressions on platforms you cannot test yourself?

Currently, only found M68K platform support that high-priority interrupt 
preempts low-priority. No other platform has such services. Therefore, 
these changes do not affect non-M68K platforms.

For M68K platform, no one report such interrupt preemption case in these 
SCSI drivers.

> BTW, how many of these legacy SCSI controllers do you have access to?

Actually, no.

> Thanks for your answers!
>
> Gr{oetje,eeting}s,
>
>                         Geert