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

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Date:   Thu, 25 Feb 2021 10:37:26 +0800
From:   Xiaofei Tan <tanxiaofei@...wei.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
CC:     "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        scsi <linux-scsi@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        <linuxarm@...neuler.org>
Subject: Re: [Linuxarm] Re: [PATCH for-next 00/32] spin lock usage
 optimization for SCSI drivers

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@...wei.com> 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
>