linux-kernel - Re: Proposal: HAVE_SEPARATE_IRQ

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Message-ID: <db056fb5-82b3-c17e-46ce-263872ef7334@imgtec.com>
Date:   Thu, 10 Nov 2016 16:36:10 +0000
From:   Matt Redfearn <matt.redfearn@...tec.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     LKML <linux-kernel@...r.kernel.org>, <linux-mips@...ux-mips.org>,
        <linux-mm@...ck.org>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>,
        <k@...ka.home.kg>
Subject: Re: Proposal: HAVE_SEPARATE_IRQ_STACK?

Hi Jason,


On 10/11/16 11:41, Jason A. Donenfeld wrote:
> On Thu, Nov 10, 2016 at 10:03 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>> If you want to go with that config, then you need
>> local_bh_disable()/enable() to fend softirqs off, which disables also
>> preemption.
> Thanks. Indeed this is what I want.
>
>>> What clever tricks do I have at my disposal, then?
>> Make MIPS use interrupt stacks.
> Yea, maybe I'll just implement this. It clearly is the most correct solution.
> @MIPS maintainers: would you merge something like this if done well?
> Are there reasons other than man-power why it isn't currently that
> way?

I don't see a reason not to do this - I'm taking a look into it.

Thanks,
Matt

>> Does the slowdown come from the kmalloc overhead or mostly from the less
>> efficient code?
>>
>> If it's mainly kmalloc, then you can preallocate the buffer once for the
>> kthread you're running in and be done with it. If it's the code, then bad
>> luck.
> I fear both. GCC can optimize stack variables in ways that it cannot
> optimize various memory reads and writes.
>
> Strangely, the solution that appeals to me most at the moment is to
> kmalloc (or vmalloc?) a new stack, copy over thread_info, and fiddle
> with the stack registers. I don't see any APIs, however, for a
> platform independent way of doing this. And maybe this is a horrible
> idea. But at least it'd allow me to keep my stack-based code the
> same...
>