linux-kernel - Re: [PATCH V4 8/8] riscv: Add config of thread stack size

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Date:   Sat, 10 Sep 2022 18:06:55 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     guoren <guoren@...nel.org>
Cc:     "Palmer Dabbelt" <palmer@...osinc.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Andy Lutomirski" <luto@...nel.org>,
        "Conor.Dooley" <conor.dooley@...rochip.com>,
        Heiko Stübner <heiko@...ech.de>,
        "Jisheng Zhang" <jszhang@...nel.org>, lazyparser@...il.com,
        falcon@...ylab.org, "Huacai Chen" <chenhuacai@...nel.org>,
        "Anup Patel" <apatel@...tanamicro.com>,
        "Atish Patra" <atishp@...shpatra.org>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        "Paul Walmsley" <paul.walmsley@...ive.com>,
        "Sebastian Andrzej Siewior" <bigeasy@...utronix.de>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        "Guo Ren" <guoren@...ux.alibaba.com>,
        "Andreas Schwab" <schwab@...e.de>
Subject: Re: [PATCH V4 8/8] riscv: Add config of thread stack size

On Sat, Sep 10, 2022, at 2:52 PM, Guo Ren wrote:
> On Thu, Sep 8, 2022 at 3:37 PM Arnd Bergmann <arnd@...db.de> wrote:
>> On Thu, Sep 8, 2022, at 4:25 AM, guoren@...nel.org wrote:
>> > From: Guo Ren <guoren@...ux.alibaba.com>
>> - When VMAP_STACK is set, make it possible to select non-power-of-two
>>   stack sizes. Most importantly, 12KB should be a really interesting
>>   choice as 8KB is probably still not enough for many 64-bit workloads,
>>   but 16KB is often more than what you need. You probably don't
>>   want to allow 64BIT/8KB without VMAP_STACK anyway since that just
>>   makes it really hard to debug, so hiding the option when VMAP_STACK
>>   is disabled may also be a good idea.
> I don't want this config to depend on VMAP_STACK. Some D1 chips would
> run with an 8K stack size and !VMAP_STACK.

That sounds like a really bad idea, why would you want to risk
using such a small stack without CONFIG_VMAP_STACK?

Are you worried about increased memory usage or something else?

>  /* thread information allocation */
> -#ifdef CONFIG_64BIT
> -#define THREAD_SIZE_ORDER      (2 + KASAN_STACK_ORDER)
> -#else
> -#define THREAD_SIZE_ORDER      (1 + KASAN_STACK_ORDER)
> -#endif
> +#define THREAD_SIZE_ORDER      CONFIG_THREAD_SIZE_ORDER
>  #define THREAD_SIZE            (PAGE_SIZE << THREAD_SIZE_ORDER)

This doesn't actually allow additional THREAD_SIZE values, as you
still round up to the nearest power of two.

I think all the non-arch code can deal with non-power-of-2
sizes, so you'd just need

#define THREAD_SIZE round_up(CONFIG_THREAD_SIZE, PAGE_SIZE)

and fix up the risc-v specific code to do the right thing
as well. I now see that THREAD_SIZE_ORDER is not actually
used anywhere with CONFIG_VMAP_STACK, so I suppose that
definition can be skipped, but you still need a THREAD_ALIGN
definition that is a power of two and at least a page larger
than THREAD_SIZE.

     Arnd