Re: [PATCH v3 0/2] Ajust lockdep static allocations for sparc

[Geert Uytterhoeven]

Hi Peter,

On Tue, Nov 29, 2016 at 12:41 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Nov 29, 2016 at 12:14:48PM +0100, Geert Uytterhoeven wrote:
>> CC linux-renesas-soc
>>
>> On Tue, Sep 27, 2016 at 9:33 PM, Babu Moger <babu.moger@xxxxxxxxxx> wrote:
>> > These patches limit the static allocations for lockdep data structures
>> > used for debugging locking correctness. For sparc, all the kernel's code,
>> > data, and bss, must have locked translations in the TLB so that we don't
>> > get TLB misses on kernel code and data. Current sparc chips have 8 TLB
>> > entries available that may be locked down, and with a 4mb page size,
>> > this gives a maximum of 32MB. With PROVE_LOCKING we could go over this
>> > limit and cause system boot-up problems. These patches limit the static
>> > allocations so that everything fits in current required size limit.
>> >
>> > patch 1 : Adds new config parameter CONFIG_PROVE_LOCKING_SMALL
>> > Patch 2 : Adjusts the sizes based on the new config parameter
>>
>> Cool, this is also useful on other platforms!
>> E.g. on r8a7791/koelsch, I cannot boot an shmobile_defconfig plus
>> CONFIG_PROVE_LOCKING kernel.
>> Forcing CONFIG_PROVE_LOCKING_SMALL=y in Kconfig fixes that.
>>
>> Should it become a user-selectable symbol?
>
> Nah, users don't need more senseless options. This is really only useful
> for dinky platforms or platforms with limited static image size (like
> sparc64).
>
> If you make this user selectable, someone will do, and then an endless
> stream of table not big enough warnings will be posted.
>
> Also, its only 4MB (IIRC), so who cares.

I care :-)

Not because of platforms with not limited memory, but because of platforms
with boot loaders that have silly kernel size limitations, and start
scribbling over the DTB or even theirselves when copying a large kernel image.

BTW, is there any particular reason these huge arrays are in BSS, and not
allocated dynamically? That would solve my problems as well...

Thanks!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds