linux-kernel - Re: [PATCH 2/4] modules: Ensure 64-bit alignment on __ksymtab

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Date: Mon, 22 Jan 2024 17:47:49 +0100
From: Helge Deller <deller@....de>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: deller@...nel.org, linux-kernel@...r.kernel.org,
 Masahiro Yamada <masahiroy@...nel.org>, Arnd Bergmann <arnd@...db.de>,
 linux-modules@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 2/4] modules: Ensure 64-bit alignment on __ksymtab_*
 sections

On 1/22/24 17:10, Luis Chamberlain wrote:
> On Sat, Dec 30, 2023 at 08:33:24AM +0100, Helge Deller wrote:
>> Your selftest code is based on perf.
>> AFAICS we don't have perf on parisc/hppa,
>
> I see!
>
>> so I can't test your selftest code
>> on that architecture.
>> I assume you tested on x86, where the CPU will transparently take care of
>> unaligned accesses. This is probably why the results are within
>> the noise.
>> But on some platforms the CPU raises an exception on unaligned accesses
>> and jumps into special exception handler assembler code inside the kernel.
>> This is much more expensive than on x86, which is why we track on parisc
>> in /proc/cpuinfo counters on how often this exception handler is called:
>> IRQ:       CPU0       CPU1
>>    3:       1332          0         SuperIO  ttyS0
>>    7:    1270013          0         SuperIO  pata_ns87415
>>   64:  320023012  320021431             CPU  timer
>>   65:   17080507   20624423             CPU  IPI
>> UAH:   10948640      58104   Unaligned access handler traps
>>
>> This "UAH" field could theoretically be used to extend your selftest.
>
> Nice!
>
>> But is it really worth it? The outcome is very much architecture and CPU
>> specific, maybe it's just within the noise as you measured.
>
> It's within the noise for x86_64, but given what you suggest
> for parisc where it is much more expensive, we should see a non-noise
> delta. Even just time on loading the module should likely result in
> a considerable delta than on x86_64. You may just need to play a bit
> with the default values at build time.

I don't know if it will be a "considerable" amount of time.

>> IMHO we should always try to natively align structures, and if we see
>> we got it wrong in kernel code, we should fix it.
>
> This was all motivated by the first review criteria of these patches
> as if they were stable worthy or not. Even if we don't consider them
> stable material, given the test is now written and easily extended to
> test on parisc with just timing information and UAH I think it would
> be nice to have this data for a few larger default factor values so we
> can compare against x86_64 while we're at it.
>
> If you don't feel like doing that test that's fine too, we can just
> ignore that.

I can do that test, but I won't have time for that in the next few weeks...

> I'll still apply the patches
Yes, please do!
Even if I don't test now, I (or others) will test at a later point.

> but, I figured I'd ask to collect information while the test was already
> written and it should now be easy to compare / contrast differences.
Ok.

Helge