Re: [PATCH v8 5/7] x86/clear_page: Introduce clear_pages()

[Ankur Arora <ankur.a.arora@oracle.com>]

Borislav Petkov <bp@alien8.de> writes:

> On Mon, Oct 27, 2025 at 01:21:07PM -0700, Ankur Arora wrote:
>> Also define ARCH_PAGE_CONTIG_NR to specify the maximum contiguous page
>> range that can be zeroed when running under cooperative preemption
>> models. This limits the worst case preemption latency.
>
> Please do not explain what the patch does in the commit message - that should
> be clear from the diff itself. Rather, concentrate on why this patch exists.

Ack.

>> +/*
>> + * When running under cooperatively scheduled preemption models limit the
>> + * maximum contiguous extent that can be cleared to pages worth 8MB.
>
> Why?

Will mention that the point is to minimize worst case preemption latency.

>> + *
>> + * With a clearing BW of ~10GBps, this should result in worst case scheduling
>
> This sounds like you have this bandwidth (please write it out - we have more
> than enough silly abbreviations) on *everything* x86 the kernel runs on. Which
> simply ain't true.
>
>> + * latency of ~1ms.
>> + */
>> +#define ARCH_PAGE_CONTIG_NR (8 << (20 - PAGE_SHIFT))
>
> And so this looks like some magic number which makes sense only on some
> uarches but most likely it doesn't on others.

The intent was to use a large enough value that enables uarchs which do
'REP; STOS' optimizations, but not too large so we end up with high
preemption latency.

> Why isn't this thing determined dynamically during boot or so, instead of
> hardcoding it this way and then having to change it again later when bandwidth
> increases?

I thought of doing that but given that the precise value doesn't matter
very much (and there's enough slack in it in either direction) it seemed
unnecessary to do at this point.

Also, I'm not sure that a boot determined value would really help given
that the 'REP; STOS' bandwidth could be high or low based on how
saturated the bus is.

Clearly some of this detail should have been in my commit message.

Let me add it there.

Thanks

--
ankur