Re: [PATCH v3 4/4] x86/folio_zero_user: multi-page clearing

[Ingo Molnar]

* Ankur Arora <ankur.a.arora@xxxxxxxxxx> wrote:

> clear_pages_rep(), clear_pages_erms() use string instructions to zero
> memory. When operating on more than a single page, we can use these
> more effectively by explicitly advertising the region-size to the
> processor, which can use that as a hint to optimize the clearing
> (ex. by eliding cacheline allocation.)
> 
> As a secondary benefit, string instructions are typically microcoded,
> and working with larger regions helps amortize the cost of the decode.

Not just the decoding, but also iterations around page-sized chunks are 
not cheap these days: there's various compiler generated mitigations 
and other overhead that applies on a typical kernel, and using larger 
sizes amortizes that per-page-iteration setup cost.

> When zeroing the 2MB page, maximize spatial locality by clearing in 
> three sections: the faulting page and its immediate neighbourhood, the
> left and the right regions, with the local neighbourhood cleared last.

s/zeroing the 2MB page
 /zeroing a 2MB page


> It's not entirely clear why the performance for pg-sz=2MB improves. 
> We decode fewer instructions and the hardware prefetcher can do a 
> better job, but the perf stats for both of those aren't convincing 
> enough to the extent of ~30%.

s/why the performance
 /why performance

> For both page-sizes, Icelakex, behaves similarly to Milan pg-sz=2MB: we
> see a drop in cycles but there's no drop in cacheline allocation.

s/Icelakex, behaves similarly
 /Icelakex behaves similarly

> Performance for preempt=none|voluntary remains unchanged.

CONFIG_PREEMPT_VOLUNTARY=y is the default on a number of major 
distributions, such as Ubuntu, and a lot of enterprise distro kernels - 
and this patch does nothing for them, for no good reason.

So could you please provide a sensible size granularity cutoff of 16MB 
or so on non-preemptible kernels, instead of this weird build-time 
all-or-nothing binary cutoff based on preemption modes?

On preempt=full/lazy the granularity limit would be infinite.

I.e the only code dependent on the preemption mode should be the size 
cutoff/limit.

On full/lazy preemption the code would, ideally, compile to something 
close to your current code.

> +obj-$(CONFIG_PREEMPTION)	+= memory.o

> +#ifndef CONFIG_HIGHMEM
> +/*
> + * folio_zero_user_preemptible(): multi-page clearing variant of folio_zero_user().

We don't care much about HIGHMEM these days I suppose, but this 
dependency still feels wrong. Is this a stealth dependency on x86-64, 
trying to avoid a new arch Kconfig for this new API, right? ;-)

Thanks,

	Ingo