Re: [PATCH v2 1/2] x86/numa: Introduce numa_fill_memblks()

[Alison Schofield]

On Wed, Jun 14, 2023 at 05:44:19AM -0700, Dan Williams wrote:
> alison.schofield@ wrote:
> > From: Alison Schofield <alison.schofield@xxxxxxxxx>
> > 
> > numa_fill_memblks() fills in the gaps in numa_meminfo memblks
> > over an HPA address range.
> > 
> > The ACPI driver will use numa_fill_memblks() to implement a new Linux
> > policy that prescribes extending proximity domains in a portion of a
> > CFMWS window to the entire window.
> > 
> > Dan Williams offered this explanation of the policy:
> > A CFWMS is an ACPI data structure that indicates *potential* locations
> > where CXL memory can be placed. It is the playground where the CXL
> > driver has free reign to establish regions. That space can be populated
> > by BIOS created regions, or driver created regions, after hotplug or
> > other reconfiguration.
> > 
> > When BIOS creates a region in a CXL Window it additionally describes
> > that subset of the Window range in the other typical ACPI tables SRAT,
> > SLIT, and HMAT. The rationale for BIOS not pre-describing the entire
> > CXL Window in SRAT, SLIT, and HMAT is that it can not predict the
> > future. I.e. there is nothing stopping higher or lower performance
> > devices being placed in the same Window. Compare that to ACPI memory
> > hotplug that just onlines additional capacity in the proximity domain
> > with little freedom for dynamic performance differentiation.
> > 
> > That leaves the OS with a choice, should unpopulated window capacity
> > match the proximity domain of an existing region, or should it allocate
> > a new one? This patch takes the simple position of minimizing proximity
> > domain proliferation by reusing any proximity domain intersection for
> > the entire Window. If the Window has no intersections then allocate a
> > new proximity domain. Note that SRAT, SLIT and HMAT information can be
> > enumerated dynamically in a standard way from device provided data.
> > Think of CXL as the end of ACPI needing to describe memory attributes,
> > CXL offers a standard discovery model for performance attributes, but
> > Linux still needs to interoperate with the old regime.
> > 
> > Reported-by: Derick Marks <derick.w.marks@xxxxxxxxx>
> > Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > Signed-off-by: Alison Schofield <alison.schofield@xxxxxxxxx>
> > Tested-by: Derick Marks <derick.w.marks@xxxxxxxxx>
> [..]
> > diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
> > index 2aadb2019b4f..fa82141d1a04 100644
> > --- a/arch/x86/mm/numa.c
> > +++ b/arch/x86/mm/numa.c
> [..]
> > @@ -961,4 +962,90 @@ int memory_add_physaddr_to_nid(u64 start)
> >  	return nid;
> >  }
> >  EXPORT_SYMBOL_GPL(memory_add_physaddr_to_nid);
> > +
> > +static int __init cmp_memblk(const void *a, const void *b)
> > +{
> > +	const struct numa_memblk *ma = *(const struct numa_memblk **)a;
> > +	const struct numa_memblk *mb = *(const struct numa_memblk **)b;
> > +
> > +	if (ma->start != mb->start)
> > +		return (ma->start < mb->start) ? -1 : 1;
> > +
> > +	/* Caller handles duplicate start addresses */
> > +	return 0;
> 
> This can be simplified to:
> 
> static int __init cmp_memblk(const void *a, const void *b)
> {
> 	const struct numa_memblk *ma = *(const struct numa_memblk **)a;
> 	const struct numa_memblk *mb = *(const struct numa_memblk **)b;
> 
> 	return ma->start - mb->start;
> }

Got it.

> 
> > +}
> > +
> > +static struct numa_memblk *numa_memblk_list[NR_NODE_MEMBLKS] __initdata;
> > +
> > +/**
> > + * numa_fill_memblks - Fill gaps in numa_meminfo memblks
> > + * @start: address to begin fill
> > + * @end: address to end fill
> > + *
> > + * Find and extend numa_meminfo memblks to cover the @start-@end
> > + * HPA address range, such that the first memblk includes @start,
> > + * the last memblk includes @end, and any gaps in between are
> > + * filled.
> > + *
> > + * RETURNS:
> > + * 0		  : Success
> > + * NUMA_NO_MEMBLK : No memblk exists in @start-@end range
> > + */
> > +
> > +int __init numa_fill_memblks(u64 start, u64 end)
> > +{
> > +	struct numa_memblk **blk = &numa_memblk_list[0];
> > +	struct numa_meminfo *mi = &numa_meminfo;
> > +	int count = 0;
> > +	u64 prev_end;
> > +
> > +	/*
> > +	 * Create a list of pointers to numa_meminfo memblks that
> > +	 * overlap start, end. Exclude (start == bi->end) since
> > +	 * end addresses in both a CFMWS range and a memblk range
> > +	 * are exclusive.
> > +	 *
> > +	 * This list of pointers is used to make in-place changes
> > +	 * that fill out the numa_meminfo memblks.
> > +	 */
> 
> Thanks for this comment, looks good.
> 
> > +	for (int i = 0; i < mi->nr_blks; i++) {
> > +		struct numa_memblk *bi = &mi->blk[i];
> > +
> > +		if (start < bi->end && end >= bi->start) {
> > +			blk[count] = &mi->blk[i];
> > +			count++;
> > +		}
> > +	}
> > +	if (!count)
> > +		return NUMA_NO_MEMBLK;
> > +
> > +	/* Sort the list of pointers in memblk->start order */
> > +	sort(&blk[0], count, sizeof(blk[0]), cmp_memblk, NULL);
> > +
> > +	/* Make sure the first/last memblks include start/end */
> > +	blk[0]->start = min(blk[0]->start, start);
> > +	blk[count - 1]->end = max(blk[count - 1]->end, end);
> > +
> > +	/*
> > +	 * Fill any gaps by tracking the previous memblks end address,
> > +	 * prev_end, and backfilling to it if needed. Avoid filling
> > +	 * overlapping memblks by making prev_end monotonically non-
> > +	 * decreasing.
> 
> I am not immediately understanding the use of the term monotonically
> non-decreasing here? I think the first sentence of this comment is
> enough, or am I missing a nuance?

Not sure it's a nuance, but if we advanced prev_end to be curr_end
at each iteration, gaps get needlessly filled, when curr is wholly
contained in prev. So the 'monotonically non-decreasing' comment is
emphasizing that prev-end will either stay the same or increase
at each iteration.

> 
> > +	 */
> > +	prev_end = blk[0]->end;
> > +	for (int i = 1; i < count; i++) {
> > +		struct numa_memblk *curr = blk[i];
> > +
> > +		if (prev_end >= curr->start) {
> > +			if (prev_end < curr->end)
> > +				prev_end = curr->end;
> > +		} else {
> > +			curr->start = prev_end;
> > +			prev_end = curr->end;
> > +		}
> > +	}
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(numa_fill_memblks);
> 
> This export is not needed. The only caller of this is
> drivers/acpi/numa/srat.c which is only ever built-in, not a module.

Got it.
Thanks for the review Dan and for helping address other reviewer
comments.

Alison