Re: [PATCH v3 2/3] x86: probe memory block size advisement value during mm init

[Gregory Price <gourry@gourry.net>]

On Tue, Oct 29, 2024 at 01:40:38PM +0100, David Hildenbrand wrote:
> On 22.10.24 23:34, Gregory Price wrote:
> > Systems with hotplug may provide an advisement value on what the
> > memblock size should be.  Probe this value when the rest of the
> > configuration values are considered.
> > 
> > The new heuristic is as follows
> > 
> > 1) set_memory_block_size_order value if already set (cmdline param)
> > 2) minimum block size if memory is less than large block limit
> > 3) if no hotplug advice: Max block size if system is bare-metal,
> >     otherwise use end of memory alignment.
> > 4) if hotplug advice: lesser of advice and end of memory alignment.
> > 
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Gregory Price <gourry@gourry.net>
> > ---
> >   arch/x86/mm/init_64.c | 14 +++++++++-----
> >   1 file changed, 9 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> > index ff253648706f..93d669f467f7 100644
> > --- a/arch/x86/mm/init_64.c
> > +++ b/arch/x86/mm/init_64.c
> > @@ -1452,13 +1452,17 @@ static unsigned long probe_memory_block_size(void)
> >   	}
> >   	/*
> > -	 * Use max block size to minimize overhead on bare metal, where
> > -	 * alignment for memory hotplug isn't a concern.
> > +	 * When hotplug alignment is not a concern, maximize blocksize
> > +	 * to minimize overhead. Otherwise, align to the lesser of advice
> > +	 * alignment and end of memory alignment.
> >   	 */
> > -	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
> > +	bz = memory_block_probe_max_size();
> > +	if (!bz) {
> >   		bz = MAX_BLOCK_SIZE;
> > -		goto done;
> > -	}
> > +		if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
> > +			goto done;
> > +	} else
> > +		bz = max(min(bz, MAX_BLOCK_SIZE), MIN_MEMORY_BLOCK_SIZE);
> >   	/* Find the largest allowed block size that aligns to memory end */
> >   	for (bz = MAX_BLOCK_SIZE; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
        ^^^^^^^^^^^^^^^^^^^^^^^^
> 
> 
> Acked-by: David Hildenbrand <david@redhat.com>

Will pick this up but wanted to point out the silly bug above.
This version completely ignores the advise lol.

Changing to below

diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index 93d669f467f7..01876629f21f 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -1465,7 +1465,7 @@ static unsigned long probe_memory_block_size(void)
                bz = max(min(bz, MAX_BLOCK_SIZE), MIN_MEMORY_BLOCK_SIZE);

        /* Find the largest allowed block size that aligns to memory end */
-       for (bz = MAX_BLOCK_SIZE; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
+       for (; bz > MIN_MEMORY_BLOCK_SIZE; bz >>= 1) {
                if (IS_ALIGNED(boot_mem_end, bz))
                        break;
        }

> 
> -- 
> Cheers,
> 
> David / dhildenb
>