Re: [PATCH v3 04/51] PCI: Optimize bus align/size calculation during sizing

[Bjorn Helgaas]

On Mon, Jul 27, 2015 at 04:29:22PM -0700, Yinghai Lu wrote:
> Current code try to get align as small as possible and use that to
> align final size. But it does not handle resource that size is bigger
> than align in optimal way, kernel only use max align for them.
> 
> For example:
>  when we have resources with align/size: 1M/2M, 512M/512M,
>    bus resource min_align/size0 will be 512M/1024M,
>    but optimal value should be 256M/768M.

I want to understand what makes 256M/768M "optimal."

This is basically a bin packing problem, and I'd like to have an
explicit statement of the constraints and the goal.  Right now the
goal is fuzzy and the code seems ad hoc.

Here are the possibilities I can see.  The placement notes are the
offsets into the allocated space:

  align  size    512M placement   2M placement    unused
  1024M  514M    [0-512]          [512-514]       [514-1024]
   512M  514M    [0-512]          [512-514]       none
   256M  768M    [256-768]        [254-256]       [0-254]
   128M  896M    [384-896]        [382-384]       [0-382]

Obviously, we would never choose 1024M/514M or any larger alignment:
it requires no more space than 512M/514M, but it fails in some cases
where 512M/514M would succeed.

Also obviously, we would never choose 128M/896M or a smaller
alignment: if we could get that, we could also get 256M/768M and it
would consume less space.

But it's not quite so obvious how to choose between 512M/514M and
256M/768M.  The former (if we can get it) consumes much less space.
The latter requires less alignment but wastes a lot of space.

> For following cases that we have resource size that is bigger
> than resource alignment:
> 1. SRIOV bar.
> 2. PCI bridges with several bridges or devices as children.

This doesn't have anything to do with how many children a bridge has,
does it?  As long as there are two or more BARs (1MB or larger) below
a bridge, we'll have the situation where the bridge window has to
contain both BARs but only needs to be aligned for (at most) the
larger one.

> We can keep on trying to allocate children devices resources under range
> [half_align, half_align + aligned_size).
> If sucesses, we can use that half_align as new min_align.
> 
> After this patch, we get:
>  align/size: 1M/2M, 2M/4M, 4M/8M, 8M/16M
>  new min_align/min_size: 4M/32M, and old is 8M/32M
> 
>  align/size: 1M/2M, 2M/4M, 4M/8M
>  new min_align/min_size: 2M/14M, and old is 4M/16M
> 
>  align/size: 1M/2M, 512M/512M
>  new min_align/min_size: 256M/768M, and old is 512M/1024M
> 
> The real result from one system with one pcie card that has
> four functions that support sriov:
>  align/size:
>    00800000/00800000
>    00800000/00800000
>    00800000/00800000
>    00800000/00800000
>    00010000/00200000
>    00010000/00200000
>    00010000/00200000
>    00010000/00200000
>    00008000/00008000
>    00008000/00008000
>    00008000/00008000
>    00008000/00008000
>    00004000/00080000
>    00004000/00080000
>    00004000/00080000
>    00004000/00080000
>  old min_align/min_size: 00400000/02c00000
>      min_align/min_size: 00100000/02b00000

I don't know if this example is essential, but if it is, it needs a
little more interpretation.  I assume the BARs above where
"align < size" are for SR-IOV, where size include multiple VFs.
In any case, please connect the dots from the raw data to the
conclusion.

> So align will be 1M instead of 4M.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=81431
> Reported-by: TJ <linux@xxxxxx>
> Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> ---
>  drivers/pci/setup-bus.c | 195 ++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 157 insertions(+), 38 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index 27cb0f0..ecdf011 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -30,6 +30,34 @@
>  
>  unsigned int pci_flags;
>  
> +static inline bool is_before(resource_size_t align1, resource_size_t size1,
> +			     resource_size_t align2, resource_size_t size2)

Can this take two struct resource pointers instead of four numbers?  I
haven't looked at all the callers, so maybe there's a caller that
doesn't have a struct resource.

> +{
> +	resource_size_t size1_left, size2_left;
> +
> +	/* big align is before small align */
> +	if (align1 > align2)
> +		return true;
> +
> +	/*
> +	 * for same align:
> +	 *   aligned is before not aligned
> +	 *   for not aligned, big remainder is before small remainder
> +	 */
> +	if (align1 == align2) {
> +		size1_left = size1 & (align1 - 1);
> +		if (!size1_left)
> +			size1_left = align1;
> +		size2_left = size2 & (align2 - 1);
> +		if (!size2_left)
> +			size2_left = align2;
> +		if (size1_left > size2_left)
> +			return true;
> +	}
> +
> +	return false;
> +}
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