TL/UCP: fix onesided a2a alg selection for 1ppn

[wfaderhold21]

What

When using 1 PPN, algorithm will return UCC_ERR_NOT_SUPPORTED. This defaults to the PUT algorithm.

[greptile-apps]

Greptile Summary

Fixed the onesided alltoall algorithm selection for 1 process per node (PPN) scenarios. Previously, when the node subgroup didn't exist (indicating 1 PPN), the function would return UCC_ERR_NOT_SUPPORTED. The fix introduces a group_size variable initialized to 1, and when the node subgroup doesn't exist with AUTO algorithm selection, it defaults to the PUT algorithm and uses group_size=1 for subsequent calculations instead of dereferencing the non-existent subgroup.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The fix is straightforward and properly handles the edge case of 1 PPN by avoiding dereferencing a non-existent subgroup. The logic correctly defaults to PUT algorithm when AUTO is selected and uses a safe default of group_size=1 for calculations.
• No files require special attention

Important Files Changed

Filename	Overview
src/components/tl/ucp/alltoall/alltoall_onesided.c	Fixed 1 PPN algorithm selection by using PUT when node subgroup doesn't exist and defaulting group_size to 1

[greptile-apps]

Additional Comments (2)

1. src/components/tl/ucp/alltoall/alltoall_onesided.c, line 282 (link)

   logic: when sbgp->status == UCC_SBGP_NOT_EXISTS (1 PPN case), sbgp->group_size is uninitialized. this line will compute ratio using garbage value, causing incorrect token calculation

2. src/components/tl/ucp/alltoall/alltoall_onesided.c, line 290-291 (link)

   logic: when sbgp->status == UCC_SBGP_NOT_EXISTS (1 PPN case), comparing uninitialized sbgp->group_size against CONGESTION_THRESHOLD will cause undefined behavior. should check sbgp status first

_{1 file reviewed, 2 comments}

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[Sergei-Lebedev]

@wfaderhold21 i think comments from greptile are relevant

[greptile-apps]

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[wfaderhold21]

/build

[Sergei-Lebedev]

/build

[Sergei-Lebedev]

/build