Set GLIBC_TUNABLES=glibc.pthread.rseq=0 on buildvariants with glibc >= 2.34. Buildvariants with this version or newer register custom rseq ABI behavior that is incompatible with the new TCMalloc, and will cause TCMalloc's rseq functionality to break and fall back to the per-thread cache behavior. Buildvariants with an older glibc version will successfully use TCMalloc's per-CPU caches. We must ensure this environment variable is set on problematic systems.

Context on the situation here.