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KVM: selftests: Use EPOLL in userfaultfd_util reader threads
With multiple reader threads POLLing a single UFFD, the demand paging test suffers from the thundering herd problem: performance degrades as the number of reader threads is increased. Solve this issue [1] by switching the the polling mechanism to EPOLL + EPOLLEXCLUSIVE. Also, change the error-handling convention of uffd_handler_thread_fn. Instead of just printing errors and returning early from the polling loop, check for them via TEST_ASSERT(). "return NULL" is reserved for a successful exit from uffd_handler_thread_fn, i.e. one triggered by a write to the exit pipe. Performance samples generated by the command in [2] are given below. Num Reader Threads, Paging Rate (POLL), Paging Rate (EPOLL) 1 249k 185k 2 201k 235k 4 186k 155k 16 150k 217k 32 89k 198k [1] Single-vCPU performance does suffer somewhat. [2] ./demand_paging_test -u MINOR -s shmem -v 4 -o -r <num readers> Signed-off-by: Anish Moorthy <amoorthy@google.com> Acked-by: James Houghton <jthoughton@google.com> Link: https://lore.kernel.org/r/20240215235405.368539-13-amoorthy@google.com Signed-off-by: Sean Christopherson <seanjc@google.com>
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