Module Name: src Committed By: riastradh Date: Thu May 25 14:30:03 UTC 2023
Modified Files: src/lib/libpthread: pthread_int.h pthread_spin.c src/lib/libpthread/arch/aarch64: pthread_md.h src/lib/libpthread/arch/arm: pthread_md.h src/lib/libpthread/arch/i386: pthread_md.h src/lib/libpthread/arch/x86_64: pthread_md.h Log Message: libpthread: New pthread__smt_wait to put CPU in low power for spin. This is now distinct from pthread__smt_pause, which is for spin lock backoff with no paired wakeup. On Arm, there is a single-bit event register per CPU, and there are two instructions to manage it: - wfe, wait for event -- if event register is clear, enter low power mode and wait until event register is set; then exit low power mode and clear event register - sev, signal event -- sets event register on all CPUs (other circumstances like interrupts also set the event register and cause wfe to wake) These can be used to reduce the power consumption of spinning for a lock, but only if they are actually paired -- if there's no sev, wfe might hang indefinitely. Currently only pthread_spin(3) actually pairs them; the other lock primitives (internal lock, mutex, rwlock) do not -- they have spin lock backoff loops, but no corresponding wakeup to cancel a wfe. It may be worthwhile to teach the other lock primitives to pair wfe/sev, but that requires some performance measurement to verify it's actually worthwhile. So for now, we just make sure not to use wfe when there's no sev, and keep everything else the same -- this should fix severe performance degredation in libpthread on Arm without hurting anything else. No change in the generated code on amd64 and i386. No change in the generated code for pthread_spin.c on arm and aarch64 -- changes only the generated code for pthread_lock.c, pthread_mutex.c, and pthread_rwlock.c, as intended. PR port-arm/57437 XXX pullup-10 To generate a diff of this commit: cvs rdiff -u -r1.110 -r1.111 src/lib/libpthread/pthread_int.h cvs rdiff -u -r1.10 -r1.11 src/lib/libpthread/pthread_spin.c cvs rdiff -u -r1.1 -r1.2 src/lib/libpthread/arch/aarch64/pthread_md.h cvs rdiff -u -r1.12 -r1.13 src/lib/libpthread/arch/arm/pthread_md.h cvs rdiff -u -r1.20 -r1.21 src/lib/libpthread/arch/i386/pthread_md.h cvs rdiff -u -r1.12 -r1.13 src/lib/libpthread/arch/x86_64/pthread_md.h Please note that diffs are not public domain; they are subject to the copyright notices on the relevant files.