From c793a62823d1ce8f70d9cfc7803e3ea436277cda Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Mon, 27 May 2024 17:34:48 -0700 Subject: sched/core: Drop spinlocks on contention iff kernel is preemptible Use preempt_model_preemptible() to detect a preemptible kernel when deciding whether or not to reschedule in order to drop a contended spinlock or rwlock. Because PREEMPT_DYNAMIC selects PREEMPTION, kernels built with PREEMPT_DYNAMIC=y will yield contended locks even if the live preemption model is "none" or "voluntary". In short, make kernels with dynamically selected models behave the same as kernels with statically selected models. Somewhat counter-intuitively, NOT yielding a lock can provide better latency for the relevant tasks/processes. E.g. KVM x86's mmu_lock, a rwlock, is often contended between an invalidation event (takes mmu_lock for write) and a vCPU servicing a guest page fault (takes mmu_lock for read). For _some_ setups, letting the invalidation task complete even if there is mmu_lock contention provides lower latency for *all* tasks, i.e. the invalidation completes sooner *and* the vCPU services the guest page fault sooner. But even KVM's mmu_lock behavior isn't uniform, e.g. the "best" behavior can vary depending on the host VMM, the guest workload, the number of vCPUs, the number of pCPUs in the host, why there is lock contention, etc. In other words, simply deleting the CONFIG_PREEMPTION guard (or doing the opposite and removing contention yielding entirely) needs to come with a big pile of data proving that changing the status quo is a net positive. Opportunistically document this side effect of preempt=full, as yielding contended spinlocks can have significant, user-visible impact. Fixes: c597bfddc9e9 ("sched: Provide Kconfig support for default dynamic preempt mode") Signed-off-by: Sean Christopherson Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Ankur Arora Reviewed-by: Chen Yu Link: https://lore.kernel.org/kvm/ef81ff36-64bb-4cfe-ae9b-e3acf47bff24@proxmox.com --- include/linux/spinlock.h | 14 ++++++-------- 1 file changed, 6 insertions(+), 8 deletions(-) (limited to 'include/linux/spinlock.h') diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h index 3fcd20de6ca8..63dd8cf3c3c2 100644 --- a/include/linux/spinlock.h +++ b/include/linux/spinlock.h @@ -462,11 +462,10 @@ static __always_inline int spin_is_contended(spinlock_t *lock) */ static inline int spin_needbreak(spinlock_t *lock) { -#ifdef CONFIG_PREEMPTION + if (!preempt_model_preemptible()) + return 0; + return spin_is_contended(lock); -#else - return 0; -#endif } /* @@ -479,11 +478,10 @@ static inline int spin_needbreak(spinlock_t *lock) */ static inline int rwlock_needbreak(rwlock_t *lock) { -#ifdef CONFIG_PREEMPTION + if (!preempt_model_preemptible()) + return 0; + return rwlock_is_contended(lock); -#else - return 0; -#endif } /* -- cgit v1.2.3-58-ga151