6.10-stable review patch. If anyone has any objections, please let me know.
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From: Sean Christopherson seanjc@google.com
[ Upstream commit c793a62823d1ce8f70d9cfc7803e3ea436277cda ]
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 seanjc@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Ankur Arora ankur.a.arora@oracle.com Reviewed-by: Chen Yu yu.c.chen@intel.com Link: https://lore.kernel.org/kvm/ef81ff36-64bb-4cfe-ae9b-e3acf47bff24@proxmox.com Signed-off-by: Sasha Levin sashal@kernel.org --- Documentation/admin-guide/kernel-parameters.txt | 4 +++- include/linux/spinlock.h | 14 ++++++-------- 2 files changed, 9 insertions(+), 9 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 27ec49af1bf27..2569e7f19b476 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4749,7 +4749,9 @@ none - Limited to cond_resched() calls voluntary - Limited to cond_resched() and might_sleep() calls full - Any section that isn't explicitly preempt disabled - can be preempted anytime. + can be preempted anytime. Tasks will also yield + contended spinlocks (if the critical section isn't + explicitly preempt disabled beyond the lock itself).
print-fatal-signals= [KNL] debug: print fatal signals diff --git a/include/linux/spinlock.h b/include/linux/spinlock.h index 3fcd20de6ca88..63dd8cf3c3c2b 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 }
/*