Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> 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.
Agreed. This behaviour makes sense. Should also be useful for PREEMPT_AUTO.
The only thing that gives me pause is that now there is an extra
call+ret even when we don't yield the lock.
But maybe that could be addressed separately by converting
preempt_model_* to use a static key or similar.
> 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")
> Link: https://lore.kernel.org/kvm/ef81ff36-64bb-4cfe-ae9b-e3acf47bff24@xxxxxxxxxxx
> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: David Matlack <dmatlack@xxxxxxxxxx>
> Cc: Friedrich Weber <f.weber@xxxxxxxxxxx>
> Cc: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
> 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 825398d66c69..fdeddb066439 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4689,7 +4689,9 @@
> 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).
This seems to read a bit better:
+ can be preempted anytime. Tasks will also yield
+ contended spinlocks (unless the critical section is
+ explicitly preempt disabled beyond the lock itself).
Ankur
> print-fatal-signals=
> [KNL] debug: print fatal signals
> 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
> }
>
> /*
