linux-kernel - Re: [PATCH] sched/core: Drop spinlocks on contention iff kernel is preemptible

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Message-ID: <22def35f-5b8d-4424-a03b-c90e9174a14d@redhat.com>
Date: Mon, 13 May 2024 16:09:22 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>
Cc: linux-kernel@...r.kernel.org,
 Valentin Schneider <valentin.schneider@....com>,
 Marco Elver <elver@...gle.com>, Frederic Weisbecker <frederic@...nel.org>,
 David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] sched/core: Drop spinlocks on contention iff kernel is
 preemptible

On 1/10/24 22:47, Sean Christopherson wrote:
> 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.

Peter, looks like this patch fell through the cracks.  Could this be 
applied for 6.10?

There is a slightly confusing line in the commit message below, so that 
it reads more like an RFC; but the patch fixes a CONFIG_PREEMPT_DYNAMIC 
regression wrt static preemption models and has no functional change for 
!CONFIG_PREEMPT_DYNAMIC.

> 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
                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This should be "deleting the preempt_model_preemptible() guard" given 
that the patch does delete CONFIG_PREEMPTION, and only leaves 
preempt_model_preemptible() in place.

Thanks,

Paolo

> 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.
> 
> Cc: Valentin Schneider <valentin.schneider@....com>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Frederic Weisbecker <frederic@...nel.org>
> Cc: David Matlack <dmatlack@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   include/linux/sched.h | 14 ++++++--------
>   1 file changed, 6 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 03bfe9ab2951..51550e64ce14 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2229,11 +2229,10 @@ static inline bool preempt_model_preemptible(void)
>    */
>   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
>   }
>   
>   /*
> @@ -2246,11 +2245,10 @@ 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
>   }
>   
>   static __always_inline bool need_resched(void)
> 
> base-commit: cdb3033e191fd03da2d7da23b9cd448dfa180a8e