linux-kernel - Re: [PATCH v2 06/28] locking/rwlocks: Add contention detection for rwlocks

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Message-ID: <6287ff89-d869-e5ed-3e64-11621cc4796a@redhat.com>
Date:   Tue, 9 Feb 2021 22:32:28 -0500
From:   Waiman Long <longman@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH v2 06/28] locking/rwlocks: Add contention detection for
 rwlocks

On 2/2/21 1:57 PM, Ben Gardon wrote:
> rwlocks do not currently have any facility to detect contention
> like spinlocks do. In order to allow users of rwlocks to better manage
> latency, add contention detection for queued rwlocks.
>
> CC: Ingo Molnar <mingo@...hat.com>
> CC: Will Deacon <will@...nel.org>
> Acked-by: Peter Zijlstra <peterz@...radead.org>
> Acked-by: Davidlohr Bueso <dbueso@...e.de>
> Acked-by: Waiman Long <longman@...hat.com>
> Acked-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
>   include/asm-generic/qrwlock.h | 24 ++++++++++++++++++------
>   include/linux/rwlock.h        |  7 +++++++
>   2 files changed, 25 insertions(+), 6 deletions(-)
>
> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 84ce841ce735..0020d3b820a7 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -14,6 +14,7 @@
>   #include <asm/processor.h>
>   
>   #include <asm-generic/qrwlock_types.h>
> +#include <asm-generic/qspinlock.h>

As said in another thread, qspinlock and qrwlock can be independently 
enabled for an architecture. So we shouldn't include qspinlock.h here. 
Instead, just include the regular linux/spinlock.h file to make sure 
that arch_spin_is_locked() is available.


>   
>   /*
>    * Writer states & reader shift and bias.
> @@ -116,15 +117,26 @@ static inline void queued_write_unlock(struct qrwlock *lock)
>   	smp_store_release(&lock->wlocked, 0);
>   }
>   
> +/**
> + * queued_rwlock_is_contended - check if the lock is contended
> + * @lock : Pointer to queue rwlock structure
> + * Return: 1 if lock contended, 0 otherwise
> + */
> +static inline int queued_rwlock_is_contended(struct qrwlock *lock)
> +{
> +	return arch_spin_is_locked(&lock->wait_lock);
> +}
> +
>   /*
>    * Remapping rwlock architecture specific functions to the corresponding
>    * queue rwlock functions.
>    */
> -#define arch_read_lock(l)	queued_read_lock(l)
> -#define arch_write_lock(l)	queued_write_lock(l)
> -#define arch_read_trylock(l)	queued_read_trylock(l)
> -#define arch_write_trylock(l)	queued_write_trylock(l)
> -#define arch_read_unlock(l)	queued_read_unlock(l)
> -#define arch_write_unlock(l)	queued_write_unlock(l)
> +#define arch_read_lock(l)		queued_read_lock(l)
> +#define arch_write_lock(l)		queued_write_lock(l)
> +#define arch_read_trylock(l)		queued_read_trylock(l)
> +#define arch_write_trylock(l)		queued_write_trylock(l)
> +#define arch_read_unlock(l)		queued_read_unlock(l)
> +#define arch_write_unlock(l)		queued_write_unlock(l)
> +#define arch_rwlock_is_contended(l)	queued_rwlock_is_contended(l)
>   
>   #endif /* __ASM_GENERIC_QRWLOCK_H */
> diff --git a/include/linux/rwlock.h b/include/linux/rwlock.h
> index 3dcd617e65ae..7ce9a51ae5c0 100644
> --- a/include/linux/rwlock.h
> +++ b/include/linux/rwlock.h
> @@ -128,4 +128,11 @@ do {								\
>   	1 : ({ local_irq_restore(flags); 0; }); \
>   })
>   
> +#ifdef arch_rwlock_is_contended
> +#define rwlock_is_contended(lock) \
> +	 arch_rwlock_is_contended(&(lock)->raw_lock)
> +#else
> +#define rwlock_is_contended(lock)	((void)(lock), 0)
> +#endif /* arch_rwlock_is_contended */
> +
>   #endif /* __LINUX_RWLOCK_H */

Cheers,
Longman