Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII;
 format=flowed
Content-Transfer-Encoding: 7bit
Date:   Wed, 22 Apr 2020 11:14:28 +0100
From:   Marc Zyngier <maz@misterjones.org>
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     tglx@linutronix.de, pbonzini@redhat.com, kvm@vger.kernel.org,
        Davidlohr Bueso <dbueso@suse.de>, peterz@infradead.org,
        torvalds@linux-foundation.org, bigeasy@linutronix.de,
        linux-kernel@vger.kernel.org, rostedt@goodmis.org,
        linux-mips@vger.kernel.org, Paul Mackerras <paulus@ozlabs.org>,
        joel@joelfernandes.org, will@kernel.org,
        kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH 4/5] kvm: Replace vcpu->swait with rcuwait
In-Reply-To: <20200422040739.18601-5-dave@stgolabs.net>
References: <20200422040739.18601-1-dave@stgolabs.net>
 <20200422040739.18601-5-dave@stgolabs.net>
Message-ID: <a6e848e46f3c865f6b9d2771c8aad37a@misterjones.org>
User-Agent: Roundcube Webmail/1.3.10
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 2020-04-22 05:07, Davidlohr Bueso wrote:
> The use of any sort of waitqueue (simple or regular) for
> wait/waking vcpus has always been an overkill and semantically
> wrong. Because this is per-vcpu (which is blocked) there is
> only ever a single waiting vcpu, thus no need for any sort of
> queue.
> 
> As such, make use of the rcuwait primitive, with the following
> considerations:
> 
>   - rcuwait already provides the proper barriers that serialize
>   concurrent waiter and waker.
> 
>   - Task wakeup is done in rcu read critical region, with a
>   stable task pointer.
> 
>   - Because there is no concurrency among waiters, we need
>   not worry about rcuwait_wait_event() calls corrupting
>   the wait->task. As a consequence, this saves the locking
>   done in swait when modifying the queue. This also applies
>   to per-vcore wait for powerpc kvm-hv.
> 
> The x86 tscdeadline_latency test mentioned in 8577370fb0cb
> ("KVM: Use simple waitqueue for vcpu->wq") shows that, on avg,
> latency is reduced by around 15-20% with this change.
> 
> Cc: Paul Mackerras <paulus@ozlabs.org>
> Cc: kvmarm@lists.cs.columbia.edu
> Cc: linux-mips@vger.kernel.org
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

Reviewed-by: Marc Zyngier <maz@kernel.org>

Thanks,

         M.
-- 
Who you jivin' with that Cosmik Debris?