Date:   Tue, 14 Feb 2023 16:49:51 +0000
Message-ID: <86zg9gxkcw.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Usama Arif <usama.arif@bytedance.com>
Cc:     linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
        linux-doc@vger.kernel.org,
        virtualization@lists.linux-foundation.org, linux@armlinux.org.uk,
        yezengruan@huawei.com, catalin.marinas@arm.com, will@kernel.org,
        steven.price@arm.com, mark.rutland@arm.com, bagasdotme@gmail.com,
        pbonzini@redhat.com, fam.zheng@bytedance.com, liangma@liangbit.com,
        punit.agrawal@bytedance.com
Subject: Re: [v3 0/6] KVM: arm64: implement vcpu_is_preempted check
In-Reply-To: <5976b0c9-d4e7-7561-6ce0-790e2460d1ef@bytedance.com>
References: <20230117102930.1053337-1-usama.arif@bytedance.com>
        <5976b0c9-d4e7-7561-6ce0-790e2460d1ef@bytedance.com>
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On Tue, 14 Feb 2023 16:06:26 +0000,
Usama Arif <usama.arif@bytedance.com> wrote:
> 
> 
> 
> On 17/01/2023 10:29, Usama Arif wrote:
> > This patchset adds support for vcpu_is_preempted in arm64, which allows the guest
> > to check if a vcpu was scheduled out, which is useful to know incase it was
> > holding a lock. vcpu_is_preempted is well integrated in core kernel code and can
> > be used to improve performance in locking (owner_on_cpu usage in mutex_spin_on_owner,
> > mutex_can_spin_on_owner, rtmutex_spin_on_owner and osq_lock) and scheduling
> > (available_idle_cpu which is used in several places in kernel/sched/fair.c
> > for e.g. in wake_affine to determine which CPU can run soonest).
> > 
> > This patchset shows significant improvement on overcommitted hosts (vCPUs > pCPUS),
> > as waiting for preempted vCPUs reduces performance.
> > 
> 
> Hi,
> 
> Just wanted to check if there are any comments for this?

Not a lot, I'm afraid. My concerns with this thing are still the same:

- it is KVM-specific
- it doesn't work with nested virtualisation
- its correctness is unproven on arm64

I'm also not going to entertain any of this without the core arm64
maintainers saying that they will enable this.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.