linux-kernel - Re: [PATCH] KVM: LAPIC: Recalculate apic map in batch

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Date:   Wed, 19 Feb 2020 08:47:56 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: LAPIC: Recalculate apic map in batch

On Tue, 18 Feb 2020 at 20:24, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Wanpeng Li <kernellwp@...il.com> writes:
>
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > In the vCPU reset and set APIC_BASE MSR path, the apic map will be recalculated
> > several times, each time it will consume 10+ us observed by ftrace in my
> > non-overcommit environment since the expensive memory allocate/mutex/rcu etc
> > operations. This patch optimizes it by recaluating apic map in batch, I hope
> > this can benefit the serverless scenario which can frequently create/destroy
> > VMs.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
>
> An alternative idea: instead of making every caller return bool and
> every call site handle the result (once) just add a
> KVM_REQ_APIC_MAP_RECALC flag or a boolean flag to struct kvm. I
> understand it may not be that easy as it sounds as we may be conunting
> on valid mapping somewhere before we actually get to handiling

Yes.

> KVM_REQ_APIC_MAP_RECALC but we may preserve *some*
> recalculate_apic_map() calls (and make it reset KVM_REQ_APIC_MAP_RECALC).

Paolo, keep the caller return bool or add a booleen flag to struct
kvm, what do you think?

    Wanpeng