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bh=0gXXVZ1M1SPzD/8mviUXXiKMlrNVIcsqsHOKxhlwRek=; b=G1PA1ucuiB4g1VrcrH6nFW0KnMDWQ55BP1PKevICK+aLU4sOkU4f563k3jaurGpXP2OG4L 6K61BlzVfhaiz+y+IkuTt9dVckhXsmNWNuntz7ftup5bsyHbkQW7M6U8C/3F/bGtSut2P6 3SxM7xGGcUz4pfEc7kKVGTjbWmk5UxE= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-395-UU3N_ykSOO6SiL7u12Utfg-1; Wed, 27 Oct 2021 10:36:46 -0400 X-MC-Unique: UU3N_ykSOO6SiL7u12Utfg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1095C112C391; Wed, 27 Oct 2021 14:35:56 +0000 (UTC) Received: from starship (unknown [10.40.194.243]) by smtp.corp.redhat.com (Postfix) with ESMTP id 83585794A9; Wed, 27 Oct 2021 14:35:46 +0000 (UTC) Message-ID: <3a0c3397302d59ea313e079435a18bf1b9a43474.camel@redhat.com> Subject: Re: [PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline From: Maxim Levitsky To: Paolo Bonzini , Sean Christopherson , Marc Zyngier , Huacai Chen , Aleksandar Markovic , Paul Mackerras , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank Date: Wed, 27 Oct 2021 17:35:45 +0300 In-Reply-To: <0072221e-02e8-4d60-9b0f-80d8c423bf4e@redhat.com> References: <20211009021236.4122790-1-seanjc@google.com> <20211009021236.4122790-17-seanjc@google.com> <0072221e-02e8-4d60-9b0f-80d8c423bf4e@redhat.com> User-Agent: Evolution 3.36.5 (3.36.5-2.fc32) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mailman-Approved-At: Wed, 27 Oct 2021 12:49:56 -0400 Cc: Wanpeng Li , kvm@vger.kernel.org, David Hildenbrand , linux-kernel@vger.kernel.org, Atish Patra , linux-riscv@lists.infradead.org, Claudio Imbrenda , kvmarm@lists.cs.columbia.edu, Joerg Roedel , kvm-ppc@vger.kernel.org, David Matlack , linux-arm-kernel@lists.infradead.org, Jim Mattson , Cornelia Huck , linux-mips@vger.kernel.org, kvm-riscv@lists.infradead.org, Vitaly Kuznetsov X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Mon, 2021-10-25 at 16:26 +0200, Paolo Bonzini wrote: > On 09/10/21 04:12, Sean Christopherson wrote: > > Calculate the halt-polling "stop" time using "cur" instead of redoing > > ktime_get(). In the happy case where hardware correctly predicts > > do_halt_poll, "cur" is only a few cycles old. And if the branch is > > mispredicted, arguably that extra latency should count toward the > > halt-polling time. > > > > In all likelihood, the numbers involved are in the noise and either > > approach is perfectly ok. > > Using "start" makes the change even more obvious, so: > > Calculate the halt-polling "stop" time using "start" instead of redoing > ktime_get(). In practice, the numbers involved are in the noise (e.g., > in the happy case where hardware correctly predicts do_halt_poll and > there are no interrupts, "start" is probably only a few cycles old) > and either approach is perfectly ok. But it's more precise to count > any extra latency toward the halt-polling time. > > Paolo > Agreed. Reviewed-by: Maxim Levitsky _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm