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Wed, 24 Jan 2024 12:14:41 GMT Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 16EB520040; Wed, 24 Jan 2024 12:14:41 +0000 (GMT) Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B5B1220043; Wed, 24 Jan 2024 12:14:40 +0000 (GMT) Received: from [9.152.224.222] (unknown [9.152.224.222]) by smtpav07.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 24 Jan 2024 12:14:40 +0000 (GMT) Message-ID: Date: Wed, 24 Jan 2024 13:14:40 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v2 0/4] KVM: irqchip: synchronize srcu only if needed To: Yi Wang , seanjc@google.com, pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, wanpengli@tencent.com, foxywang@tencent.com, oliver.upton@linux.dev, maz@kernel.org, anup@brainfault.org, atishp@atishpatra.org, frankja@linux.ibm.com, imbrenda@linux.ibm.com References: <20240121111730.262429-1-foxywang@tencent.com> Content-Language: en-US From: Christian Borntraeger In-Reply-To: <20240121111730.262429-1-foxywang@tencent.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: cbW_U5WPhzOWf19FHy2Hl1GFoRPKfTjm X-Proofpoint-ORIG-GUID: HmjjZcFKC3F87LftISToZPyjK4682bMp X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.1011,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2024-01-24_06,2024-01-24_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 phishscore=0 adultscore=0 suspectscore=0 clxscore=1015 mlxscore=0 bulkscore=0 mlxlogscore=676 priorityscore=1501 lowpriorityscore=0 spamscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311290000 definitions=main-2401240088 Am 21.01.24 um 12:17 schrieb Yi Wang: > From: Yi Wang > > We found that it may cost more than 20 milliseconds very accidentally > to enable cap of KVM_CAP_SPLIT_IRQCHIP on a host which has many vms > already. > > The reason is that when vmm(qemu/CloudHypervisor) invokes > KVM_CAP_SPLIT_IRQCHIP kvm will call synchronize_srcu_expedited() and > might_sleep and kworker of srcu may cost some delay during this period. > One way makes sence is setup empty irq routing when creating vm and > so that x86/s390 don't need to setup empty/dummy irq routing. > > Note: I have no s390 machine so the s390 patch has not been tested. I just did a quick sniff and it still seems to work. No performance check etc.