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Fri, 17 May 2019 02:06:34 +0000 Received: from pps.filterd (aserp3030.oracle.com [127.0.0.1]) by aserp3030.oracle.com (8.16.0.27/8.16.0.27) with SMTP id x4H25SXB035632; Fri, 17 May 2019 02:06:33 GMT Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by aserp3030.oracle.com with ESMTP id 2sggeu154u-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 17 May 2019 02:06:33 +0000 Received: from abhmp0020.oracle.com (abhmp0020.oracle.com [141.146.116.26]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x4H26WY6014020; Fri, 17 May 2019 02:06:32 GMT Received: from [192.168.0.110] (/70.36.60.91) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 16 May 2019 19:06:32 -0700 Subject: Re: [PATCH] sched: introduce configurable delay before entering idle To: Wanpeng Li Cc: Marcelo Tosatti , kvm-devel , LKML , Thomas Gleixner , Ingo Molnar , Andrea Arcangeli , Bandan Das , Paolo Bonzini References: <20190507185647.GA29409@amt.cnet> <20190514135022.GD4392@amt.cnet> <7e390fef-e0df-963f-4e18-e44ac2766be3@oracle.com> From: Ankur Arora Message-ID: <265675b1-07e2-f5dd-6de8-5e47fa91be32@oracle.com> Date: Thu, 16 May 2019 19:06:30 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9259 signatures=668687 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905170011 X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9259 signatures=668687 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1905170011 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019-05-15 6:07 p.m., Wanpeng Li wrote: > On Thu, 16 May 2019 at 02:42, Ankur Arora wrote: >> >> On 5/14/19 6:50 AM, Marcelo Tosatti wrote: >>> On Mon, May 13, 2019 at 05:20:37PM +0800, Wanpeng Li wrote: >>>> On Wed, 8 May 2019 at 02:57, Marcelo Tosatti wrote: >>>>> >>>>> >>>>> Certain workloads perform poorly on KVM compared to baremetal >>>>> due to baremetal's ability to perform mwait on NEED_RESCHED >>>>> bit of task flags (therefore skipping the IPI). >>>> >>>> KVM supports expose mwait to the guest, if it can solve this? >>>> >>>> Regards, >>>> Wanpeng Li >>> >>> Unfortunately mwait in guest is not feasible (uncompatible with multiple >>> guests). Checking whether a paravirt solution is possible. >> >> Hi Marcelo, >> >> I was also looking at making MWAIT available to guests in a safe manner: >> whether through emulation or a PV-MWAIT. My (unsolicited) thoughts > > MWAIT emulation is not simple, here is a research > https://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/mwait.html Agreed. I had outlined my attempt to do that below and come to the conclusion that we would need a PV-MWAIT. Ankur > > Regards, > Wanpeng Li > >> follow. >> >> We basically want to handle this sequence: >> >> monitor(monitor_address); >> if (*monitor_address == base_value) >> mwaitx(max_delay); >> >> Emulation seems problematic because, AFAICS this would happen: >> >> guest hypervisor >> ===== ==== >> >> monitor(monitor_address); >> vmexit ===> monitor(monitor_address) >> if (*monitor_address == base_value) >> mwait(); >> vmexit ====> mwait() >> >> There's a context switch back to the guest in this sequence which seems >> problematic. Both the AMD and Intel specs list system calls and >> far calls as events which would lead to the MWAIT being woken up: >> "Voluntary transitions due to fast system call and far calls (occurring >> prior to issuing MWAIT but after setting the monitor)". >> >> >> We could do this instead: >> >> guest hypervisor >> ===== ==== >> >> monitor(monitor_address); >> vmexit ===> cache monitor_address >> if (*monitor_address == base_value) >> mwait(); >> vmexit ====> monitor(monitor_address) >> mwait() >> >> But, this would miss the "if (*monitor_address == base_value)" check in >> the host which is problematic if *monitor_address changed simultaneously >> when monitor was executed. >> (Similar problem if we cache both the monitor_address and >> *monitor_address.) >> >> >> So, AFAICS, the only thing that would work is the guest offloading the >> whole PV-MWAIT operation. >> >> AFAICS, that could be a paravirt operation which needs three parameters: >> (monitor_address, base_value, max_delay.) >> >> This would allow the guest to offload this whole operation to >> the host: >> monitor(monitor_address); >> if (*monitor_address == base_value) >> mwaitx(max_delay); >> >> I'm guessing you are thinking on similar lines? >> >> >> High level semantics: If the CPU doesn't have any runnable threads, then >> we actually do this version of PV-MWAIT -- arming a timer if necessary >> so we only sleep until the time-slice expires or the MWAIT max_delay does. >> >> If the CPU has any runnable threads then this could still finish its >> time-quanta or we could just do a schedule-out. >> >> >> So the semantics guaranteed to the host would be that PV-MWAIT returns >> after >= max_delay OR with the *monitor_address changed. >> >> >> >> Ankur