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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 1 Apr 2019 06:11:42 +0100 Received: from d06av23.portsmouth.uk.ibm.com (d06av23.portsmouth.uk.ibm.com [9.149.105.59]) by b06cxnps4074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x315Bf5441746650 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 1 Apr 2019 05:11:41 GMT Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B31B0A405D; Mon, 1 Apr 2019 05:11:41 +0000 (GMT) Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 2052EA4053; Mon, 1 Apr 2019 05:11:38 +0000 (GMT) Received: from oc0383214508.ibm.com (unknown [9.85.70.33]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTP; Mon, 1 Apr 2019 05:11:37 +0000 (GMT) Subject: Re: [PATCH 1/2] cpuidle : auto-promotion for cpuidle states To: Daniel Lezcano , "Rafael J. Wysocki" References: <20190322072942.8038-1-huntbag@linux.vnet.ibm.com> <20190322072942.8038-2-huntbag@linux.vnet.ibm.com> From: Abhishek Date: Mon, 1 Apr 2019 10:41:35 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-TM-AS-GCONF: 00 x-cbid: 19040105-0008-0000-0000-000002D4AD02 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19040105-0009-0000-0000-00002240B281 Message-Id: <50f62972-dfce-52bf-d26b-32e6d2a367e2@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:, , definitions=2019-04-01_03:, , signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound 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-1904010038 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Rafael J. Wysocki" , linuxppc-dev , Linux Kernel Mailing List , Linux PM Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 03/22/2019 06:56 PM, Daniel Lezcano wrote: > On 22/03/2019 10:45, Rafael J. Wysocki wrote: >> On Fri, Mar 22, 2019 at 8:31 AM Abhishek Goel >> wrote: >>> Currently, the cpuidle governors (menu /ladder) determine what idle state >>> an idling CPU should enter into based on heuristics that depend on the >>> idle history on that CPU. Given that no predictive heuristic is perfect, >>> there are cases where the governor predicts a shallow idle state, hoping >>> that the CPU will be busy soon. However, if no new workload is scheduled >>> on that CPU in the near future, the CPU will end up in the shallow state. >>> >>> In case of POWER, this is problematic, when the predicted state in the >>> aforementioned scenario is a lite stop state, as such lite states will >>> inhibit SMT folding, thereby depriving the other threads in the core from >>> using the core resources. >>> >>> To address this, such lite states need to be autopromoted. The cpuidle- >>> core can queue timer to correspond with the residency value of the next >>> available state. Thus leading to auto-promotion to a deeper idle state as >>> soon as possible. >> Isn't the tick stopping avoidance sufficient for that? > I was about to ask the same :) > > > > Thanks for the review. I performed experiments for three scenarios to collect some data. case 1 : Without this patch and without tick retained, i.e. in a upstream kernel, It would spend more than even a second to get out of stop0_lite. case 2 : With tick retained(as suggested) - Generally, we have a sched tick at 4ms(CONF_HZ = 250). Ideally I expected it to take 8 sched tick to get out of stop0_lite. Experimentally, observation was =================================== min            max            99percentile 4ms            12ms          4ms =================================== *ms = milliseconds It would take atleast one sched tick to get out of stop0_lite. case 2 :  With this patch (not stopping tick, but explicitly queuing a timer) min            max              99.5percentile =============================== 144us       192us              144us =============================== *us = microseconds In this patch, we queue a timer just before entering into a stop0_lite state. The timer fires at (residency of next available state + exit latency of next available state * 2). Let's say if next state(stop0) is available which has residency of 20us, it should get out in as low as (20+2*2)*8 [Based on the forumla (residency + 2xlatency)*history length] microseconds = 192us. Ideally we would expect 8 iterations, it was observed to get out in 6-7 iterations. Even if let's say stop2 is next available state(stop0 and stop1 both are unavailable), it would take (100+2*10)*8 = 960us to get into stop2. So, We are able to get out of stop0_lite generally in 150us(with this patch) as compared to 4ms(with tick retained). As stated earlier, we do not want to get stuck into stop0_lite as it inhibits SMT folding for other sibling threads, depriving them of core resources. Current patch is using auto-promotion only for stop0_lite, as it gives performance benefit(primary reason) along with lowering down power consumption. We may extend this model for other states in future. --Abhishek