Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   "Chris Mason" <clm@fb.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
CC:     Rik van Riel <riel@surriel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] fix scheduler regression from "sched/fair: Rework
 load_balance()"
Date:   Mon, 26 Oct 2020 12:48:30 -0400
Message-ID: <B23038A4-945F-48E9-8D38-EABE8204F208@fb.com>
In-Reply-To: <20201026162029.GA11367@vingu-book>
References: <DB4481A8-FD4E-4879-9CD2-275ABAFC09CF@fb.com>
 <CAKfTPtBiOFXwV9SkZ=YBw16xoS6LSrKVR4sFX6r2hZPZ9_5-+A@mail.gmail.com>
 <0014CA62-A632-495A-92B0-4B14C8CA193C@fb.com>
 <20201026142455.GA13495@vingu-book>
 <465597a2250d69346cff73dd07817794d3e80244.camel@surriel.com>
 <CAKfTPtCVzass7GM5oj3o3y0ru4HQViWZc2+D-RpFoLvg=__FrA@mail.gmail.com>
 <334f491d2887a6ed7c5347d5125412849feb8a0a.camel@surriel.com>
 <CAKfTPtAfKn0jzOpPNR4NUb0zLs02iLQq2_UCDSCEwhTB2LDAig@mail.gmail.com>
 <20201026120445.6a5dbbbe@imladris.surriel.com>
 <20201026162029.GA11367@vingu-book>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
X-MS-Exchange-MessageSentRepresentingType: 1
Received: from [100.109.75.124] (2620:10d:c091:480::1:2ebd) by MN2PR01CA0023.prod.exchangelabs.com (2603:10b6:208:10c::36) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3499.18 via Frontend Transport; Mon, 26 Oct 2020 16:48:31 +0000
X-MS-PublicTrafficType: Email
X-MS-Office365-Filtering-Correlation-Id: 5e142689-dd85-47a4-24b7-08d879cef938
X-MS-TrafficTypeDiagnostic: MN2PR15MB3118:
X-Microsoft-Antispam-PRVS: <MN2PR15MB31182FE3772799B90E202F5DD3190@MN2PR15MB3118.namprd15.prod.outlook.com>
X-FB-Source: Internal
X-MS-Oob-TLC-OOBClassifiers: OLM:4714;
X-MS-Exchange-SenderADCheck: 1
X-Microsoft-Antispam: BCL:0;
X-Microsoft-Antispam-Message-Info: L6tR70XslCa/YLuER6irb5k6uk3fQh1b2lpCarrv/0CD2itBMcgabpshvTv3j8AhQputZEKegocc1XDJ3nLtG32P+fs3LqEGl60zDzLpyTd/QrIuA89yoFomg5qGAEWQiiIi/iq+H7j+4o8heIlyhZzgPMUZ6jAN6+D66uiSsF9mNxmPQZvPYBuHMna3p11zy007zlNHsGscKYHA2xAq7RAvGBH3rzEoRANLcQYTOim1VEwCuQbWDUmnGojIwVBs7uf6dAqERxlWOK4BkfgcWqRXLL7+476IfjSsmyAtPlO40P/2+5CyaaT8NDm6y0PB/Oy2Ln+1KUxV66VWDUW2D/Ei+S0TN0mZLDxcYuNtY+3PaWM1xmqBG2hHyfmrNydV
X-Forefront-Antispam-Report: CIP:255.255.255.255;CTRY:;LANG:en;SCL:1;SRV:;IPV:NLI;SFV:NSPM;H:MN2PR15MB2878.namprd15.prod.outlook.com;PTR:;CAT:NONE;SFS:(39860400002)(136003)(346002)(376002)(366004)(396003)(66476007)(66946007)(316002)(83380400001)(5660300002)(86362001)(33656002)(54906003)(8676002)(186003)(2906002)(4326008)(6486002)(16526019)(956004)(6916009)(36756003)(66556008)(52116002)(8936002)(2616005)(53546011)(478600001)(78286007);DIR:OUT;SFP:1102;
X-MS-Exchange-AntiSpam-MessageData: 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
X-MS-Exchange-CrossTenant-Network-Message-Id: 5e142689-dd85-47a4-24b7-08d879cef938
X-MS-Exchange-CrossTenant-AuthSource: MN2PR15MB2878.namprd15.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 26 Oct 2020 16:48:32.6733
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: REALg09xVlZslrNwvBKTDyqRYgpvmhQDE1s18E7nEdwXrs5oTgN4gDXeSiIQe3MW
X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR15MB3118
X-OriginatorOrg: fb.com
X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.235,18.0.737
 definitions=2020-10-26_08:2020-10-26,2020-10-26 signatures=0
X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 suspectscore=0
 phishscore=0 priorityscore=1501 impostorscore=0 clxscore=1015
 mlxlogscore=999 spamscore=0 malwarescore=0 bulkscore=0 adultscore=0
 mlxscore=0 lowpriorityscore=0 classifier=spam adjust=0 reason=mlx
 scancount=1 engine=8.12.0-2009150000 definitions=main-2010260113
X-FB-Internal: deliver
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 26 Oct 2020, at 12:20, Vincent Guittot wrote:

> Le lundi 26 oct. 2020 à 12:04:45 (-0400), Rik van Riel a écrit :
>> On Mon, 26 Oct 2020 16:42:14 +0100
>> Vincent Guittot <vincent.guittot@linaro.org> wrote:
>>> On Mon, 26 Oct 2020 at 16:04, Rik van Riel <riel@surriel.com> wrote:
>>
>>>> Could utilization estimates be off, either lagging or
>>>> simply having a wrong estimate for a task, resulting
>>>> in no task getting pulled sometimes, while doing a
>>>> migrate_task imbalance always moves over something?
>>>
>>> task and cpu utilization are not always up to fully synced and may 
>>> lag
>>> a bit which explains that sometimes LB can fail to migrate for a 
>>> small
>>> diff
>>
>> OK, running with this little snippet below, I see latencies
>> improve back to near where they used to be:
>>
>> Latency percentiles (usec) runtime 150 (s)
>>         50.0th: 13
>>         75.0th: 31
>>         90.0th: 69
>>         95.0th: 90
>>         *99.0th: 761
>>         99.5th: 2268
>>         99.9th: 9104
>>         min=1, max=16158
>>
>> I suspect the right/cleaner approach might be to use
>> migrate_task more in !CPU_NOT_IDLE cases?
>>
>> Running a task to an idle CPU immediately, instead of refusing
>> to have the load balancer move it, improves latencies for fairly
>> obvious reasons.
>>
>> I am not entirely clear on why the load balancer should need to
>> be any more conservative about moving tasks than the wakeup
>> path is in eg. select_idle_sibling.
>
>
> what you are suggesting is something like:
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4978964e75e5..3b6fbf33abc2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9156,7 +9156,8 @@ static inline void calculate_imbalance(struct 
> lb_env *env, struct sd_lb_stats *s
>          * emptying busiest.
>          */
>         if (local->group_type == group_has_spare) {
> -               if (busiest->group_type > group_fully_busy) {
> +               if ((busiest->group_type > group_fully_busy) &&
> +                   !(env->sd->flags & SD_SHARE_PKG_RESOURCES)) {
>                         /*
>                          * If busiest is overloaded, try to fill spare
>                          * capacity. This might end up creating spare 
> capacity
>
> which also fixes the problem for me and alignes LB with wakeup path 
> regarding the migration
> in the LLC

Vincent’s patch on top of 5.10-rc1 looks pretty great:

Latency percentiles (usec) runtime 90 (s) (3320 total samples)
         50.0th: 161 (1687 samples)
         75.0th: 200 (817 samples)
         90.0th: 228 (488 samples)
         95.0th: 254 (164 samples)
         *99.0th: 314 (131 samples)
         99.5th: 330 (17 samples)
         99.9th: 356 (13 samples)
         min=29, max=358

Next we test in prod, which probably won’t have answers until 
tomorrow.  Thanks again Vincent!

-chris