Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 507DCC74A44 for ; Tue, 14 Mar 2023 11:04:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230334AbjCNLEC (ORCPT ); Tue, 14 Mar 2023 07:04:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57664 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230152AbjCNLD5 (ORCPT ); Tue, 14 Mar 2023 07:03:57 -0400 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E4C1E62308 for ; Tue, 14 Mar 2023 04:03:15 -0700 (PDT) Received: from dggpeml500018.china.huawei.com (unknown [172.30.72.55]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4PbVtr2WMzzHwmV; Tue, 14 Mar 2023 19:00:52 +0800 (CST) Received: from [10.67.111.186] (10.67.111.186) by dggpeml500018.china.huawei.com (7.185.36.186) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.21; Tue, 14 Mar 2023 19:03:02 +0800 Message-ID: <55754a59-a01f-206a-43f6-d07ea37300dd@huawei.com> Date: Tue, 14 Mar 2023 19:03:02 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.1.1 Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated To: Vincent Guittot CC: , , , , , , , , , , Peter Zijlstra References: <20230306132418.50389-1-zhangqiao22@huawei.com> <20230309130524.GA273121@hirez.programming.kicks-ass.net> <20230309142825.GB273121@hirez.programming.kicks-ass.net> From: Zhang Qiao In-Reply-To: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.111.186] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpeml500018.china.huawei.com (7.185.36.186) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2023/3/13 22:23, Vincent Guittot 写道: > On Sat, 11 Mar 2023 at 10:57, Zhang Qiao wrote: >> >> >> >> 在 2023/3/10 22:29, Vincent Guittot 写道: >>> Le jeudi 09 mars 2023 à 16:14:38 (+0100), Vincent Guittot a écrit : >>>> On Thu, 9 Mar 2023 at 15:37, Peter Zijlstra wrote: >>>>> >>>>> On Thu, Mar 09, 2023 at 03:28:25PM +0100, Peter Zijlstra wrote: >>>>>> On Thu, Mar 09, 2023 at 02:34:05PM +0100, Vincent Guittot wrote: >>>>>> >>>>>>> Then, even if we don't clear exec_start before migrating and keep >>>>>>> current value to be used in place_entity on the new cpu, we can't >>>>>>> compare the rq_clock_task(rq_of(cfs_rq)) of 2 different rqs AFAICT >>>>>> >>>>>> Blergh -- indeed, irq and steal time can skew them between CPUs :/ >>>>>> I suppose we can fudge that... wait_start (which is basically what we're >>>>>> making it do) also does that IIRC. >>>>>> >>>>>> I really dislike having this placement muck spreadout like proposed. >>>>> >>>>> Also, I think we might be over-engineering this, we don't care about >>>>> accuracy at all, all we really care about is 'long-time'. >>>> >>>> you mean taking the patch 1/2 that you mentioned here to add a >>>> migrated field: >>>> https://lore.kernel.org/all/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@linux.intel.com/T/#ma5637eb8010f3f4a4abff778af8db705429d003b >>>> >>>> And assume that the divergence between the rq_clock_task() can be ignored ? >>>> >>>> That could probably work but we need to replace the (60LL * >>>> NSEC_PER_SEC) by ((1ULL << 63) / NICE_0_LOAD) because 60sec divergence >>>> would not be unrealistic. >>>> and a comment to explain why it's acceptable >>> >>> Zhang, >>> >>> Could you try the patch below ? >>> This is a rebase/merge/update of: >>> -patch 1/2 above and >>> -https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@amazon.de/ >> >> >> I applyed and tested this patch, and it make hackbench slower. >> According to my previous test results. The good result is 82.1(s). >> But the result of this patch is 108.725(s). > > By "the result of this patch is 108.725(s)", you mean the result of > https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@amazon.de/ > alone, don't you ? No, with your patch, the test results is 108.725(s), git diff: diff --git a/include/linux/sched.h b/include/linux/sched.h index 63d242164b1a..93a3909ae4c4 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -550,6 +550,7 @@ struct sched_entity { struct rb_node run_node; struct list_head group_node; unsigned int on_rq; + unsigned int migrated; u64 exec_start; u64 sum_exec_runtime; diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index ff4dbbae3b10..e60defc39f6e 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se) /* * We are starting a new run period: */ + se->migrated = 0; se->exec_start = rq_clock_task(rq_of(cfs_rq)); } @@ -4690,9 +4691,9 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) * inversed due to s64 overflow. */ sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start; - if ((s64)sleep_time > 60LL * NSEC_PER_SEC) + if ((s64)sleep_time > (1ULL << 63) / scale_load_down(NICE_0_LOAD) / 2) { se->vruntime = vruntime; - else + } else se->vruntime = max_vruntime(se->vruntime, vruntime); } @@ -7658,8 +7659,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) se->avg.last_update_time = 0; /* We have migrated, no longer consider this task hot */ - se->exec_start = 0; - + se->migrated = 1; update_scan_period(p, new_cpu); } @@ -8343,6 +8343,8 @@ static int task_hot(struct task_struct *p, struct lb_env *env) if (sysctl_sched_migration_cost == 0) return 0; + if (p->se.migrated) + return 0; delta = rq_clock_task(env->src_rq) - p->se.exec_start; > >> >> >>> version1: v6.2 >>> version2: v6.2 + commit 829c1651e9c4 >>> version3: v6.2 + commit 829c1651e9c4 + this patch >>> >>> ------------------------------------------------- >>> version1 version2 version3 >>> test1 81.0 118.1 82.1 >>> test2 82.1 116.9 80.3 >>> test3 83.2 103.9 83.3 >>> avg(s) 82.1 113.0 81.9 > > Ok, it looks like we are back to normal figures > >>> >>> ------------------------------------------------- >>> >>> The proposal accepts a divergence of up to 52 days between the 2 rqs. >>> >>> If this work, we will prepare a proper patch >>> >>> diff --git a/include/linux/sched.h b/include/linux/sched.h >>> index 63d242164b1a..cb8af0a137f7 100644 >>> --- a/include/linux/sched.h >>> +++ b/include/linux/sched.h >>> @@ -550,6 +550,7 @@ struct sched_entity { >>> struct rb_node run_node; >>> struct list_head group_node; >>> unsigned int on_rq; >>> + unsigned int migrated; >>> >>> u64 exec_start; >>> u64 sum_exec_runtime; >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 7a1b1f855b96..36acd9598b40 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se) >>> /* >>> * We are starting a new run period: >>> */ >>> + se->migrated = 0; >>> se->exec_start = rq_clock_task(rq_of(cfs_rq)); >>> } >>> >>> @@ -4684,13 +4685,23 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) >>> >>> /* >>> * Pull vruntime of the entity being placed to the base level of >>> - * cfs_rq, to prevent boosting it if placed backwards. If the entity >>> - * slept for a long time, don't even try to compare its vruntime with >>> - * the base as it may be too far off and the comparison may get >>> - * inversed due to s64 overflow. >>> + * cfs_rq, to prevent boosting it if placed backwards. >>> + * However, min_vruntime can advance much faster than real time, with >>> + * the exterme being when an entity with the minimal weight always runs >>> + * on the cfs_rq. If the new entity slept for long, its vruntime >>> + * difference from min_vruntime may overflow s64 and their comparison >>> + * may get inversed, so ignore the entity's original vruntime in that >>> + * case. >>> + * The maximal vruntime speedup is given by the ratio of normal to >>> + * minimal weight: NICE_0_LOAD / MIN_SHARES, so cutting off on the >> >> why not is `scale_load_down(NICE_0_LOAD) / MIN_SHARES` here ? > > yes, you are right. > >> >> >>> + * sleep time of 2^63 / NICE_0_LOAD should be safe. >>> + * When placing a migrated waking entity, its exec_start has been set >>> + * from a different rq. In order to take into account a possible >>> + * divergence between new and prev rq's clocks task because of irq and >> >> This divergence might be larger, it cause `sleep_time` maybe negative. > > AFAICT, we are safe with ((1ULL << 63) / scale_load_down(NICE_0_LOAD) > / 2) as long as the divergence between the 2 rqs clocks task is lower > than 2^52nsec. Do you expect a divergence higher than 2^52 nsec > (around 52 days)? > > We can probably keep using (1ULL << 63) / scale_load_down(NICE_0_LOAD) > which is already half the max value if needed. > > the fact that sleep_time can be negative is not a problem as > s64)sleep_time > will take care of this. In my opinion, when comparing signed with unsigned, the compiler converts the signed value to unsigned. So, if sleep_time < 0, "(s64)sleep_time > (1ULL << 63) / NICE_0_LOAD / 2" will be true. > >> >>> + * stolen time, we take an additional margin. >>> */ >>> sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start; >>> - if ((s64)sleep_time > 60LL * NSEC_PER_SEC) >>> + if ((s64)sleep_time > (1ULL << 63) / NICE_0_LOAD / 2)> se->vruntime = vruntime; >>> else >>> se->vruntime = max_vruntime(se->vruntime, vruntime); >>> @@ -7658,7 +7669,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) >>> se->avg.last_update_time = 0; >>> >>> /* We have migrated, no longer consider this task hot */ >>> - se->exec_start = 0; >>> + se->migrated = 1; >>> >>> update_scan_period(p, new_cpu); >>> } >>> @@ -8344,6 +8355,9 @@ static int task_hot(struct task_struct *p, struct lb_env *env) >>> if (sysctl_sched_migration_cost == 0) >>> return 0; >>> >>> + if (p->se.migrated) >>> + return 0; >>> + >>> delta = rq_clock_task(env->src_rq) - p->se.exec_start; >>> >>> return delta < (s64)sysctl_sched_migration_cost; >>> >>> >>> >>>> >>>> >>>>> >>>>> >>> . >>> > . >