From mboxrd@z Thu Jan 1 00:00:00 1970 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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7A1B2C433EF for ; Tue, 5 Oct 2021 14:13:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 61D396115B for ; Tue, 5 Oct 2021 14:13:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236300AbhJEOPS (ORCPT ); Tue, 5 Oct 2021 10:15:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40906 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235259AbhJEOOI (ORCPT ); Tue, 5 Oct 2021 10:14:08 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CCAD3C061767; Tue, 5 Oct 2021 07:12:13 -0700 (PDT) Date: Tue, 05 Oct 2021 14:12:11 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1633443132; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8A72kVAZT9zwFB9tUhmCi/Ygs+9Eka8rLl/cBRSqrRs=; b=dWBcXunEVjxEKmCqDv4KWgJdFYbaRmTfgnzxoskL5/SLKWzAsbqt5SPReQSrDGW8r69x6U ERm1CrQrIHEih00ZGAPm7i+uscCaW5CYwICM4Gsq3XkTq8Z5LW55SEB5qqsK5kiJ/9Qqzm KYyxuzRmhOLb8vFZ/kYhM3QDqHzFfKrIyzYUtIbducpwgKtKXNskrnoDXwQnNxVf2iH7L1 5vdpzbG5YpVlt9pFkUehjaJk2NhEMMgB+ShcXARPR5QW7TJe0KRfJwUFwvpgqWaugqj9WA I3CraM2Duuxr3usdT8N0s+hr0piCwsgiL+IkwQNiT3iluZ9Kui5t2kGNmswMWw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1633443132; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8A72kVAZT9zwFB9tUhmCi/Ygs+9Eka8rLl/cBRSqrRs=; b=IpIpX8rTW0gQUSg/orPMKRAcSru4NZI6RCakdMGIkI3uOzRwT2wvVJoSZxzG5eluIbEV8d Iu4+8TzXflXb5aDg== From: "tip-bot2 for Huaixin Chang" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: sched/core] sched/fair: Add document for burstable CFS bandwidth Cc: Shanpei Chen , Tianchen Ding , Huaixin Chang , "Peter Zijlstra (Intel)" , Daniel Jordan , Tejun Heo , x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20210830032215.16302-3-changhuaixin@linux.alibaba.com> References: <20210830032215.16302-3-changhuaixin@linux.alibaba.com> MIME-Version: 1.0 Message-ID: <163344313162.25758.8989405226028414673.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the sched/core branch of tip: Commit-ID: d73df887b6b8174dfbb7f5f878fbd1e0e2eb3f08 Gitweb: https://git.kernel.org/tip/d73df887b6b8174dfbb7f5f878fbd1e0e2eb3f08 Author: Huaixin Chang AuthorDate: Mon, 30 Aug 2021 11:22:15 +08:00 Committer: Peter Zijlstra CommitterDate: Tue, 05 Oct 2021 15:51:41 +02:00 sched/fair: Add document for burstable CFS bandwidth Basic description of usage and effect for CFS Bandwidth Control Burst. Co-developed-by: Shanpei Chen Signed-off-by: Shanpei Chen Co-developed-by: Tianchen Ding Signed-off-by: Tianchen Ding Signed-off-by: Huaixin Chang Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Daniel Jordan Acked-by: Tejun Heo Link: https://lore.kernel.org/r/20210830032215.16302-3-changhuaixin@linux.alibaba.com --- Documentation/admin-guide/cgroup-v2.rst | 8 ++- Documentation/scheduler/sched-bwc.rst | 84 +++++++++++++++++++++--- 2 files changed, 83 insertions(+), 9 deletions(-) diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst index babbe04..d5b0e8a 100644 --- a/Documentation/admin-guide/cgroup-v2.rst +++ b/Documentation/admin-guide/cgroup-v2.rst @@ -1016,6 +1016,8 @@ All time durations are in microseconds. - nr_periods - nr_throttled - throttled_usec + - nr_bursts + - burst_usec cpu.weight A read-write single value file which exists on non-root @@ -1047,6 +1049,12 @@ All time durations are in microseconds. $PERIOD duration. "max" for $MAX indicates no limit. If only one number is written, $MAX is updated. + cpu.max.burst + A read-write single value file which exists on non-root + cgroups. The default is "0". + + The burst in the range [0, $MAX]. + cpu.pressure A read-write nested-keyed file. diff --git a/Documentation/scheduler/sched-bwc.rst b/Documentation/scheduler/sched-bwc.rst index 1fc7355..173c141 100644 --- a/Documentation/scheduler/sched-bwc.rst +++ b/Documentation/scheduler/sched-bwc.rst @@ -22,9 +22,52 @@ cfs_quota units at each period boundary. As threads consume this bandwidth it is transferred to cpu-local "silos" on a demand basis. The amount transferred within each of these updates is tunable and described as the "slice". +Burst feature +------------- +This feature borrows time now against our future underrun, at the cost of +increased interference against the other system users. All nicely bounded. + +Traditional (UP-EDF) bandwidth control is something like: + + (U = \Sum u_i) <= 1 + +This guaranteeds both that every deadline is met and that the system is +stable. After all, if U were > 1, then for every second of walltime, +we'd have to run more than a second of program time, and obviously miss +our deadline, but the next deadline will be further out still, there is +never time to catch up, unbounded fail. + +The burst feature observes that a workload doesn't always executes the full +quota; this enables one to describe u_i as a statistical distribution. + +For example, have u_i = {x,e}_i, where x is the p(95) and x+e p(100) +(the traditional WCET). This effectively allows u to be smaller, +increasing the efficiency (we can pack more tasks in the system), but at +the cost of missing deadlines when all the odds line up. However, it +does maintain stability, since every overrun must be paired with an +underrun as long as our x is above the average. + +That is, suppose we have 2 tasks, both specify a p(95) value, then we +have a p(95)*p(95) = 90.25% chance both tasks are within their quota and +everything is good. At the same time we have a p(5)p(5) = 0.25% chance +both tasks will exceed their quota at the same time (guaranteed deadline +fail). Somewhere in between there's a threshold where one exceeds and +the other doesn't underrun enough to compensate; this depends on the +specific CDFs. + +At the same time, we can say that the worst case deadline miss, will be +\Sum e_i; that is, there is a bounded tardiness (under the assumption +that x+e is indeed WCET). + +The interferenece when using burst is valued by the possibilities for +missing the deadline and the average WCET. Test results showed that when +there many cgroups or CPU is under utilized, the interference is +limited. More details are shown in: +https://lore.kernel.org/lkml/5371BD36-55AE-4F71-B9D7-B86DC32E3D2B@linux.alibaba.com/ + Management ---------- -Quota and period are managed within the cpu subsystem via cgroupfs. +Quota, period and burst are managed within the cpu subsystem via cgroupfs. .. note:: The cgroupfs files described in this section are only applicable @@ -32,29 +75,37 @@ Quota and period are managed within the cpu subsystem via cgroupfs. :ref:`Documentation/admin-guide/cgroup-v2.rst `. - cpu.cfs_quota_us: the total available run-time within a period (in - microseconds) +- cpu.cfs_quota_us: run-time replenished within a period (in microseconds) - cpu.cfs_period_us: the length of a period (in microseconds) - cpu.stat: exports throttling statistics [explained further below] +- cpu.cfs_burst_us: the maximum accumulated run-time (in microseconds) The default values are:: cpu.cfs_period_us=100ms - cpu.cfs_quota=-1 + cpu.cfs_quota_us=-1 + cpu.cfs_burst_us=0 A value of -1 for cpu.cfs_quota_us indicates that the group does not have any bandwidth restriction in place, such a group is described as an unconstrained bandwidth group. This represents the traditional work-conserving behavior for CFS. -Writing any (valid) positive value(s) will enact the specified bandwidth limit. -The minimum quota allowed for the quota or period is 1ms. There is also an -upper bound on the period length of 1s. Additional restrictions exist when -bandwidth limits are used in a hierarchical fashion, these are explained in -more detail below. +Writing any (valid) positive value(s) no smaller than cpu.cfs_burst_us will +enact the specified bandwidth limit. The minimum quota allowed for the quota or +period is 1ms. There is also an upper bound on the period length of 1s. +Additional restrictions exist when bandwidth limits are used in a hierarchical +fashion, these are explained in more detail below. Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit and return the group to an unconstrained state once more. +A value of 0 for cpu.cfs_burst_us indicates that the group can not accumulate +any unused bandwidth. It makes the traditional bandwidth control behavior for +CFS unchanged. Writing any (valid) positive value(s) no larger than +cpu.cfs_quota_us into cpu.cfs_burst_us will enact the cap on unused bandwidth +accumulation. + Any updates to a group's bandwidth specification will result in it becoming unthrottled if it is in a constrained state. @@ -74,7 +125,7 @@ for more fine-grained consumption. Statistics ---------- -A group's bandwidth statistics are exported via 3 fields in cpu.stat. +A group's bandwidth statistics are exported via 5 fields in cpu.stat. cpu.stat: @@ -82,6 +133,9 @@ cpu.stat: - nr_throttled: Number of times the group has been throttled/limited. - throttled_time: The total time duration (in nanoseconds) for which entities of the group have been throttled. +- nr_bursts: Number of periods burst occurs. +- burst_time: Cumulative wall-time (in nanoseconds) that any CPUs has used + above quota in respective periods This interface is read-only. @@ -179,3 +233,15 @@ Examples By using a small period here we are ensuring a consistent latency response at the expense of burst capacity. + +4. Limit a group to 40% of 1 CPU, and allow accumulate up to 20% of 1 CPU + additionally, in case accumulation has been done. + + With 50ms period, 20ms quota will be equivalent to 40% of 1 CPU. + And 10ms burst will be equivalent to 20% of 1 CPU. + + # echo 20000 > cpu.cfs_quota_us /* quota = 20ms */ + # echo 50000 > cpu.cfs_period_us /* period = 50ms */ + # echo 10000 > cpu.cfs_burst_us /* burst = 10ms */ + + Larger buffer setting (no larger than quota) allows greater burst capacity.