From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 717843D3490 for ; Mon, 25 May 2026 07:26:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779693969; cv=none; b=WlSkbEPP9TN3Bvc47sdCqqyvffywXczKe4zRW1UR4Vl7u37WlaYmt5ipqPh4pKx2md55vBujHEV7AZbf0OKG+zpa0achD/c1i9YFH29WVSGzw3szzMKsjzQqwwkSTYFhl/vfCbDrgOMRnixGEa2MuuZegpvyaW3leINzzjUB4bw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779693969; c=relaxed/simple; bh=4k7tHLDS4qQTJBKOq4UgraBlkPp9Af7jUCNriS95KCU=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=bmjhmlKrn0eOPbgRt9bUQksRMeQMaojWOfX6ClQwEVGTnmB3roRDU6hxABbUnImN6z3bcztjxPr9yElgFSPezzR2RTGt/5F7vfSuKv1rYIFovr9t9+tOZK64zLKyvP9GgpqRtrMaML02kOlhUCG4oA1QG1pumSkfCKx05MN/YUE= ARC-Authentication-Results:i=1; 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h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=GNR2CfKxb8187spVUjwqymabnWkP/lgA6dJ53MYKekzbdv7+2AOOczqT67gvczRIu qs0WJOlSAV/tJj3yAPjE4xg6v3WSAnsOMLll388k9rJw6HCdcxghWfq4ENNht9oCjf V7Qhwhotm7RnmCWfYwR2uPh1la5maeru0fm8y3TU= Message-ID: <8b82ef04-6b8c-4e39-ae9a-8cdaa1e97cf1@arm.com> Date: Mon, 25 May 2026 08:25:58 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: sched/deadline: Use revised wakeup rule for dl_server From: Christian Loehle To: Andreas Ziegler Cc: Peter Zijlstra , Juri Lelli , linux-kernel@vger.kernel.org, Dietmar Eggemann , John Stultz , Gabriele Monaco References: <496e4b3329fe258da9618b9f05b18fcf@umbiko.net> <97d9e04fd9d222f1a64f1ecfda8b81d7@umbiko.net> <50156878-265d-4025-9b36-c819c80b7493@arm.com> Content-Language: en-US In-Reply-To: <50156878-265d-4025-9b36-c819c80b7493@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 5/11/26 10:47, Christian Loehle wrote: > On 5/9/26 12:42, Andreas Ziegler wrote: >> Hi Christian, Everyone, >> >> On 2026-05-08 14:13, Christian Loehle wrote: >>> On 5/8/26 13:06, Andreas Ziegler wrote: >>>> Hi Christian, >>>> >>>> On 2026-05-08 09:20, Christian Loehle wrote: >>>>> On 5/8/26 09:09, Andreas Ziegler wrote: >>>>>> Linux kernel version: 6.12 >>>>>>   CONFIG_PREEMPT_RT (w/ PREEMPT_RT patch applied) >>>>>> Architecture: aarch64 >>>>>> Platform: Raspberry Pi 4 >>>>>> >>>>>> Hi everyone, >>>>>> >>>>>> Commit d66792919d4f (sched/deadline: Use revised wakeup rule for dl_server) [1] introduced a marked degradation in scheduling latency for real-time tasks in the presence of heavy I/O load. >>>>>> >>>>>> --- a/kernel/sched/deadline.c >>>>>> +++ b/kernel/sched/deadline.c >>>>>> @@ -1079,7 +1079,7 @@ static void update_dl_entity(struct sched_dl_entity *dl_se) >>>>>>      if (dl_time_before(dl_se->deadline, rq_clock(rq)) || >>>>>>          dl_entity_overflow(dl_se, rq_clock(rq))) { >>>>>> >>>>>> -        if (unlikely(!dl_is_implicit(dl_se) && >>>>>> +        if (unlikely((!dl_is_implicit(dl_se) || dl_se->dl_defer) && >>>>>>                   !dl_time_before(dl_se->deadline, rq_clock(rq)) && >>>>>>                   !is_dl_boosted(dl_se))) { >>>>>>              update_dl_revised_wakeup(dl_se, rq); >>>>>> >>>>>> This was observed using a modified version of Con Kolivas' interactivity benchmark [2]; kernel bisection eventually pointed to the above mentioned commit. >>>>>> >>>>>> Benchmark results before d66792919d4f: >>>>>> >>>>>> --- Benchmarking simulated cpu of Audio real time in the presence of simulated --- >>>>>> Load    Latency +/- SD   median  max [100n]    Desired CPU  Deadlines met [%] >>>>>> None      76.6 +/- 8.3654    76  166 >>>>>> Video      78.5 +/- 3.9433    78  107 >>>>>> X      76.4 +/- 8.123     75  157 >>>>>> Burn      72.0 +/- 6.4733    71  127 >>>>>> Write     255.3 +/- 26.627   252  331 >>>>>> Read     226.6 +/- 12.38    227  262 >>>>>> Ring      84.2 +/- 6.6207    83  125 >>>>>> Compile     225.3 +/- 23.949   222  328 >>>>>> >>>>>>      136.8 +/- 78.462        331 >>>>>> >>>>>> Benchmark results after d66792919d4f: >>>>>> >>>>>> --- Benchmarking simulated cpu of Audio real time in the presence of simulated --- >>>>>> Load    Latency +/- SD   median  max [100n]    Desired CPU  Deadlines met [%] >>>>>> None      68.4 +/- 9.7864    67  169 >>>>>> Video      74.4 +/- 3.724     74   97 >>>>>> X      72.0 +/- 6.5681    71  129 >>>>>> Burn      66.9 +/- 5.9059    66  117 >>>>>> Write    9576.9 +/- 67639    250500418        98.1         98.1 >>>>>> Read     209.3 +/- 11.018   209  267 >>>>>> Ring      80.5 +/- 8.0993    78  125 >>>>>> Compile     239.0 +/- 29.447   234  372 >>>>>> >>>>>>     1298.4 +/- 24118       500418 >>>>>> >>>>>> Reverting this commit obviously solves the issue for me. I have no idea why this issue appears exclusively with heavy write loads in the background. >>>>>> >>>>>> Is this a scheduler issue, or rather something in the background? >>>>>> >>>>> >>>>> Hi Andreas, >>>>> You're using cpufreq schedutil for your tests I'm assuming? >>>>> Is there a difference in cpufreq behavior (avg cpufreq or OPP residencies?) >>>>> Does the regression also happen on powersave/performance governor? >>>> >>>> Actually this is a very stripped-down system. The 'performance' cpufreq governor is the only one compiled in, the processor cores run on a fixed frequency. CONFIG_PM_OPP is not set. >>> >>> That certainly makes the analysis easier. >>> I couldn't reproduce the issue so far on my system but it does seem like the dl server >>> would get potentially unbounded running time with very frequent >>> starting and stopping of the dlserver (which presumably happens because of >>> the writeback) reset the runtime, which then leads to your 25s observed latency. >>> Peter, how is the revised wakeup rule supposed to behave here? >>> >>>> [snip] >> >> This seems to be a case of runtime starvation. If I change sched_rt_runtime_us to a smaller value, the benchmark returns reasonable latency values. >> >> # echo "980000" > /proc/sys/kernel/sched_rt_runtime_us >> >> I could live with this workaround, since it seems not to impact overall latency values in a noticeable way. >> > > Not a very stable workaround unfortunately :/ > While I try to reproduce this, what you're observing should imply that the > background SCHED_NORMAL work is enough to fully utilize the system, right? > interbench Write does 4k (buffered) writes of a 1GB file and then close+open > and repeat, nothing fancy really. Does this actually produce significant CPU > utilization for you? Can you just run the background work and see what that > looks like? > (What you're seeing looks like a bug in any case, just so I'm not going down > a wrong path when trying to reproduce here). I'd be interested if you can still reproduce with this fix: https://lore.kernel.org/lkml/20260522125833.264145-1-gmonaco@redhat.com/