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Mon, 20 Oct 2025 04:58:53 -0700 (PDT) X-Google-Smtp-Source: AGHT+IG3bJRkDnTNsDoLX27C6wNA7H7qPmCVso88dp9ehs0+zp2XESQa1XmzDa+Csu2dj2eVZorh8A== X-Received: by 2002:a05:600c:828a:b0:46e:39e1:fc3c with SMTP id 5b1f17b1804b1-4711787617amr91944415e9.5.1760961533236; Mon, 20 Oct 2025 04:58:53 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([176.206.13.103]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-471144c900asm229469775e9.16.2025.10.20.04.58.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Oct 2025 04:58:52 -0700 (PDT) Date: Mon, 20 Oct 2025 13:58:50 +0200 From: Juri Lelli To: Andrea Righi Cc: Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Joel Fernandes , Tejun Heo , David Vernet , Changwoo Min , Shuah Khan , sched-ext@lists.linux.dev, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, Luigi De Matteis Subject: Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks Message-ID: References: <20251017093214.70029-1-arighi@nvidia.com> <20251017093214.70029-7-arighi@nvidia.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251017093214.70029-7-arighi@nvidia.com> Hi! On 17/10/25 11:25, Andrea Righi wrote: > From: Joel Fernandes > > sched_ext currently suffers starvation due to RT. The same workload when > converted to EXT can get zero runtime if RT is 100% running, causing EXT > processes to stall. Fix it by adding a DL server for EXT. > > A kselftest is also provided later to verify: > > ./runner -t rt_stall > ===== START ===== > TEST: rt_stall > DESCRIPTION: Verify that RT tasks cannot stall SCHED_EXT tasks > OUTPUT: > TAP version 13 > 1..1 > ok 1 PASS: CFS task got more than 4.00% of runtime > > [ arighi: drop ->balance() now that pick_task() has an rf argument ] > > Cc: Luigi De Matteis > Co-developed-by: Andrea Righi > Signed-off-by: Andrea Righi > Signed-off-by: Joel Fernandes > --- > kernel/sched/core.c | 3 +++ > kernel/sched/deadline.c | 2 +- > kernel/sched/ext.c | 51 +++++++++++++++++++++++++++++++++++++++-- > kernel/sched/sched.h | 2 ++ > 4 files changed, 55 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 096e8d03d85e7..31a9c9381c63f 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -8679,6 +8679,9 @@ void __init sched_init(void) > hrtick_rq_init(rq); > atomic_set(&rq->nr_iowait, 0); > fair_server_init(rq); > +#ifdef CONFIG_SCHED_CLASS_EXT > + ext_server_init(rq); > +#endif > > #ifdef CONFIG_SCHED_CORE > rq->core = rq; > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index 0680e0186577a..3c1fd2190949e 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -1504,7 +1504,7 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64 > * The fair server (sole dl_server) does not account for real-time Fair server is not alone anymore. :)) Please update the comment as well. > * workload because it is running fair work. > */ > - if (dl_se == &rq->fair_server) > + if (dl_se->dl_server) > return; > > #ifdef CONFIG_RT_GROUP_SCHED ... > @@ -1487,6 +1499,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int deq_flags > sub_nr_running(rq, 1); > > dispatch_dequeue(rq, p); > + > + /* Stop the server if this was the last task */ > + if (rq->scx.nr_running == 0) > + dl_server_stop(&rq->ext_server); > + Do we want to use the delayed stop behavior for scx-server as we have for fair-server? Wonder if it's a matter of removing this explicit stop and wait for a full period to elapse as we do for fair. It should reduce timer reprogramming overhead for scx as well. Thanks, Juri