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Wed, 16 Oct 2024 09:36:59 GMT Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 5952F2004B; Wed, 16 Oct 2024 09:36:59 +0000 (GMT) Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 9B31A20040; Wed, 16 Oct 2024 09:36:55 +0000 (GMT) Received: from [9.124.220.43] (unknown [9.124.220.43]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTP; Wed, 16 Oct 2024 09:36:55 +0000 (GMT) Message-ID: Date: Wed, 16 Oct 2024 15:06:54 +0530 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: [PATCH 1/7] sched: warn for high latency with TIF_NEED_RESCHED_LAZY To: Ankur Arora , linux-kernel@vger.kernel.org Cc: peterz@infradead.org, tglx@linutronix.de, paulmck@kernel.org, mingo@kernel.org, bigeasy@linutronix.de, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, frederic@kernel.org, efault@gmx.de References: <20241009165411.3426937-1-ankur.a.arora@oracle.com> <20241009165411.3426937-2-ankur.a.arora@oracle.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <20241009165411.3426937-2-ankur.a.arora@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: JsMaHPEng6uTVByfH41hZVFNX1cOu3dv X-Proofpoint-GUID: ZQeHq2MQ5hslzMb9JPzVF9oC8q71SHMz X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1051,Hydra:6.0.680,FMLib:17.12.62.30 definitions=2024-10-15_01,2024-10-11_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 mlxlogscore=999 priorityscore=1501 bulkscore=0 suspectscore=0 malwarescore=0 spamscore=0 impostorscore=0 lowpriorityscore=0 mlxscore=0 phishscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2409260000 definitions=main-2410160061 On 10/9/24 22:24, Ankur Arora wrote: > resched_latency_warn() now also warns if TIF_NEED_RESCHED_LAZY is set > without rescheduling for more than the latency_warn_ms period. > I am bit confused here. Why do we need to warn if LAZY is set for a long time? If lazy set, the subsequent tick, it would be set to upgraded to NEED_RESCHED. Since the value of latency_warn_ms=100ms, that means even on system with HZ=100, that means 10 ticks before that warning would be printed no? IIUC, the changelog c006fac556e40 ("sched: Warn on long periods of pending need_resched") has the concern of need_resched set but if it is non-preemptible kernel it would spend a lot of time in kernel mode. In that case print a warning. If someone enables Lazy, that means it is preemptible and probably this whole notion of resched_latency_warn doesn't apply to lazy. Please correct me if i am not understanding this correctly. > Cc: Ingo Molnar > Cc: Peter Ziljstra > Cc: Juri Lelli > Cc: Vincent Guittot > Signed-off-by: Ankur Arora > --- > kernel/sched/core.c | 2 +- > kernel/sched/debug.c | 7 +++++-- > 2 files changed, 6 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 694bfcf153cb..1229766b704e 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -5571,7 +5571,7 @@ static u64 cpu_resched_latency(struct rq *rq) > if (sysctl_resched_latency_warn_once && warned_once) > return 0; > > - if (!need_resched() || !latency_warn_ms) > + if ((!need_resched() && !tif_need_resched_lazy()) || !latency_warn_ms) > return 0; > > if (system_state == SYSTEM_BOOTING) > diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c > index 9abcc6ead11b..f0d551ba64bb 100644 > --- a/kernel/sched/debug.c > +++ b/kernel/sched/debug.c > @@ -1293,9 +1293,12 @@ void proc_sched_set_task(struct task_struct *p) > void resched_latency_warn(int cpu, u64 latency) > { > static DEFINE_RATELIMIT_STATE(latency_check_ratelimit, 60 * 60 * HZ, 1); > + char *nr; > + > + nr = tif_need_resched() ? "need_resched" : "need_resched_lazy"; > > WARN(__ratelimit(&latency_check_ratelimit), > - "sched: CPU %d need_resched set for > %llu ns (%d ticks) " > + "sched: CPU %d %s set for > %llu ns (%d ticks) " > "without schedule\n", > - cpu, latency, cpu_rq(cpu)->ticks_without_resched); > + cpu, nr, latency, cpu_rq(cpu)->ticks_without_resched); > }