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Tue, 22 Oct 2024 05:41:23 GMT Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3A8D02004B; Tue, 22 Oct 2024 05:41:23 +0000 (GMT) Received: from smtpav01.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E52B220040; Tue, 22 Oct 2024 05:41:19 +0000 (GMT) Received: from [9.124.208.49] (unknown [9.124.208.49]) by smtpav01.fra02v.mail.ibm.com (Postfix) with ESMTP; Tue, 22 Oct 2024 05:41:19 +0000 (GMT) Message-ID: <19a072e1-cfd8-43e1-9e8e-aa1950b20533@linux.ibm.com> Date: Tue, 22 Oct 2024 11:11:18 +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 Cc: linux-kernel@vger.kernel.org, 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> <87iktli7o6.fsf@oracle.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: <87iktli7o6.fsf@oracle.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: usf6-aVYRlsWImRD6O-ARk41k-Uy-5y1 X-Proofpoint-GUID: iEgEjaitKUNbnWhJGrjzIRJDkU4aAHXV 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 spamscore=0 clxscore=1015 phishscore=0 impostorscore=0 priorityscore=1501 lowpriorityscore=0 bulkscore=0 mlxscore=0 adultscore=0 mlxlogscore=999 suspectscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.19.0-2409260000 definitions=main-2410220035 On 10/22/24 00:51, Ankur Arora wrote: > > Shrikanth Hegde writes: > >> 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? > > That's a fair point. However, the assumption there is that there are no > bugs in upgrade on tick or that there's no situation in which the tick > is off for a prolonged period. > ok. But if tick is off, then ticks_without_resched isn't incremented either. IIUC, this check is for situation when NR is set and tick is on. > Ankur > >> 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); >>> } > > > -- > ankur