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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: YO6DPr54H68vTp3Rp9BcbwS53YODe0vdO0AajcTIDs69nKnD+Q0vrZPwbiFMmUhTCiOFQTneOvQTkteVTypGZxeR8JB26XqR0HmPIfTyA+0= X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH0PR10MB7440 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1057,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2024-11-29_16,2024-11-28_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 malwarescore=0 spamscore=0 adultscore=0 mlxlogscore=999 suspectscore=0 mlxscore=0 phishscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2411120000 definitions=main-2411290154 X-Proofpoint-GUID: k5giO209QE2sNcghGTUoc9yTwKwSVW4N X-Proofpoint-ORIG-GUID: k5giO209QE2sNcghGTUoc9yTwKwSVW4N Frederic Weisbecker writes: > Le Thu, Nov 28, 2024 at 09:03:56PM -0800, Ankur Arora a =C3=A9crit : >> >> Frederic Weisbecker writes: >> >> > Le Wed, Nov 06, 2024 at 12:17:57PM -0800, Ankur Arora a =C3=A9crit : >> >> To reduce RCU noise for nohz_full configurations, osnoise depends >> >> on cond_resched() providing quiescent states for PREEMPT_RCU=3Dn >> >> configurations. And, for PREEMPT_RCU=3Dy configurations does this >> >> by directly calling rcu_momentary_eqs(). >> >> >> >> With PREEMPT_LAZY=3Dy, however, we can have configurations with >> >> (PREEMPTION=3Dy, PREEMPT_RCU=3Dn), which means neither of the above >> >> can help. >> >> >> >> Handle that by fallback to the explicit quiescent states via >> >> rcu_momentary_eqs(). >> >> >> >> Cc: Paul E. McKenney >> >> Cc: Daniel Bristot de Oliveira >> >> Cc: Steven Rostedt >> >> Suggested-by: Paul E. McKenney >> >> Acked-by: Daniel Bristot de Oliveira >> >> Signed-off-by: Ankur Arora >> >> --- >> >> kernel/trace/trace_osnoise.c | 22 ++++++++++++---------- >> >> 1 file changed, 12 insertions(+), 10 deletions(-) >> >> >> >> diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnois= e.c >> >> index a50ed23bee77..15e9600d231d 100644 >> >> --- a/kernel/trace/trace_osnoise.c >> >> +++ b/kernel/trace/trace_osnoise.c >> >> @@ -1538,18 +1538,20 @@ static int run_osnoise(void) >> >> /* >> >> * In some cases, notably when running on a nohz_full CPU with >> >> * a stopped tick PREEMPT_RCU has no way to account for QSs. >> >> - * This will eventually cause unwarranted noise as PREEMPT_RCU >> >> - * will force preemption as the means of ending the current >> >> - * grace period. We avoid this problem by calling >> >> - * rcu_momentary_eqs(), which performs a zero duration >> >> - * EQS allowing PREEMPT_RCU to end the current grace period. >> >> - * This call shouldn't be wrapped inside an RCU critical >> >> - * section. >> >> + * This will eventually cause unwarranted noise as RCU forces >> >> + * preemption as the means of ending the current grace period. >> >> + * We avoid this by calling rcu_momentary_eqs(), which performs >> >> + * a zero duration EQS allowing RCU to end the current grace >> >> + * period. This call shouldn't be wrapped inside an RCU >> >> + * critical section. >> >> * >> >> - * Note that in non PREEMPT_RCU kernels QSs are handled through >> >> - * cond_resched() >> >> + * For non-PREEMPT_RCU kernels with cond_resched() (non- >> >> + * PREEMPT_LAZY configurations), QSs are handled through >> >> + * cond_resched(). For PREEMPT_LAZY kernels, we fallback to >> >> + * the zero duration QS via rcu_momentary_eqs(). >> >> */ >> >> - if (IS_ENABLED(CONFIG_PREEMPT_RCU)) { >> >> + if (IS_ENABLED(CONFIG_PREEMPT_RCU) || >> >> + (!IS_ENABLED(CONFIG_PREEMPT_RCU) && IS_ENABLED(CONFIG_PREEMPTI= ON))) { >> >> if (!disable_irq) >> >> local_irq_disable(); >> > >> > How about making this unconditional so it works everywhere and doesn't >> > rely on cond_resched() Kconfig/preempt-dynamic mood? >> >> I think it's a minor matter given that this isn't a hot path, but >> we don't really need it for the !PREEMPT_RCU configuration. > > Well if you make it unconditional, cond_resched() / rcu_all_qs() won't do= its > own rcu_momentary_qs(), because rcu_data.rcu_urgent_qs should > be false. So that essentially unify the behaviours for all configurations= . Ah, yes. That makes sense. Ankur >> >> Still, given that both of those clauses imply CONFIG_PREEMPTION, we >> can just simplify this to (with an appropriately adjusted comment): >> >> --- a/kernel/trace/trace_osnoise.c >> +++ b/kernel/trace/trace_osnoise.c >> @@ -1543,7 +1543,7 @@ static int run_osnoise(void) >> * Note that in non PREEMPT_RCU kernels QSs are handled = through >> * cond_resched() >> */ >> - if (IS_ENABLED(CONFIG_PREEMPT_RCU)) { >> + if (IS_ENABLED(CONFIG_PREEMPTION)) { >> if (!disable_irq) >> local_irq_disable(); >> >> -- >> ankur -- ankur