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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: A0isSSjvIzCf1uAilaG6pMJtIVSh/IISTiNoKfLXgZIjEOSmZzFW2eTzjkEo59yTdoAag94g/clE/ndO3PkX4+BAwTRvDunk/65R6Qg40U0= X-MS-Exchange-Transport-CrossTenantHeadersStamped: SJ0PR10MB6350 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_02,2024-11-28_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 malwarescore=0 suspectscore=0 spamscore=0 phishscore=0 mlxscore=0 mlxlogscore=999 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2411120000 definitions=main-2411290035 X-Proofpoint-GUID: XuFeNBSgac0qHUSv097W31KAZWTewBsK X-Proofpoint-ORIG-GUID: XuFeNBSgac0qHUSv097W31KAZWTewBsK Frederic Weisbecker writes: > Le Tue, Nov 26, 2024 at 10:19:05PM -0800, Ankur Arora a =C3=A9crit : >> >> Ankur Arora writes: >> >> > Frederic Weisbecker writes: >> > >> >> Le Mon, Nov 25, 2024 at 01:40:39PM -0800, Ankur Arora a =C3=A9crit : >> >>> >> >>> Frederic Weisbecker writes: >> >>> >> >>> > Le Wed, Nov 06, 2024 at 12:17:55PM -0800, Ankur Arora a =C3=A9crit= : >> >>> >> PREEMPT_LAZY can be enabled stand-alone or alongside PREEMPT_DYNA= MIC >> >>> >> which allows for dynamic switching of preemption models. >> >>> >> >> >>> >> The choice of PREEMPT_RCU or not, however, is fixed at compile ti= me. >> >>> >> >> >>> >> Given that PREEMPT_RCU makes some trade-offs to optimize for late= ncy >> >>> >> as opposed to throughput, configurations with limited preemption >> >>> >> might prefer the stronger forward-progress guarantees of PREEMPT_= RCU=3Dn. >> >>> >> >> >>> >> Accordingly, explicitly limit PREEMPT_RCU=3Dy to the latency orie= nted >> >>> >> preemption models: PREEMPT, PREEMPT_RT, and the runtime configura= ble >> >>> >> model PREEMPT_DYNAMIC. >> >>> >> >> >>> >> This means the throughput oriented models, PREEMPT_NONE, >> >>> >> PREEMPT_VOLUNTARY and PREEMPT_LAZY will run with PREEMPT_RCU=3Dn. >> >>> >> >> >>> >> Cc: Paul E. McKenney >> >>> >> Cc: Peter Zijlstra >> >>> >> Signed-off-by: Ankur Arora >> >>> >> --- >> >>> >> kernel/rcu/Kconfig | 2 +- >> >>> >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >>> >> >> >>> >> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig >> >>> >> index 5a7ff5e1cdcb..9d52f87fac27 100644 >> >>> >> --- a/kernel/rcu/Kconfig >> >>> >> +++ b/kernel/rcu/Kconfig >> >>> >> @@ -18,7 +18,7 @@ config TREE_RCU >> >>> >> >> >>> >> config PREEMPT_RCU >> >>> >> bool >> >>> >> - default y if PREEMPTION >> >>> >> + default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC) >> >>> >> select TREE_RCU >> >>> >> help >> >>> >> This option selects the RCU implementation that is >> >>> > >> >>> > Reviewed-by: Frederic Weisbecker >> >>> > >> >>> > But looking at !CONFIG_PREEMPT_RCU code on tree_plugin.h, I see >> >>> > some issues now that the code can be preemptible. Well I think >> >>> > it has always been preemptible but PREEMPTION && !PREEMPT_RCU >> >>> > has seldom been exerciced (or was it even possible?). >> >>> > >> >>> > For example rcu_read_unlock_strict() can be called with preemption >> >>> > enabled so we need the following otherwise the rdp is unstable, th= e >> >>> > norm value becomes racy (though automagically fixed in rcu_report_= qs_rdp()) >> >>> > and rcu_report_qs_rdp() might warn. >> >>> > >> >>> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h >> >>> > index 58d84c59f3dd..368f00267d4e 100644 >> >>> > --- a/include/linux/rcupdate.h >> >>> > +++ b/include/linux/rcupdate.h >> >>> > @@ -95,9 +95,9 @@ static inline void __rcu_read_lock(void) >> >>> > >> >>> > static inline void __rcu_read_unlock(void) >> >>> > { >> >>> > - preempt_enable(); >> >>> > if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) >> >>> > rcu_read_unlock_strict(); >> >>> > + preempt_enable(); >> >>> > } >> >>> > >> >>> > static inline int rcu_preempt_depth(void) >> >>> >> >>> Based on the discussion on the thread, how about keeping this and >> >>> changing the preempt_count check in rcu_read_unlock_strict() instead= ? >> >>> >> >>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h >> >>> index 1c7cbd145d5e..8fc67639d3a7 100644 >> >>> --- a/kernel/rcu/tree_plugin.h >> >>> +++ b/kernel/rcu/tree_plugin.h >> >>> @@ -831,8 +831,15 @@ dump_blkd_tasks(struct rcu_node *rnp, int nchec= k) >> >>> void rcu_read_unlock_strict(void) >> >>> { >> >>> struct rcu_data *rdp; >> >>> + int pc =3D ((preempt_count() & PREEMPT_MASK) >> PREEMPT_SHIF= T); >> >> >> >> This should be in_atomic_preempt_off(), otherwise softirqs and IRQs a= re >> >> spuriously accounted as quiescent states. >> > >> > Not sure I got that. Won't ((preempt_count() & PREEMPT_MASK) >> PREEMP= T_SHIFT) >> > give us task only preempt count? >> >> Oh wait. I see your point now. My check is too narrow. >> >> Great. This'll work: >> >> - if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread) >> + if (irqs_disabled() || in_atomic_preempt_off()|| !rcu_state.gp_k= thread) >> >> Thanks > > Do you plan to integrate this in a further version of your set? Or should= I send > a patch? Sure. I can add it to v3. Okay, if I add your suggested-by? -- ankur