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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: 8OAE1p0qTSeRVFnSRxediC4ErHxPhF0jtIOMgQa2lNBkAqRe10/QXHcrWbGHyGgOkNk+FZHi3YNE0NSQqjLEBAx2Af/yyHGDw+ahQnp5zPU= X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR10MB7530 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1057,Hydra:6.0.680,FMLib:17.12.62.30 definitions=2024-11-13_16,2024-11-13_01,2024-09-30_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 spamscore=0 adultscore=0 mlxlogscore=999 mlxscore=0 bulkscore=0 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2409260000 definitions=main-2411140000 X-Proofpoint-ORIG-GUID: kSE_nZXqCO5miBAMXlmjNbXQ6OUD4oUc X-Proofpoint-GUID: kSE_nZXqCO5miBAMXlmjNbXQ6OUD4oUc 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_DYNAMIC >> which allows for dynamic switching of preemption models. >> >> The choice of PREEMPT_RCU or not, however, is fixed at compile time. >> >> Given that PREEMPT_RCU makes some trade-offs to optimize for latency >> 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 oriented >> preemption models: PREEMPT, PREEMPT_RT, and the runtime configurable >> 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 Thanks! > 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, the > norm value becomes racy I think I see your point about rdp being racy, but given that rcu_read_unlock_strict() would always be called with a non-zero preemption count (with CONFIG_PREEMPTION), wouldn't the preempt_count() check defeat any calls to rcu_read_unlock_strict()? void rcu_read_unlock_strict(void) { struct rcu_data *rdp; if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread= ) return; Or am I missing something? Ankur > (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) > > > Let me audit further if we missed something else... > > Thanks. > >> -- >> 2.43.5 >>