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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 29 Sep 2025 04:37:38.8809 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 68ffed1c-c2cb-4d1e-7c43-08ddff11eba1 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: BN1PEPF00005FFC.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN6PR12MB8492 Hello Jemmy, On 9/28/2025 8:44 PM, Jemmy Wong wrote: > The TIF_NEED_RESCHED_LAZY flag can be set multiple times in a single > call path. For example: > > entity_tick() > update_curr(cfs_rq); > resched_curr_lazy(rq); > resched_curr_lazy(rq_of(cfs_rq)); > > Add a check in resched_curr_lazy() to return early if the flag is > already set, avoiding redundant operations. That would have been a decent idea but then you decided to put that check in __resched_curr() which makes it plain wrong. [..snip..] > --- a/include/linux/thread_info.h > +++ b/include/linux/thread_info.h > @@ -67,6 +67,8 @@ enum syscall_work_bit { > #define _TIF_NEED_RESCHED_LAZY _TIF_NEED_RESCHED > #endif > > +#define _TIF_NEED_RESCHED_MUSK (_TIF_NEED_RESCHED | _TIF_NEED_RESCHED_LAZY) s/MUSK/MASK/g > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -1108,7 +1108,7 @@ static void __resched_curr(struct rq *rq, int tif) > if (is_idle_task(curr) && tif == TIF_NEED_RESCHED_LAZY) > tif = TIF_NEED_RESCHED; > > - if (cti->flags & ((1 << tif) | _TIF_NEED_RESCHED)) > + if (cti->flags & ((1 << tif) | _TIF_NEED_RESCHED_MUSK)) > return; __resched_curr() is used to set both TIF_NEED_RESCHED_LAZY and TIF_NEED_RESCHED. By putting this check here, any effort to set NEED_RESCHED and force an early preemption will bail out if NEED_RESCHED_LAZY is already set which will delay the preemption. An example: /* New fair task wakes up. */ check_preempt_wakeup_fair() resched_curr_lazy() __resched_curr(TIF_NEED_RESCHED_LAZY) /* New RT task wakes up. */ wakeup_preempt() resched_curr() __resched_curr(TIF_NEED_RESCHED) /* Sees NEED_RESCHED_LAZY is already set. */ /* Does not do a set_preempt_need_resched() */ ... /* Added latency */ sched_tick() if (tif_test_bit(TIF_NEED_RESCHED_LAZY)) resched_curr() __resched_curr(TIF_NEED_RESCHED) /* Again bails out early! */ ... /* More latency! */ So, the tick doesn't even upgrade the LAZY flag to a full NEED_RESCHED and the only time you actually schedule is either at exit to user mode or if a kthread decides to yield. Going back to your commit message, something like: diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 7f1e5cb94c53..3275abce9ca2 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1164,6 +1164,9 @@ static __always_inline int get_lazy_tif_bit(void) void resched_curr_lazy(struct rq *rq) { + if (task_thread_info(rq->curr)->flags & TIF_NEED_RESCHED_MASK) + return; + __resched_curr(rq, get_lazy_tif_bit()); } probably fits the bill better. > > cpu = cpu_of(rq); > -- > 2.50.1 (Apple Git-155) > -- Thanks and Regards, Prateek