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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 05 Dec 2025 13:05:48.3470 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 57c01f97-0b52-4981-2aed-08de33ff027c 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=[satlexmb08.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ1PEPF00002327.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR12MB4456 On 12/5/2025 6:26 PM, Peter Zijlstra wrote: > On Fri, Dec 05, 2025 at 06:19:07PM +0530, K Prateek Nayak wrote: >> On 12/5/2025 2:29 PM, Peter Zijlstra wrote: >>>>> Why not use move_queued_task() ? >>>> >>>> double_lock_balance() can fail and prevent being blocked waiting for >>>> new rq whereas move_queued_task() will wait, won't it ? >>>> >>>> Do you think move_queued_task() would be better ? >>> >>> No, double_lock_balance() never fails, the return value indicates if the >>> currently held rq-lock, (the first argument) was unlocked while >>> attaining both -- this is required when the first rq is a higher address >>> than the second. >>> >>> double_lock_balance() also puts the wait-time and hold time of the >>> second inside the hold time of the first, which gets you a quadric term >>> in the rq hold times IIRC. Something that's best avoided. >>> >>> move_queued_task() OTOH takes the task off the runqueue you already hold >>> locked, drops this lock, acquires the second, puts the task there, and >>> returns with the dst rq locked. >> >> So I was experimenting with: >> >> deactivate_task(rq, p, 0); >> set_task_cpu(p, target_cpu); >> __ttwu_queue_wakelist(p, target_cpu, 0); >> >> and nothing has screamed at me yet during the benchmark runs. >> Would this be any good instead of the whole lock juggling? > > This will get schedstats and any class with ->task_woken confused I > think. The IPI handler (sched_ttwu_pending / ttwu_do_activate) is > currently only geared towards doing the remote bit of ttwu. > > This is fixable of course. I believe a simple enough check could be: task_on_rq_migrating() -> queued (vs) blocked -> queued during activation. > >> Since this CPU is found to be going overloaded, pushing via an >> IPI vs taking the overhead ourselves seems to make more sense >> to me from EAS standpoint. > > The performance characteristics here are very platform dependent. > Sometimes raising an IPI can be very expensive in and of itself. Ack! But I assume those platforms already disable TTWU_QUEUE? Or maybe the ttwu_queue_cond() takes care of it indirectly. We can add some guards if necessary and use the same fallback as PREEMPT_RT. -- Thanks and Regards, Prateek