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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: =?us-ascii?Q?r8UWd2j65Gj1RsRnmpyHCyLeVoPAbEtj97DPWFLf6HhvJJm1TehIqIdPvUjp?= =?us-ascii?Q?GYWBKtsueZx1rRKfpYdZFed9RvjSs2oI40YFAUA0DJ7ihLNLO4mCfqBWglM8?= =?us-ascii?Q?8rCsJl6Y8dYNmy4zJ5pEay3vr2gMBxrsr0pNq+O2U5MiTtDHroPmW6dNcVtM?= =?us-ascii?Q?4CYBt4ho/b+V49Y4awHXE2hw7sEyvNcVV2Wwx8L/tN3VUQ9gDjeVsNSIzneC?= =?us-ascii?Q?rATzkoAe4Kpgxy7Ey5Aso4z5j60z7X9XUS/Anaw3QjA4AIPtd9Cw5RL9R2gr?= =?us-ascii?Q?J/6JvdjpwNzN3D+ToACo32I9XvND8BmjHbcVdAxgQQe4CccymRo0YEwZuZqN?= =?us-ascii?Q?EkKNbQHX3XEIWfZU4LqJgSVMi1FDdllGSa+ZsGG+NWJF/nW5q9APwtvpmLcd?= =?us-ascii?Q?qX81SNEPDql27KlhCsRJsQSQkG/Sh70Q/qB4rD8Iw6p6ig+oTM0WxQZ6dXv2?= =?us-ascii?Q?1hYRzHPvTgDWw1TMmUzKxmAwvl/7vyylnwg5w2FU/x7PY+ZLoHh7cw9CeD4w?= =?us-ascii?Q?TjDbM425FGHQj9BwXqEviTKXmD11sNtB0sBOYCFKs28SOezEmN0hMOruRp4D?= =?us-ascii?Q?ZkqHsCq+kfoS1iSABmH/MaAdG0vabHX2KiYg06boci0kh4Se0u2fYP9SHid+?= =?us-ascii?Q?nMG4//NjqPqr7qn9quGf1Fsia1WXBIJX3GaSOcTnyxC7rPEYx/+smuTBH16v?= =?us-ascii?Q?mPzThN/QhVGbAxpcsQ0Y4PjWOAiA6AhhfBZ3uEhdFa50J9zkf/yXczypcSIg?= =?us-ascii?Q?svfwN9xpJXb6hAdEQBntW6T7dfURoDm44cNkuc6Bkq9dj+99VDlu+AliCmW/?= =?us-ascii?Q?93y3J0KC9N2UqeTx22/heFo88LWh4VeKrlZcCgkyMBN9qKqToK1fa/OWRreE?= =?us-ascii?Q?T/ndsitJM6lfIS+MSWgfWfh8Xz8+zr/Qqflh52jacTmfI0pis07ty4p3KEAS?= =?us-ascii?Q?y/93JwB2Y7dY35fB1Zdv4ffFHwWplvUplUV6yeFsUX64tjyAscJLgz2vSwBm?= =?us-ascii?Q?PB7Fz33mlrx1uu7pKU8ecURn9/BFXzt/N3xwVwpRZBZNpjLMdccE87rbVmyY?= =?us-ascii?Q?VtDEjcvfrzQfTp+0y3xfObms9+YZKVmpT1LOsBeqct3HU+JfCUom18zwF//G?= =?us-ascii?Q?/ryz83pdgBwfSPcgXnBjsTHLX4dNaKPbfr3j7r4AawxMSgyCm3OZsLdGkm/p?= =?us-ascii?Q?FRY4BPmCxDI1+aokLQ/aMgcYEWnPzAtKzs0uhf+3LEcJHpBqLYUKNQXn87Il?= =?us-ascii?Q?OXdjjEPFm146Xsi1NNEwHIuAiI31BLnyPxJUgBc4Yf40vO+mFvxo6xQXyOue?= =?us-ascii?Q?/xxbD2RoeATcGFZ0BQxgMOHzp5y8oZSr984mu6LsXn9syPsAt3GewIEi4NqG?= =?us-ascii?Q?J8JU+jcQQAe6UIVK4ZZZGJfFqnTLxcuGLY8S9IJTkq75xK4H7sJFlb6p5h4v?= =?us-ascii?Q?hSw14v71VB4iB2FHbcV6vZGvCHZJrXPWudcVMszwSaCveANHu4GqpIbPdwZ1?= =?us-ascii?Q?b2HQo4Ad7PBnqtsDje30+SpICnCnBd13zPG+kQa8OV9PBeFjHDKr5hwsrgqS?= =?us-ascii?Q?WaUvOouMqtCfN7CGrT5ZD2BIyvTcimrulH926yQV?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: 479d8d94-44cf-4c20-6e61-08dd657817d9 X-MS-Exchange-CrossTenant-AuthSource: CY5PR12MB6405.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Mar 2025 17:21:03.0107 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: DNxtSn/HjV4Cp6pw4osDCos/t4/ZcVx/3jmYKswinLmTc+C18DKiRpOHjMUtjVTVugzhEEnPkdZG0gyomGQ+yA== X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR12MB6467 Hi Joel, On Mon, Mar 17, 2025 at 04:28:02AM -0400, Joel Fernandes wrote: > Consider that the previous CPU is cache affined to the waker's CPU and > is idle. Currently, scx's default select function only selects the > previous CPU in this case if WF_SYNC request is also made to wakeup on the > waker's CPU. > > This means, without WF_SYNC, the previous CPU being cache affined to the > waker and is idle is not considered. This seems extreme. WF_SYNC is not > normally passed to the wakeup path outside of some IPC drivers but it is > very possible that the task is cache hot on previous CPU and shares > cache with the waker CPU. Lets avoid too many migrations and select the > previous CPU in such cases. > > This change is consistent with the fair scheduler's behavior as well. In > select_idle_sibling(), the previous CPU is selected if it is cache > affined with the target. This is done regardless of WF_SYNC and before > any scanning of fully idle cores is done. > > One difference still exists though between SCX and CFS in this regard, in CFS > we first check if the target CPU is idle before checking if the previous CPU is > idle. However that could be a matter of choice and in the future, that behavior > could also be unified. > > Signed-off-by: Joel Fernandes > --- > kernel/sched/ext.c | 24 +++++++++++------------- > 1 file changed, 11 insertions(+), 13 deletions(-) > > diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c > index 5a81d9a1e31f..3b1a489e2aaf 100644 > --- a/kernel/sched/ext.c > +++ b/kernel/sched/ext.c > @@ -3479,7 +3479,7 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, > { > const struct cpumask *llc_cpus = NULL; > const struct cpumask *numa_cpus = NULL; > - s32 cpu; > + s32 cpu = smp_processor_id(); > > *found = false; > > @@ -3507,22 +3507,20 @@ static s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, > llc_cpus = llc_span(prev_cpu); > } > > + /* > + * If the waker's CPU is cache affine and prev_cpu is idle, then avoid > + * a migration. > + */ > + if (cpus_share_cache(cpu, prev_cpu) && > + test_and_clear_cpu_idle(prev_cpu)) { > + cpu = prev_cpu; > + goto cpu_found; > + } > + One potential issue that I see is that when SMT is enabled, you may end up using prev_cpu also when the other sibling is busy. Maybe we should check if prev_cpu is set in the SMT idle cpumask. Also, last time I tried a similar change I was regressing a lot of benchmarks. Maybe we should repeat the tests and get some numbers. > /* > * If WAKE_SYNC, try to migrate the wakee to the waker's CPU. > */ > if (wake_flags & SCX_WAKE_SYNC) { > - cpu = smp_processor_id(); > - > - /* > - * If the waker's CPU is cache affine and prev_cpu is idle, > - * then avoid a migration. > - */ > - if (cpus_share_cache(cpu, prev_cpu) && > - test_and_clear_cpu_idle(prev_cpu)) { > - cpu = prev_cpu; > - goto cpu_found; > - } > - > /* > * If the waker's local DSQ is empty, and the system is under > * utilized, try to wake up @p to the local DSQ of the waker. > -- > 2.43.0 > Thanks, -Andrea