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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 11 Sep 2025 17:06:26.5683 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 92184cae-d401-46c6-6495-08ddf1558b3d 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: SJ1PEPF0000231D.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR12MB7892 Hello Shrikanth, On 9/11/2025 10:22 PM, Shrikanth Hegde wrote: >>> +    if (is_cpu_paravirt(cpu)) >>> +        push_current_from_paravirt_cpu(rq); >> >> Does this mean paravirt CPU is capable of handling an interrupt but may >> not be continuously available to run a task? > > When i run hackbench which involves fair bit of IRQ stuff, it moves out. > > For example, > > echo 600-710 > /sys/devices/system/cpu/paravirt > > 11:31:54 AM  CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest  %gnice   %idle > 11:31:57 AM  598    2.04    0.00   77.55    0.00   18.37    0.00    1.02    0.00    0.00    1.02 > 11:31:57 AM  599    1.01    0.00   79.80    0.00   17.17    0.00    1.01    0.00    0.00    1.01 > 11:31:57 AM  600    0.00    0.00    0.00    0.00    0.00    0.00    0.99    0.00    0.00   99.01 > 11:31:57 AM  601    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00 > 11:31:57 AM  602    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00  100.00 > > > There could some workloads which doesn't move irq's out, for which needs irqbalance change. > Looking into it. > >  Or is the VMM expected to set >> the CPU on the paravirt mask and give the vCPU sufficient time to move the >> task before yanking it away from the pCPU? >> > > If the vCPU is running something, it is going to run at some point on pCPU. > hypervisor will give the cycles to this vCPU by preempting some other vCPU. > > It is that using this infra, there is should be nothing on that paravirt vCPU. > That way collectively VMM gets only limited request for pCPU which it can satify > without vCPU preemption. Ack! Just wanted to understand the usage. P.S. I remember discussions during last LPC where we could communicate this unavailability via CPU capacity. Was that problematic for some reason? Sorry if I didn't follow this discussion earlier. [..snip..] >>> +    local_irq_save(flags); >>> +    preempt_disable(); >> >> Disabling IRQs implies preemption is disabled. >> > > Most of the places stop_one_cpu_nowait called with preemption & irq disabled. > stopper runs at the next possible opportunity. But is there any reason to do both local_irq_save() and preempt_disable()? include/linux/preempt.h defines preemptible() as: #define preemptible() (preempt_count() == 0 && !irqs_disabled()) so disabling IRQs should be sufficient right or am I missing something? > > stop_one_cpu_nowait >  ->queues the task into stopper list >     -> wake_up_process(stopper) >        -> set need_resched >          -> stopper runs as early as possible. >          -- Thanks and Regards, Prateek