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SubjectRe: [PATCH v2 1/2] x86/fpu: Extend kernel_fpu_begin_mask() to initialize AMX state
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On 5/8/2024 7:40 AM, Dave Hansen wrote:
> On 5/7/24 16:53, Chang S. Bae wrote:
>
>> However, due to resource constraints in storage, AMX state is excluded
>> from the scope of state recovery. Consequently, AMX state must be in its
>> initialized state for the IFS test to run.
>
> This doesn't mention how this issue got introduced. Are we all bad at
> reading the SDM? :)

Ah, I'd rather zap out this SDM sentence.

>> When AMX workloads are running, an active user AMX state remains even
>> after a context switch, optimizing to reduce the state reload cost. In
>> such cases, the test cannot proceed if it is scheduled.
>
> This is a bit out of the blue. What does scheduling have do do with IFS?

$ echo <cpu#> > /sys/devices/virtual/misc/intel_ifs_0/run_test

Then,
run_test_store()
-> do_core_test()
-> ifs_test_core()
-> stop_core_cpuslocked()
-> stop_cpus()
-> queue_stop_cpus_work()
-> cpu_stop_queue_work()
-> wake_q_add()
-> wake_up_q()

So, the CPU stopper threads for <cpu#> and its sibling to execute
doscan() are queued up with the highest priority.

queue_stop_cpus_work() has

for_each_cpu(cpu, cpumask) {
work = &per_cpu(cpu_stopper.stop_work, cpu);
work->fn = fn;
work->arg = arg;
work->done = done;
work->caller = _RET_IP_;
if (cpu_stop_queue_work(cpu, work))
queued = true;
}

Those threads are created during early boot via
smpboot_register_percpu_thread().

> I'm not sure those last two paragraphs add much value. I'd try to
> banish most of that content to *after* you talk about the solution. Or
> maybe put it in the cover letter.

It looks like lots of distractions coming from bunch of alternatives in
different levels.

Thanks,
Chang

PS: Let me respond the solution discussion separately. I do want to
experiment the init-track behavior a bit.

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