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    SubjectRe: [RFC PATCH 12/23] thermal: intel: hfi: Convert table_lock to use flags-handling variants
    On Tue, Sep 27, 2022 at 01:36:53PM +0200, Peter Zijlstra wrote:
    > On Tue, Sep 27, 2022 at 01:34:07PM +0200, Peter Zijlstra wrote:
    > > On Fri, Sep 09, 2022 at 04:11:54PM -0700, Ricardo Neri wrote:
    > >
    > > > --- a/drivers/thermal/intel/intel_hfi.c
    > > > +++ b/drivers/thermal/intel/intel_hfi.c
    > > > @@ -175,9 +175,10 @@ static struct workqueue_struct *hfi_updates_wq;
    > > > static void get_hfi_caps(struct hfi_instance *hfi_instance,
    > > > struct thermal_genl_cpu_caps *cpu_caps)
    > > > {
    > > > + unsigned long flags;
    > > > int cpu, i = 0;
    > > >
    > > > - raw_spin_lock_irq(&hfi_instance->table_lock);
    > > > + raw_spin_lock_irqsave(&hfi_instance->table_lock, flags);
    > > > for_each_cpu(cpu, hfi_instance->cpus) {
    > > > struct hfi_cpu_data *caps;
    > > > s16 index;
    > >
    > > ^^^^ Anti-pattern alert!
    > >
    > > Now your IRQ latency depends on nr_cpus -- which is a fair fail. The
    > > existing code is already pretty crap in that it has the preemption
    > > latency depend on nr_cpus.
    > >
    > > While I'm here looking at the HFI stuff, did they fix that HFI interrupt
    > > broadcast mess already? Sending an interrupt to *all* CPUs is quite
    > > insane.
    >
    > Anyway; given the existence of this here loop; why not have something
    > like:
    >
    > DEFINE_PER_CPU(int, hfi_ipc_class);
    >
    > class = // extract from HFI mess
    > WRITE_ONCE(per_cpu(hfi_ipc_class, cpu), class);
    >
    > And then have the tick use this_cpu_read(hfi_ipc_class)? No extra
    > locking required.

    Thanks Peter. I think this is a good solution. I will implement it.

    BR,
    Ricardo

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