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Date:   Tue, 27 Sep 2022 13:36:53 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ricardo Neri <ricardo.neri@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Len Brown <len.brown@intel.com>, Mel Gorman <mgorman@suse.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Valentin Schneider <vschneid@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, "Tim C . Chen" <tim.c.chen@intel.com>
Subject: Re: [RFC PATCH 12/23] thermal: intel: hfi: Convert table_lock to use
 flags-handling variants
Message-ID: <YzLgVckjbAAfC7jg@hirez.programming.kicks-ass.net>
References: <20220909231205.14009-1-ricardo.neri-calderon@linux.intel.com>
 <20220909231205.14009-13-ricardo.neri-calderon@linux.intel.com>
 <YzLfr49woc1PMOxO@hirez.programming.kicks-ass.net>
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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.