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SubjectRe: PATCH] ftrace: Add a C/P state tracer to help power optimization

On Mon, 6 Oct 2008, Andi Kleen wrote:
>
> > +void trace_power_end(struct power_trace *it)
> > +{
> > + struct ring_buffer_event *event;
> > + struct trace_power *entry;
> > + struct trace_array_cpu *data;
> > + unsigned long irq_flags;
> > + struct trace_array *tr = power_trace;
> > +
> > + if (!trace_power_enabled)
> > + return;
> > +
> > + preempt_disable();
> > + it->end = ktime_get();
> > + data = tr->data[smp_processor_id()];
> > +
> > + event = ring_buffer_lock_reserve(tr->buffer, sizeof(*entry),
> > + &irq_flags);
> > + if (!event)
> > + goto out;
> > + entry = ring_buffer_event_data(event);
> > + tracing_generic_entry_update(&entry->ent, 0, 0);
> > + entry->ent.type = TRACE_POWER;
> > + entry->state_data = *it;
> > + ring_buffer_unlock_commit(tr->buffer, event, irq_flags);
>
> When ring_buffer_lock_reserve really disables interrupts (I haven't
> checked since it's not in 2.6.27rc8) you could avoid the
> preempt_disable by moving the data = tr->data ... one below.

The ring_buffer_lock_reserve use to disable interrupts (I'll be removing
the flags argument soon). Now it just does a minimum of preempt_disable.
So your suggestion about moving the smp_processor_id() calls below that
is still valid. The reserve will always disable preemption.

-- Steve

>
> Similar comments to trace_power_mark. Also it would be probably good
> to use a common function instead of duplicating so much code.
>
>


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