linux-kernel - Re: [RFC PATCH 1/9] cxl/mem: Implement Get Event Records command

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Date:   Tue, 16 Aug 2022 16:11:12 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Dan Williams <dan.j.williams@...el.com>,
        Alison Schofield <alison.schofield@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "Ben Widawsky" <bwidawsk@...nel.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        <linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>
Subject: Re: [RFC PATCH 1/9] cxl/mem: Implement Get Event Records command

On Tue, Aug 16, 2022 at 12:41:28PM -0400, Steven Rostedt wrote:
> On Tue, 16 Aug 2022 12:39:58 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > > +		record_count = le16_to_cpu(payload.record_count);
> > > +		if (record_count > 0)
> > > +			trace_cxl_event(dev_name(cxlds->dev), type,
> > > +					&payload.record);
> > > +
> > > +		if (payload.flags & CXL_GET_EVENT_FLAG_OVERFLOW)
> > > +			trace_cxl_event_overflow(dev_name(cxlds->dev), type,
> > > +						 &payload);  
> > 
> > If you want to avoid the compare operations when the tracepoints are not
> > enabled, you can add:
> > 
> > 		if (trace_cxl_event_enabled()) {
> > 			if (record_count > 0)
> > 				trace_cxl_event(dev_name(cxlds->dev), type,
> > 						&payload.record);
> > 		}
> > 
> > 		if (trace_cxl_event_overflow_enabled()) {
> > 			if (payload.flags & CXL_GET_EVENT_FLAG_OVERFLOW)
> > 				trace_cxl_event_overflow(dev_name(cxlds->dev), type,
> > 							 &payload);
> > 		}
> > 
> > Those "<tracepoint>_enabled()" functions are static branches. Which means
> > when not enabled it's a nop that skips this code, and when either is
> > enabled, it turns into a jump to the contents of the if block.
> 
> Ignore this suggestion. I see in the second patch you add more logic to the
> if condition.

Correct.

> Only use this suggestion if the logic is only for when the
> tracepoint is enabled.

This could apply to the overflow trace.  I think it is more likely that either
all of these traces will be enabled or none.

So other than doing:

	if (trace_cxl_event_enabled() ||
	    trace_cxl_event_overflow_enabled() ||
	    trace_cxl_gen_media_event() ||
	    <every event defined>
	    ...) {

Is there a way to know if 

	/sys/kernel/tracing/events/cxl_events/enable

is != 0?

I feel like the real optimization will be to shut this entire functionality
down if no one is listening.

Ira