linux-kernel - Re: [PATCH V2 2/3] perf/x86/intel/bm.c: Add Intel Branch Monitoring support

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Date:   Tue, 12 Dec 2017 23:32:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Megha Dey <megha.dey@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, andriy.shevchenko@...ux.intel.com,
        kstewart@...uxfoundation.org, yu-cheng.yu@...el.com,
        len.brown@...el.com, gregkh@...uxfoundation.org, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, vikas.shivappa@...ux.intel.com,
        pombredanne@...b.com, me@...ehuey.com, bp@...e.de,
        grzegorz.andrejczuk@...el.com, tony.luck@...el.com, corbet@....net,
        ravi.v.shankar@...el.com
Subject: Re: [PATCH V2 2/3] perf/x86/intel/bm.c: Add Intel Branch Monitoring
 support

On Tue, Dec 12, 2017 at 01:10:57PM -0800, Megha Dey wrote:
> On Mon, 2017-11-20 at 12:57 +0100, Peter Zijlstra wrote:
> > On Fri, Nov 17, 2017 at 05:54:05PM -0800, Megha Dey wrote:
> > > +	mutex_lock(&bm_counter_mutex);
> > > +	for (i = 0; i < BM_MAX_COUNTERS; i++) {
> > > +		if (bm_counter_owner[i] == NULL) {
> > > +			counter_to_use = i;
> > > +			bm_counter_owner[i] = event;
> > > +			break;
> > > +		}
> > > +	}
> > > +	mutex_unlock(&bm_counter_mutex);
> > > +
> > > +	if (counter_to_use == -1)
> > > +		return -EBUSY;
> > 
> > > +static struct pmu intel_bm_pmu = {
> > > +	.task_ctx_nr     = perf_sw_context,
> > > +	.attr_groups     = intel_bm_attr_groups,
> > > +	.event_init      = intel_bm_event_init,
> > > +	.add             = intel_bm_event_add,
> > > +	.del             = intel_bm_event_del,
> > > +};
> > 
> > Still horrid.. still no.
> 
> It seems like perf_invalid_context does not support per task monitoring:
> find_get_context():
>  ctxn = pmu->task_ctx_nr;
>         if (ctxn < 0)
>                 goto errout;
> 
> Also, perf_hw_context is to be used only for core PMU, correct?
> 
> That leaves us with only perf_sw_context to be used. Not sure if a new
> context needs to be implemented.

There's work on the way to allow multiple HW PMUs. You'll either have to
wait for that or help in making that happen. What you do not do is
silently hack around it.