Re: [PATCH V1 2/3] perf/x86/intel/bm.c: Add Intel Branch Monitoring support

[Megha Dey <megha.dey@intel.com>]

On Mon, 2017-11-13 at 21:25 +0100, Thomas Gleixner wrote:
> On Mon, 13 Nov 2017, Dey, Megha wrote:
> > >-----Original Message-----
> > >From: Peter Zijlstra [mailto:peterz@infradead.org]
> > >Sent: Monday, November 13, 2017 1:00 AM
> > >To: Megha Dey <megha.dey@linux.intel.com>
> > >Cc: x86@kernel.org; linux-kernel@vger.kernel.org; linux-
> 
> Please fix your mail client so it does not add this complete useless
> information to the reply.

Will fix this.
> 
> > >On Sat, Nov 11, 2017 at 01:20:05PM -0800, Megha Dey wrote:
> > >> +/*
> > >> + * Unmask the NMI bit of the local APIC the first time task is
> > >> +scheduled
> > >> + * on a particular CPU.
> > >> + */
> > >> +static void intel_bm_unmask_nmi(void) {
> > >> +	this_cpu_write(bm_unmask_apic, 0);
> > >> +
> > >> +	if (!(this_cpu_read(bm_unmask_apic))) {
> > >> +		apic_write(APIC_LVTPC, APIC_DM_NMI);
> > >> +		this_cpu_inc(bm_unmask_apic);
> > >> +	}
> > >> +}
> > >
> > >What? Why?
> > 
> 
> > Normally, other drivers using perf create an event on every CPU (thereby
> > calling perf_init on every CPU), where this bit(APIC_DM_NMI)is explicitly
> > unmasked.  In our driver, we do not do this (since we are worried only
> > about a particular task) and hence this bit is only disabled on the local
> > APIC where the perf event is initialized.
> >
> > As such, if the task is scheduled out to some other CPU, this bit is set
> > and hence would stop the interrupt from reaching the processing core.
> 
> Still that code makes no sense at all and certainly does not do what you
> claim it does:
> 
> > >> +	this_cpu_write(bm_unmask_apic, 0);
> > >> +
> > >> +	if (!(this_cpu_read(bm_unmask_apic))) {
> 
> So first you write the per cpu variable to 0 and then you check whether it
> is zero, which is pointless obviously.

yes, I see your point. The logic is flawed. Will fix this.
> 
> > >
> > >> +static int intel_bm_event_add(struct perf_event *event, int mode) {
> 
> Please move the opening bracket of the function into the next line. See the
> kernel coding style documentation.

Will do.
> 
> Thanks,
> 
> 	tglx