linux-kernel - Re: [PATCH] drivers: CCI: fix used_mask init in validate

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Date:	Thu, 09 Apr 2015 10:26:40 -0400
From:	Mark Salter <msalter@...hat.com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] drivers: CCI: fix used_mask init in validate_group()

On Thu, 2015-04-09 at 15:20 +0100, Mark Rutland wrote:
> On Thu, Apr 09, 2015 at 03:11:43PM +0100, Mark Salter wrote:
> > On Thu, 2015-04-09 at 14:51 +0100, Mark Rutland wrote:
> > > On Wed, Apr 08, 2015 at 07:21:24PM +0100, Mark Salter wrote:
> > > > Currently in validate_group(), there is a static initializer
> > > > for fake_pmu.used_mask which is based on CPU_BITS_NONE but
> > > > the used_mask array size is based on CCI_PMU_MAX_HW_EVENTS.
> > > > CCI_PMU_MAX_HW_EVENTS is not based on NR_CPUS, so CPU_BITS_NONE
> > > > is not correct and will cause a build failure if NR_CPUS
> > > > is set high enough to make CPU_BITS_NONE larger than used_mask.
> > > 
> > > Whoops. My bad.
> > > 
> > > > This patch changes the used_mask initialization to be runtime
> > > > based on the actual size of the array.
> > > > 
> > > > Signed-off-by: Mark Salter <msalter@...hat.com>
> > > > ---
> > > >  drivers/bus/arm-cci.c | 14 +++++++-------
> > > >  1 file changed, 7 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/drivers/bus/arm-cci.c b/drivers/bus/arm-cci.c
> > > > index 84fd660..1d83072 100644
> > > > --- a/drivers/bus/arm-cci.c
> > > > +++ b/drivers/bus/arm-cci.c
> > > > @@ -679,13 +679,13 @@ static int
> > > >  validate_group(struct perf_event *event)
> > > >  {
> > > >  	struct perf_event *sibling, *leader = event->group_leader;
> > > > -	struct cci_pmu_hw_events fake_pmu = {
> > > > -		/*
> > > > -		 * Initialise the fake PMU. We only need to populate the
> > > > -		 * used_mask for the purposes of validation.
> > > > -		 */
> > > > -		.used_mask = CPU_BITS_NONE,
> > > 
> > > Can we not simply change this to:
> > > 
> > > 		.used_mask = { 0 },
> > > 
> > > That should result in the entire array being zeroed.
> > 
> > It does, but it also causes the whole struct to be cleared.
> 
> Sure, but it's also the minimal diff, and it's easier to read. This was
> what the code was intended to be initially.
> 
> > With the memset, only used_mask gets cleared.
> 
> Is there an appreciable difference between the two performance-wise?

I dunno. It is 3 strp insns vs 1 str.
If you want the static init, I'll send another patch.


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