linux-kernel - Re: [PATCH 07/34] perf/arm: optimize opencoded atomic find

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Date:   Tue, 21 Nov 2023 16:17:42 +0000
From:   Will Deacon <will@...nel.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, Jan Kara <jack@...e.cz>,
        Mirsad Todorovac <mirsad.todorovac@....unizg.hr>,
        Matthew Wilcox <willy@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Alexey Klimov <klimov.linux@...il.com>
Subject: Re: [PATCH 07/34] perf/arm: optimize opencoded atomic find_bit() API

On Tue, Nov 21, 2023 at 08:16:13AM -0800, Yury Norov wrote:
> On Tue, Nov 21, 2023 at 03:53:44PM +0000, Will Deacon wrote:
> > On Sat, Nov 18, 2023 at 07:50:38AM -0800, Yury Norov wrote:
> > > Switch subsystem to use atomic find_bit() or atomic iterators as
> > > appropriate.
> > > 
> > > Signed-off-by: Yury Norov <yury.norov@...il.com>
> > > ---
> > >  drivers/perf/arm-cci.c        | 23 +++++------------------
> > >  drivers/perf/arm-ccn.c        | 10 ++--------
> > >  drivers/perf/arm_dmc620_pmu.c |  9 ++-------
> > >  drivers/perf/arm_pmuv3.c      |  8 ++------
> > >  4 files changed, 11 insertions(+), 39 deletions(-)
> > > 
> > > diff --git a/drivers/perf/arm-cci.c b/drivers/perf/arm-cci.c
> > > index 61de861eaf91..70fbf9d09d37 100644
> > > --- a/drivers/perf/arm-cci.c
> > > +++ b/drivers/perf/arm-cci.c
> > > @@ -320,12 +320,8 @@ static int cci400_get_event_idx(struct cci_pmu *cci_pmu,
> > >  		return CCI400_PMU_CYCLE_CNTR_IDX;
> > >  	}
> > >  
> > > -	for (idx = CCI400_PMU_CNTR0_IDX; idx <= CCI_PMU_CNTR_LAST(cci_pmu); ++idx)
> > > -		if (!test_and_set_bit(idx, hw->used_mask))
> > > -			return idx;
> > > -
> > > -	/* No counters available */
> > > -	return -EAGAIN;
> > > +	idx = find_and_set_bit(hw->used_mask, CCI_PMU_CNTR_LAST(cci_pmu) + 1);
> > 
> > CCI400_PMU_CNTR0_IDX is defined as 1, so isn't this wrong?
> 
> You're right. Will fix in v2
>  
> > [...]
> > 
> > > diff --git a/drivers/perf/arm_dmc620_pmu.c b/drivers/perf/arm_dmc620_pmu.c
> > > index 30cea6859574..e41c84dabc3e 100644
> > > --- a/drivers/perf/arm_dmc620_pmu.c
> > > +++ b/drivers/perf/arm_dmc620_pmu.c
> > > @@ -303,13 +303,8 @@ static int dmc620_get_event_idx(struct perf_event *event)
> > >  		end_idx = DMC620_PMU_MAX_COUNTERS;
> > >  	}
> > >  
> > > -	for (idx = start_idx; idx < end_idx; ++idx) {
> > > -		if (!test_and_set_bit(idx, dmc620_pmu->used_mask))
> > > -			return idx;
> > > -	}
> > > -
> > > -	/* The counters are all in use. */
> > > -	return -EAGAIN;
> > > +	idx = find_and_set_next_bit(dmc620_pmu->used_mask, end_idx, start_idx);
> > 
> > It might just be me, but I'd find this a tonne easier to read if you swapped
> > the last two arguments around so that the offset came before the limit in
> > the new function.
> 
> I personally agree, but we already have find_next_*_bit(addr, nbits, offset)
> functions, and having atomic versions of the same with different order
> of arguments will make it even more messy...

Urgh, and there's loads of them too :(

Will