linux-kernel - Re: [PATCH 08/34] drivers/perf: optimize ali_drw_get_counter_idx() by using find

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Message-ID: <20231121155450.GC1938@willie-the-truck>
Date:   Tue, 21 Nov 2023 15:54:50 +0000
From:   Will Deacon <will@...nel.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Shuai Xue <xueshuai@...ux.alibaba.com>,
        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 08/34] drivers/perf: optimize ali_drw_get_counter_idx()
 by using find_bit()

On Sat, Nov 18, 2023 at 07:50:39AM -0800, Yury Norov wrote:
> The function searches used_mask for a set bit in a for-loop bit by bit.
> We can do it faster by using atomic find_and_set_bit().
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>  drivers/perf/alibaba_uncore_drw_pmu.c | 10 ++--------
>  1 file changed, 2 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/perf/alibaba_uncore_drw_pmu.c b/drivers/perf/alibaba_uncore_drw_pmu.c
> index 19d459a36be5..2a3b7701d568 100644
> --- a/drivers/perf/alibaba_uncore_drw_pmu.c
> +++ b/drivers/perf/alibaba_uncore_drw_pmu.c
> @@ -274,15 +274,9 @@ static const struct attribute_group *ali_drw_pmu_attr_groups[] = {
>  static int ali_drw_get_counter_idx(struct perf_event *event)
>  {
>  	struct ali_drw_pmu *drw_pmu = to_ali_drw_pmu(event->pmu);
> -	int idx;
> +	int idx = find_and_set_bit(drw_pmu->used_mask, ALI_DRW_PMU_COMMON_MAX_COUNTERS);
>  
> -	for (idx = 0; idx < ALI_DRW_PMU_COMMON_MAX_COUNTERS; ++idx) {
> -		if (!test_and_set_bit(idx, drw_pmu->used_mask))
> -			return idx;
> -	}
> -
> -	/* The counters are all in use. */
> -	return -EBUSY;
> +	return idx < ALI_DRW_PMU_COMMON_MAX_COUNTERS ? idx : -EBUSY;
>  }
>  
>  static u64 ali_drw_pmu_read_counter(struct perf_event *event)
> -- 
> 2.39.2

Acked-by: Will Deacon <will@...nel.org>

Will