linux-kernel - Re: [PATCH 2/3] perf/amlogic: Fix large number of counter issue

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Message-ID: <20230327141054.GC31752@willie-the-truck>
Date:   Mon, 27 Mar 2023 15:10:55 +0100
From:   Will Deacon <will@...nel.org>
To:     Jiucheng Xu <jiucheng.xu@...ogic.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Kevin Hilman <khilman@...libre.com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Jianxin Pan <jianxin.pan@...ogic.com>,
        Kelvin Zhang <kelvin.zhang@...ogic.com>,
        Chris Healy <cphealy@...il.com>,
        Chris Healy <healych@...zon.com>,
        linux-amlogic@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] perf/amlogic: Fix large number of counter issue

On Thu, Feb 09, 2023 at 07:54:02PM +0800, Jiucheng Xu wrote:
> When use 1ms interval, very large number of counter happens
> once in a while as below:
> 
> 25.968654513 281474976710655.84 MB meson_ddr_bw/chan_1_rw_bytes,arm=1/
> 26.118657346 281474976710655.88 MB meson_ddr_bw/chan_1_rw_bytes,arm=1/
> 26.180137180 281474976710655.66 MB meson_ddr_bw/chan_1_rw_bytes,arm=1/
> 
> Root cause is the race between irq handler
> and pmu.read callback. Use spin lock to protect the sw&hw
> counters.
> 
> Signed-off-by: Jiucheng Xu <jiucheng.xu@...ogic.com>
> ---
>  drivers/perf/amlogic/meson_ddr_pmu_core.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/amlogic/meson_ddr_pmu_core.c b/drivers/perf/amlogic/meson_ddr_pmu_core.c
> index 0b24dee1ed3c..9b2e5d5c0626 100644
> --- a/drivers/perf/amlogic/meson_ddr_pmu_core.c
> +++ b/drivers/perf/amlogic/meson_ddr_pmu_core.c
> @@ -14,6 +14,7 @@
>  #include <linux/perf_event.h>
>  #include <linux/platform_device.h>
>  #include <linux/printk.h>
> +#include <linux/spinlock.h>
>  #include <linux/sysfs.h>
>  #include <linux/types.h>
>  
> @@ -23,6 +24,7 @@ struct ddr_pmu {
>  	struct pmu pmu;
>  	struct dmc_info info;
>  	struct dmc_counter counters;	/* save counters from hw */
> +	spinlock_t lock;		/* protect hw/sw counter */
>  	bool pmu_enabled;
>  	struct device *dev;
>  	char *name;
> @@ -92,10 +94,12 @@ static void meson_ddr_perf_event_update(struct perf_event *event)
>  	int idx;
>  	int chann_nr = pmu->info.hw_info->chann_nr;
>  
> +	spin_lock(&pmu->lock);

Why doesn't this need the _irqsave() variant if we're racing with the irq
handler?

Will