linux-kernel - Re: [PATCH 2/2] Thunderx2, uncore: Add workaround for ThunderX2 erratum 221

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Message-ID: <20191106113730.GB50610@lakrids.cambridge.arm.com>
Date:   Wed, 6 Nov 2019 11:37:30 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Ganapatrao Prabhakerrao Kulkarni <gkulkarni@...vell.com>
Cc:     "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "will@...nel.org" <will@...nel.org>,
        "corbet@....net" <corbet@....net>,
        "gklkml16@...il.com" <gklkml16@...il.com>
Subject: Re: [PATCH 2/2] Thunderx2, uncore: Add workaround for ThunderX2
 erratum 221

On Wed, Nov 06, 2019 at 01:01:41AM +0000, Ganapatrao Prabhakerrao Kulkarni wrote:
> When perf tried with more events than the PMU supported counters, the perf
> core uses event multiplexing to accommodate all events. This results in
> burst of PMU register read and writes and causes the system hang, when
> executed along with the CPU intensive applications.

Can you please elaborate on how a burst of PMU reads/writes leads to a
hang?

I see the PMU counts DMC/L3C events -- does this occur under heavy /cpu/
load, or heavy /memory/ load?

Does this only happen with a specific timing of reads/writes, or is it
always possible that accessing the PMU can trigger a lockup, and it's
just more likely when the PMU is accessed more often?

Thanks,
Mark.

> 
> Adding software workaround by disabling event multiplexing.
> 
> Signed-off-by: Ganapatrao Prabhakerrao Kulkarni <gkulkarni@...vell.com>
> ---
>  Documentation/admin-guide/perf/thunderx2-pmu.rst | 9 +++++++++
>  drivers/perf/thunderx2_pmu.c                     | 3 ++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/perf/thunderx2-pmu.rst b/Documentation/admin-guide/perf/thunderx2-pmu.rst
> index 08e33675853a..fff65382c887 100644
> --- a/Documentation/admin-guide/perf/thunderx2-pmu.rst
> +++ b/Documentation/admin-guide/perf/thunderx2-pmu.rst
> @@ -40,3 +40,12 @@ Examples::
>    uncore_l3c_0/read_hit/,\
>    uncore_l3c_0/inv_request/,\
>    uncore_l3c_0/inv_hit/ sleep 1
> +
> +ThunderX2 erratum 221:
> +When perf tried with more events than the PMU supported counters, the perf core
> +uses event multiplexing to accommodate all events. This results in burst of PMU
> +registers read and write and leading to system hang when executed along with
> +CPU intensive applications.
> +
> +
> +Disabling PMUs event multiplexing capability.
> diff --git a/drivers/perf/thunderx2_pmu.c b/drivers/perf/thunderx2_pmu.c
> index 43d76c85da56..c443be8bd449 100644
> --- a/drivers/perf/thunderx2_pmu.c
> +++ b/drivers/perf/thunderx2_pmu.c
> @@ -563,7 +563,8 @@ static int tx2_uncore_pmu_register(
>  		.start		= tx2_uncore_event_start,
>  		.stop		= tx2_uncore_event_stop,
>  		.read		= tx2_uncore_event_read,
> -		.capabilities	= PERF_PMU_CAP_NO_EXCLUDE,
> +		.capabilities	= PERF_PMU_CAP_NO_EXCLUDE |
> +					PERF_PMU_CAP_NO_MUX_EVENTS,
>  	};
>  
>  	tx2_pmu->pmu.name = devm_kasprintf(dev, GFP_KERNEL,
> -- 
> 2.17.1
>