linux-kernel - Re: [RFC] fpga: dfl: fme: Fix cpu hotplug code

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Message-ID: <YNoXnHq+nlJhO8o6@epycbox.lan>
Date:   Mon, 28 Jun 2021 11:40:28 -0700
From:   Moritz Fischer <mdf@...nel.org>
To:     Kajol Jain <kjain@...ux.ibm.com>
Cc:     will@...nel.org, hao.wu@...el.com, mark.rutland@....com,
        trix@...hat.com, yilun.xu@...el.com, luwei.kang@...el.com,
        mdf@...nel.org, linux-fpga@...r.kernel.org,
        maddy@...ux.vnet.ibm.com, atrajeev@...ux.vnet.ibm.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        rnsastry@...ux.ibm.com
Subject: Re: [RFC] fpga: dfl: fme: Fix cpu hotplug code

On Mon, Jun 28, 2021 at 12:45:46PM +0530, Kajol Jain wrote:
> Commit 724142f8c42a ("fpga: dfl: fme: add performance
> reporting support") added performance reporting support
> for FPGA management engine via perf.
> 
> It also added cpu hotplug feature but it didn't add
> pmu migration call in cpu offline function.
> This can create an issue incase the current designated
> cpu being used to collect fme pmu data got offline,
> as based on current code we are not migrating fme pmu to
> new target cpu. Because of that perf will still try to
> fetch data from that offline cpu and hence we will not
> get counter data.
> 
> Patch fixed this issue by adding pmu_migrate_context call
> in fme_perf_offline_cpu function.
> 
> Fixes: 724142f8c42a ("fpga: dfl: fme: add performance reporting support")
> Signed-off-by: Kajol Jain <kjain@...ux.ibm.com>

You might want to Cc: stable@...r.kernel.org if it fixes an actual bug.
> ---
>  drivers/fpga/dfl-fme-perf.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> ---
> - This fix patch is not tested (as I don't have required environment).
>   But issue mentioned in the commit msg can be re-created, by starting any
>   fme_perf event and while its still running, offline current designated
>   cpu pointed by cpumask file. Since current code didn't migrating pmu,
>   perf gonna try getting counts from that offlined cpu and hence we will
>   not get event data.
> ---
> diff --git a/drivers/fpga/dfl-fme-perf.c b/drivers/fpga/dfl-fme-perf.c
> index 4299145ef347..b9a54583e505 100644
> --- a/drivers/fpga/dfl-fme-perf.c
> +++ b/drivers/fpga/dfl-fme-perf.c
> @@ -953,6 +953,10 @@ static int fme_perf_offline_cpu(unsigned int cpu, struct hlist_node *node)
>  		return 0;
>  
>  	priv->cpu = target;
> +
> +	/* Migrate fme_perf pmu events to the new target cpu */
> +	perf_pmu_migrate_context(&priv->pmu, cpu, target);
> +
>  	return 0;
>  }
>  
> -- 
> 2.31.1
> 
- Moritz