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Date: Thu, 22 Feb 2024 10:11:08 -0500
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Marcelo Tosatti <mtosatti@redhat.com>, 
 netdev@vger.kernel.org, 
 linux-kernel@vger.kernel.org
Cc: Jakub Kicinski <kuba@kernel.org>, 
 Eric Dumazet <edumazet@google.com>, 
 Frederic Weisbecker <frederic@kernel.org>, 
 Valentin Schneider <vschneid@redhat.com>
Message-ID: <65d7640c7983b_2bd671294c3@willemb.c.googlers.com.notmuch>
In-Reply-To: <ZdSAWAwUxc5R46NH@tpad>
References: <ZdSAWAwUxc5R46NH@tpad>
Subject: Re: [PATCH] net/core/dev.c: enable timestamp static key if CPU
 isolation is configured
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Marcelo Tosatti wrote:
> For systems that use CPU isolation (via nohz_full), creating or destroying
> a socket with  timestamping (SOF_TIMESTAMPING_OPT_TX_SWHW) might cause a
> static key to be enabled/disabled. This in turn causes undesired 
> IPIs to isolated CPUs.

This refers to SOF_TIMESTAMPING_RX_SOFTWARE, not SOF_TIMESTAMPING_OPT_TX_SWHW.
See also sock_set_timestamping.
> 
> So enable the static key unconditionally, if CPU isolation is enabled,
> thus avoiding the IPIs.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 0d548431f3fa..cc9a77b4aa4e 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -153,6 +153,7 @@
>  #include <linux/prandom.h>
>  #include <linux/once_lite.h>
>  #include <net/netdev_rx_queue.h>
> +#include <linux/sched/isolation.h>
>  
>  #include "dev.h"
>  #include "net-sysfs.h"
> @@ -11601,3 +11602,14 @@ static int __init net_dev_init(void)
>  }
>  
>  subsys_initcall(net_dev_init);
> +
> +static int __init net_dev_late_init(void)
> +{
> +	/* avoid static key IPIs to isolated CPUs */
> +	if (housekeeping_enabled(HK_TYPE_MISC))
> +		net_enable_timestamp();
> +
> +	return 0;
> +}
> +
> +late_initcall(net_dev_late_init);
>