linux-kernel - Re: [PATCH] net/core/dev.c: enable timestamp static key if CPU isolation is configured

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Date: Thu, 22 Feb 2024 13:42:16 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Marcelo Tosatti <mtosatti@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Jakub Kicinski <kuba@...nel.org>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Frederic Weisbecker <frederic@...nel.org>, 
 Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH] net/core/dev.c: enable timestamp static key if CPU
 isolation is configured

Marcelo Tosatti wrote:
> On Thu, Feb 22, 2024 at 10:11:08AM -0500, Willem de Bruijn wrote:
> > 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.
> 
> Willem,
> 
> This test program does trigger the static key change:
> 
> int main (void)
> {
>         int option = SOF_TIMESTAMPING_OPT_TX_SWHW;
>         int sock_fd;
>         int ret;
>         int pid_fd;
>         pid_t pid;
>         char buf[50];
> 
> ...
> 
>         /* set the timestamping option
>          * this is to trigger the IPIs that notify all cpus of the change
>          */
>         if (setsockopt(sock_fd, SOL_SOCKET, SO_TIMESTAMP, &option, sizeof (option)) < 0) {
>                 printf("Could not enable timestamping option %x", (unsigned int)option);
>                 close(sock_fd);
>                 return 0;
>         }
> ...
> 

That is because you call SO_TIMESTAMP, which interprets option as a
boolean. The SOF_ flags apply to setsockopt SO_TIMESTAMPING.