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

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Date: Thu, 22 Feb 2024 14:52:11 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: 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

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;
        }
...