netdev - Re: [PATCH net] net: busy-poll: use ktime_get_ns() instead of local

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Message-ID: <Zs2_TzLC1najJ2Ip@LQ3V64L9R2>
Date: Tue, 27 Aug 2024 12:58:07 +0100
From: Joe Damato <jdamato@...tly.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	netdev@...r.kernel.org, eric.dumazet@...il.com,
	Mina Almasry <almasrymina@...gle.com>,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] net: busy-poll: use ktime_get_ns() instead of
 local_clock()

On Tue, Aug 27, 2024 at 11:49:16AM +0000, Eric Dumazet wrote:
> Typically, busy-polling durations are below 100 usec.
> 
> When/if the busy-poller thread migrates to another cpu,
> local_clock() can be off by +/-2msec or more for small
> values of HZ, depending on the platform.
> 
> Use ktimer_get_ns() to ensure deterministic behavior,
> which is the whole point of busy-polling.
> 
> Fixes: 060212928670 ("net: add low latency socket poll")
> Fixes: 9a3c71aa8024 ("net: convert low latency sockets to sched_clock()")
> Fixes: 37089834528b ("sched, net: Fixup busy_loop_us_clock()")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Mina Almasry <almasrymina@...gle.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> Cc: Joe Damato <jdamato@...tly.com>
> ---
>  include/net/busy_poll.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/net/busy_poll.h b/include/net/busy_poll.h
> index 9b09acac538eed8dbaa2576bf2af926ecd98eb44..522f1da8b747ac73578d8fd93301d31835a6dae0 100644
> --- a/include/net/busy_poll.h
> +++ b/include/net/busy_poll.h
> @@ -68,7 +68,7 @@ static inline bool sk_can_busy_loop(struct sock *sk)
>  static inline unsigned long busy_loop_current_time(void)
>  {
>  #ifdef CONFIG_NET_RX_BUSY_POLL
> -	return (unsigned long)(local_clock() >> 10);
> +	return (unsigned long)(ktime_get_ns() >> 10);
>  #else
>  	return 0;
>  #endif
> -- 
> 2.46.0.295.g3b9ea8a38a-goog

Makes sense to me, thanks.

Reviewed-by: Joe Damato <jdamato@...tly.com>