Re: [PATCH 1/2] tcp: Reset ACK_TO_TAP_DUE flag whenever an ACK isn't needed anymore

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Nov 19, 2024 at 08:53:43PM +0100, Stefano Brivio wrote:
> We enter the timer handler with the ACK_TO_TAP_DUE flag, call
> tcp_prepare_flags() with ACK_IF_NEEDED, and realise that we
> acknowledged everything meanwhile, so we return early, but we also
> need to reset that flag to avoid unnecessarily scheduling the timer
> over and over again until more pending data appears.
> 
> I'm not sure if this fixes any real issue, but I've spotted this
> in several logs reported by users, including one where we have some
> unexpected bursts of high CPU load during TCP transfers at low rates,
> from https://github.com/containers/podman/issues/23686.
> 
> Link: https://github.com/containers/podman/discussions/24572
> Link: https://github.com/containers/podman/issues/23686
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  tcp.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/tcp.c b/tcp.c
> index 6a98dfa..f357920 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -1235,8 +1235,10 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
>  	int s = conn->sock;
>  
>  	if (SEQ_GE(conn->seq_ack_to_tap, conn->seq_from_tap) &&
> -	    !flags && conn->wnd_to_tap)
> +	    !flags && conn->wnd_to_tap) {
> +		conn_flag(c, conn, ~ACK_TO_TAP_DUE);
>  		return 0;
> +	}
>  
>  	if (getsockopt(s, SOL_TCP, TCP_INFO, &tinfo, &sl)) {
>  		conn_event(c, conn, CLOSED);

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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