Re: [PATCH] tcp_splice: Don't wake up on input data if we can't write it anywhere

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

[-- Attachment #1: Type: text/plain, Size: 2410 bytes --]

On Sun, Feb 16, 2025 at 11:12:16PM +0100, Stefano Brivio wrote:
> If we set the OUT_WAIT_* flag (waiting on EPOLLOUT) for a side of a
> given flow, it means that we're blocked, waiting for the receiver to
> actually receive data, with a full pipe.
> 
> In that case, if we keep EPOLLIN set for the socket on the other side
> (our receiving side), we'll get into a loop such as:
> 
>   41.0230:          pasta: epoll event on connected spliced TCP socket 108 (events: 0x00000001)
>   41.0230:          Flow 1 (TCP connection (spliced)): -1 from read-side call
>   41.0230:          Flow 1 (TCP connection (spliced)): -1 from write-side call (passed 8192)
>   41.0230:          Flow 1 (TCP connection (spliced)): event at tcp_splice_sock_handler:577
>   41.0230:          pasta: epoll event on connected spliced TCP socket 108 (events: 0x00000001)
>   41.0230:          Flow 1 (TCP connection (spliced)): -1 from read-side call
>   41.0230:          Flow 1 (TCP connection (spliced)): -1 from write-side call (passed 8192)
>   41.0230:          Flow 1 (TCP connection (spliced)): event at tcp_splice_sock_handler:577
> 
> leading to 100% CPU usage, of course.
> 
> Drop EPOLLIN on our receiving side as long when we're waiting for
> output readiness on the other side.
> 
> Link: https://github.com/containers/podman/issues/23686#issuecomment-2661036584
> Link: https://www.reddit.com/r/podman/comments/1iph50j/pasta_high_cpu_on_podman_rootless_container/
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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

> ---
>  tcp_splice.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/tcp_splice.c b/tcp_splice.c
> index f1a9223..8a39a6f 100644
> --- a/tcp_splice.c
> +++ b/tcp_splice.c
> @@ -131,8 +131,12 @@ static void tcp_splice_conn_epoll_events(uint16_t events,
>  		ev[1].events = EPOLLOUT;
>  	}
>  
> -	flow_foreach_sidei(sidei)
> -		ev[sidei].events |= (events & OUT_WAIT(sidei)) ? EPOLLOUT : 0;
> +	flow_foreach_sidei(sidei) {
> +		if (events & OUT_WAIT(sidei)) {
> +			ev[sidei].events |= EPOLLOUT;
> +			ev[!sidei].events &= ~EPOLLIN;
> +		}
> +	}
>  }
>  
>  /**

-- 
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

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]