Re: [PATCH v4 4/7] tcp, flow: Replace per-connection in_epoll flag with threadnb in flow_common

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

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On Fri, Oct 17, 2025 at 12:31:26PM +0200, Laurent Vivier wrote:
> The in_epoll boolean flag in tcp_tap_conn and tcp_splice_conn only tracked
> whether a connection was registered with epoll, not which epoll instance.
> This limited flexibility for future multi-epoll support.
> 
> Replace the boolean with a threadnb field in flow_common that identifies
> which thread (and thus which epoll instance) the flow is registered with.
> Use FLOW_THREADNB_INVALID to indicate when a flow is not registered with
> any epoll instance. A threadnb_to_epollfd[] mapping table translates
> thread numbers to their corresponding epoll file descriptors.
> 
> Add helper functions:
> - flow_in_epoll() to check if a flow is registered with epoll
> - flow_epollfd() to retrieve the epoll fd for a flow's thread
> - flow_thread_register() to register an epoll fd with a thread
> - flow_thread_set() to set the thread number of a flow
> 
> This change also simplifies tcp_timer_ctl() and conn_flag_do() by removing
> the need to pass the context 'c', since the epoll fd is now directly
> accessible from the flow structure via flow_epollfd().
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Some minor queries only.

> ---
>  flow.c       | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>  flow.h       | 12 ++++++++++++
>  passt.c      |  1 +
>  tcp.c        | 39 ++++++++++++++++++++------------------
>  tcp_conn.h   |  8 +-------
>  tcp_splice.c | 24 ++++++++++++------------
>  6 files changed, 99 insertions(+), 38 deletions(-)
> 
> diff --git a/flow.c b/flow.c
> index b14e9d8b63ff..d56bae776239 100644
> --- a/flow.c
> +++ b/flow.c
> @@ -116,6 +116,7 @@ static_assert(ARRAY_SIZE(flow_proto) == FLOW_NUM_TYPES,
>  unsigned flow_first_free;
>  union flow flowtab[FLOW_MAX];
>  static const union flow *flow_new_entry; /* = NULL */
> +static int threadnb_to_epollfd[FLOW_THREADNB_SIZE];

Same comments as in the previous version about "thread" in the name.

>  
>  /* Hash table to index it */
>  #define FLOW_HASH_LOAD		70		/* % */
> @@ -347,6 +348,55 @@ static void flow_set_state(struct flow_common *f, enum flow_state state)
>  	flow_log_details_(f, LOG_DEBUG, MAX(state, oldstate));
>  }
>  
> +/**
> + * flow_in_epoll() - Check if flow is registered with an epoll instance
> + * @f:		Flow to check
> + *
> + * Return: true if flow is registered with epoll, false otherwise
> + */
> +bool flow_in_epoll(const struct flow_common *f)
> +{
> +	return f->threadnb != FLOW_THREADNB_INVALID;
> +}
> +
> +/**
> + * flow_epollfd() - Get the epoll file descriptor for a flow
> + * @f:		Flow to query
> + *
> + * Return: epoll file descriptor associated with the flow's thread
> + */
> +int flow_epollfd(const struct flow_common *f)
> +{
> +	ASSERT(f->threadnb < FLOW_THREADNB_MAX);
> +
> +	return threadnb_to_epollfd[f->threadnb];
> +}
> +
> +/**
> + * flow_thread_set() - Associate a flow with a thread
> + * @f:		Flow to update
> + * @threadnb:	Thread number to associate with this flow
> + */
> +void flow_thread_set(struct flow_common *f, int threadnb)
> +{
> +	ASSERT(threadnb < FLOW_THREADNB_MAX);
> +
> +	f->threadnb = threadnb;
> +}
> +
> +/**
> + * flow_thread_register() - Initialize the threadnb -> epollfd mapping
> + * @threadnb:	Thread number to associate to
> + * @epollfd:	epoll file descriptor for the thread
> + */
> +void flow_thread_register(int threadnb, int epollfd)
> +{
> +	ASSERT(threadnb < FLOW_THREADNB_MAX);
> +	ASSERT(epollfd >= 0);

Not sure about the second assert.  It seems like an error people are
unlikely to make by accident, whereas attempting to deliberately clear
/ delete an entry by setting its fd to -1 seems like a reasonable
thing to do.

> +
> +	threadnb_to_epollfd[threadnb] = epollfd;
> +}
> +
>  /**
>   * flow_initiate_() - Move flow to INI, setting pif[INISIDE]
>   * @flow:	Flow to change state
> @@ -548,6 +598,7 @@ union flow *flow_alloc(void)
>  
>  	flow_new_entry = flow;
>  	memset(flow, 0, sizeof(*flow));
> +	flow->f.threadnb = FLOW_THREADNB_INVALID;
>  	flow_set_state(&flow->f, FLOW_STATE_NEW);
>  
>  	return flow;
> @@ -827,7 +878,7 @@ void flow_defer_handler(const struct ctx *c, const struct timespec *now)
>  		case FLOW_TCP_SPLICE:
>  			closed = tcp_splice_flow_defer(&flow->tcp_splice);
>  			if (!closed && timer)
> -				tcp_splice_timer(c, &flow->tcp_splice);
> +				tcp_splice_timer(&flow->tcp_splice);
>  			break;
>  		case FLOW_PING4:
>  		case FLOW_PING6:
> diff --git a/flow.h b/flow.h
> index ef138b83add8..700d8b32c990 100644
> --- a/flow.h
> +++ b/flow.h
> @@ -177,6 +177,8 @@ int flowside_connect(const struct ctx *c, int s,
>   * @type:	Type of packet flow
>   * @pif[]:	Interface for each side of the flow
>   * @side[]:	Information for each side of the flow
> + * @threadnb:	Thread number flow is registered with
> + *		(FLOW_THREADNB_INVALID if not)
>   */
>  struct flow_common {
>  #ifdef __GNUC__
> @@ -192,8 +194,14 @@ struct flow_common {
>  #endif
>  	uint8_t		pif[SIDES];
>  	struct flowside	side[SIDES];
> +#define FLOW_THREADNB_BITS 8
> +	unsigned int	threadnb:FLOW_THREADNB_BITS;
>  };
>  
> +#define FLOW_THREADNB_SIZE	(1 << FLOW_THREADNB_BITS)
> +#define FLOW_THREADNB_MAX	(FLOW_THREADNB_SIZE - 1)
> +#define FLOW_THREADNB_INVALID	FLOW_THREADNB_MAX
> +
>  #define FLOW_INDEX_BITS		17	/* 128k - 1 */
>  #define FLOW_MAX		MAX_FROM_BITS(FLOW_INDEX_BITS)
>  
> @@ -249,6 +257,10 @@ flow_sidx_t flow_lookup_sa(const struct ctx *c, uint8_t proto, uint8_t pif,
>  union flow;
>  
>  void flow_init(void);
> +bool flow_in_epoll(const struct flow_common *f);
> +int flow_epollfd(const struct flow_common *f);
> +void flow_thread_set(struct flow_common *f, int threadnb);
> +void flow_thread_register(int threadnb, int epollfd);
>  void flow_defer_handler(const struct ctx *c, const struct timespec *now);
>  int flow_migrate_source_early(struct ctx *c, const struct migrate_stage *stage,
>  			      int fd);
> diff --git a/passt.c b/passt.c
> index af928111786b..37f2c897be84 100644
> --- a/passt.c
> +++ b/passt.c
> @@ -285,6 +285,7 @@ int main(int argc, char **argv)
>  	c.epollfd = epoll_create1(EPOLL_CLOEXEC);
>  	if (c.epollfd == -1)
>  		die_perror("Failed to create epoll file descriptor");
> +	flow_thread_register(0, c.epollfd);
>  
>  	if (getrlimit(RLIMIT_NOFILE, &limit))
>  		die_perror("Failed to get maximum value of open files limit");
> diff --git a/tcp.c b/tcp.c
> index db9f17c0622f..8c49852b8454 100644
> --- a/tcp.c
> +++ b/tcp.c
> @@ -504,25 +504,27 @@ static uint32_t tcp_conn_epoll_events(uint8_t events, uint8_t conn_flags)
>   */
>  static int tcp_epoll_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
>  {
> -	int m = conn->in_epoll ? EPOLL_CTL_MOD : EPOLL_CTL_ADD;
> +	int m = flow_in_epoll(&conn->f) ? EPOLL_CTL_MOD : EPOLL_CTL_ADD;
>  	union epoll_ref ref = { .type = EPOLL_TYPE_TCP, .fd = conn->sock,
>  		                .flowside = FLOW_SIDX(conn, !TAPSIDE(conn)), };
>  	struct epoll_event ev = { .data.u64 = ref.u64 };
> +	int epollfd = flow_in_epoll(&conn->f) ? flow_epollfd(&conn->f)
> +						   : c->epollfd;
>  
>  	if (conn->events == CLOSED) {
> -		if (conn->in_epoll)
> -			epoll_del(c->epollfd, conn->sock);
> +		if (flow_in_epoll(&conn->f))
> +			epoll_del(epollfd, conn->sock);
>  		if (conn->timer != -1)
> -			epoll_del(c->epollfd, conn->timer);
> +			epoll_del(epollfd, conn->timer);
>  		return 0;
>  	}
>  
>  	ev.events = tcp_conn_epoll_events(conn->events, conn->flags);
>  
> -	if (epoll_ctl(c->epollfd, m, conn->sock, &ev))
> +	if (epoll_ctl(epollfd, m, conn->sock, &ev))
>  		return -errno;
>  
> -	conn->in_epoll = true;
> +	flow_thread_set(&conn->f, 0);
>  
>  	if (conn->timer != -1) {
>  		union epoll_ref ref_t = { .type = EPOLL_TYPE_TCP_TIMER,
> @@ -531,7 +533,8 @@ static int tcp_epoll_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
>  		struct epoll_event ev_t = { .data.u64 = ref_t.u64,
>  					    .events = EPOLLIN | EPOLLET };
>  
> -		if (epoll_ctl(c->epollfd, EPOLL_CTL_MOD, conn->timer, &ev_t))
> +		if (epoll_ctl(flow_epollfd(&conn->f), EPOLL_CTL_MOD,
> +			      conn->timer, &ev_t))
>  			return -errno;
>  	}
>  
> @@ -540,12 +543,11 @@ static int tcp_epoll_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
>  
>  /**
>   * tcp_timer_ctl() - Set timerfd based on flags/events, create timerfd if needed
> - * @c:		Execution context
>   * @conn:	Connection pointer
>   *
>   * #syscalls timerfd_create timerfd_settime
>   */
> -static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
> +static void tcp_timer_ctl(struct tcp_tap_conn *conn)
>  {
>  	struct itimerspec it = { { 0 }, { 0 } };
>  
> @@ -570,7 +572,8 @@ static void tcp_timer_ctl(const struct ctx *c, struct tcp_tap_conn *conn)
>  		}
>  		conn->timer = fd;
>  
> -		if (epoll_ctl(c->epollfd, EPOLL_CTL_ADD, conn->timer, &ev)) {
> +		if (epoll_ctl(flow_epollfd(&conn->f), EPOLL_CTL_ADD,
> +			      conn->timer, &ev)) {

Possibly a question for an earlier patch, but is there a reason we
can't use epoll_add() here?

>  			flow_dbg_perror(conn, "failed to add timer");
>  			close(conn->timer);
>  			conn->timer = -1;
> @@ -628,7 +631,7 @@ void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
>  			 * flags and factor this into the logic below.
>  			 */
>  			if (flag == ACK_FROM_TAP_DUE)
> -				tcp_timer_ctl(c, conn);
> +				tcp_timer_ctl(conn);
>  
>  			return;
>  		}
> @@ -644,7 +647,7 @@ void conn_flag_do(const struct ctx *c, struct tcp_tap_conn *conn,
>  	if (flag == ACK_FROM_TAP_DUE || flag == ACK_TO_TAP_DUE		  ||
>  	    (flag == ~ACK_FROM_TAP_DUE && (conn->flags & ACK_TO_TAP_DUE)) ||
>  	    (flag == ~ACK_TO_TAP_DUE   && (conn->flags & ACK_FROM_TAP_DUE)))
> -		tcp_timer_ctl(c, conn);
> +		tcp_timer_ctl(conn);
>  }
>  
>  /**
> @@ -699,7 +702,7 @@ void conn_event_do(const struct ctx *c, struct tcp_tap_conn *conn,
>  		tcp_epoll_ctl(c, conn);
>  
>  	if (CONN_HAS(conn, SOCK_FIN_SENT | TAP_FIN_ACKED))
> -		tcp_timer_ctl(c, conn);
> +		tcp_timer_ctl(conn);
>  }
>  
>  /**
> @@ -1757,7 +1760,7 @@ static int tcp_data_from_tap(const struct ctx *c, struct tcp_tap_conn *conn,
>  				   seq, conn->seq_from_tap);
>  
>  			tcp_send_flag(c, conn, ACK);
> -			tcp_timer_ctl(c, conn);
> +			tcp_timer_ctl(conn);
>  
>  			if (p->count == 1) {
>  				tcp_tap_window_update(c, conn,
> @@ -2406,7 +2409,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
>  
>  	if (conn->flags & ACK_TO_TAP_DUE) {
>  		tcp_send_flag(c, conn, ACK_IF_NEEDED);
> -		tcp_timer_ctl(c, conn);
> +		tcp_timer_ctl(conn);
>  	} else if (conn->flags & ACK_FROM_TAP_DUE) {
>  		if (!(conn->events & ESTABLISHED)) {
>  			flow_dbg(conn, "handshake timeout");
> @@ -2428,7 +2431,7 @@ void tcp_timer_handler(const struct ctx *c, union epoll_ref ref)
>  				return;
>  
>  			tcp_data_from_sock(c, conn);
> -			tcp_timer_ctl(c, conn);
> +			tcp_timer_ctl(conn);
>  		}
>  	} else {
>  		struct itimerspec new = { { 0 }, { ACT_TIMEOUT, 0 } };
> @@ -3476,7 +3479,7 @@ int tcp_flow_migrate_source_ext(const struct ctx *c,
>  	if (c->migrate_no_linger)
>  		close(s);
>  	else
> -		epoll_del(c->epollfd, s);
> +		epoll_del(flow_epollfd(&conn->f), s);
>  
>  	/* Adjustments unrelated to FIN segments: sequence numbers we dumped are
>  	 * based on the end of the queues.
> @@ -3625,7 +3628,7 @@ static int tcp_flow_repair_connect(const struct ctx *c,
>  		return rc;
>  	}
>  
> -	conn->in_epoll = 0;
> +	conn->f.threadnb = FLOW_THREADNB_INVALID;
>  	conn->timer = -1;
>  	conn->listening_sock = -1;
>  
> diff --git a/tcp_conn.h b/tcp_conn.h
> index 38b5c541f003..81333122d531 100644
> --- a/tcp_conn.h
> +++ b/tcp_conn.h
> @@ -12,7 +12,6 @@
>  /**
>   * struct tcp_tap_conn - Descriptor for a TCP connection (not spliced)
>   * @f:			Generic flow information
> - * @in_epoll:		Is the connection in the epoll set?
>   * @retrans:		Number of retransmissions occurred due to ACK_TIMEOUT
>   * @ws_from_tap:	Window scaling factor advertised from tap/guest
>   * @ws_to_tap:		Window scaling factor advertised to tap/guest
> @@ -36,8 +35,6 @@ struct tcp_tap_conn {
>  	/* Must be first element */
>  	struct flow_common f;
>  
> -	bool		in_epoll	:1;
> -
>  #define TCP_RETRANS_BITS		3
>  	unsigned int	retrans		:TCP_RETRANS_BITS;
>  #define TCP_MAX_RETRANS			MAX_FROM_BITS(TCP_RETRANS_BITS)
> @@ -196,7 +193,6 @@ struct tcp_tap_transfer_ext {
>   * @written:		Bytes written (not fully written from one other side read)
>   * @events:		Events observed/actions performed on connection
>   * @flags:		Connection flags (attributes, not events)
> - * @in_epoll:		Is the connection in the epoll set?
>   */
>  struct tcp_splice_conn {
>  	/* Must be first element */
> @@ -220,8 +216,6 @@ struct tcp_splice_conn {
>  #define RCVLOWAT_SET(sidei_)		((sidei_) ? BIT(1) : BIT(0))
>  #define RCVLOWAT_ACT(sidei_)		((sidei_) ? BIT(3) : BIT(2))
>  #define CLOSING				BIT(4)
> -
> -	bool in_epoll	:1;
>  };
>  
>  /* Socket pools */
> @@ -245,7 +239,7 @@ int tcp_flow_migrate_target_ext(struct ctx *c, struct tcp_tap_conn *conn, int fd
>  bool tcp_flow_is_established(const struct tcp_tap_conn *conn);
>  
>  bool tcp_splice_flow_defer(struct tcp_splice_conn *conn);
> -void tcp_splice_timer(const struct ctx *c, struct tcp_splice_conn *conn);
> +void tcp_splice_timer(struct tcp_splice_conn *conn);
>  int tcp_conn_pool_sock(int pool[]);
>  int tcp_conn_sock(sa_family_t af);
>  int tcp_sock_refill_pool(int pool[], sa_family_t af);
> diff --git a/tcp_splice.c b/tcp_splice.c
> index 6f21184bdc55..703bd7610890 100644
> --- a/tcp_splice.c
> +++ b/tcp_splice.c
> @@ -149,7 +149,9 @@ static void tcp_splice_conn_epoll_events(uint16_t events,
>  static int tcp_splice_epoll_ctl(const struct ctx *c,
>  				struct tcp_splice_conn *conn)
>  {
> -	int m = conn->in_epoll ? EPOLL_CTL_MOD : EPOLL_CTL_ADD;
> +	int epollfd = flow_in_epoll(&conn->f) ? flow_epollfd(&conn->f)
> +						   : c->epollfd;
> +	int m = flow_in_epoll(&conn->f) ? EPOLL_CTL_MOD : EPOLL_CTL_ADD;
>  	const union epoll_ref ref[SIDES] = {
>  		{ .type = EPOLL_TYPE_TCP_SPLICE, .fd = conn->s[0],
>  		  .flowside = FLOW_SIDX(conn, 0) },
> @@ -161,25 +163,24 @@ static int tcp_splice_epoll_ctl(const struct ctx *c,
>  
>  	tcp_splice_conn_epoll_events(conn->events, ev);
>  
> -	if (epoll_ctl(c->epollfd, m, conn->s[0], &ev[0]) ||
> -	    epoll_ctl(c->epollfd, m, conn->s[1], &ev[1])) {
> +
> +	if (epoll_ctl(epollfd, m, conn->s[0], &ev[0]) ||
> +	    epoll_ctl(epollfd, m, conn->s[1], &ev[1])) {
>  		int ret = -errno;
>  		flow_perror(conn, "ERROR on epoll_ctl()");
>  		return ret;
>  	}
> -
> -	conn->in_epoll = true;
> +	flow_thread_set(&conn->f, 0);
>  
>  	return 0;
>  }
>  
>  /**
>   * conn_flag_do() - Set/unset given flag, log, update epoll on CLOSING flag
> - * @c:		Execution context
>   * @conn:	Connection pointer
>   * @flag:	Flag to set, or ~flag to unset
>   */
> -static void conn_flag_do(const struct ctx *c, struct tcp_splice_conn *conn,
> +static void conn_flag_do(struct tcp_splice_conn *conn,
>  			 unsigned long flag)
>  {
>  	if (flag & (flag - 1)) {
> @@ -204,15 +205,15 @@ static void conn_flag_do(const struct ctx *c, struct tcp_splice_conn *conn,
>  	}
>  
>  	if (flag == CLOSING) {
> -		epoll_del(c->epollfd, conn->s[0]);
> -		epoll_del(c->epollfd, conn->s[1]);
> +		epoll_del(flow_epollfd(&conn->f), conn->s[0]);
> +		epoll_del(flow_epollfd(&conn->f), conn->s[1]);
>  	}
>  }
>  
>  #define conn_flag(c, conn, flag)					\
>  	do {								\
>  		flow_trace(conn, "flag at %s:%i", __func__, __LINE__);	\
> -		conn_flag_do(c, conn, flag);				\
> +		conn_flag_do(conn, flag);				\
>  	} while (0)
>  
>  /**
> @@ -751,10 +752,9 @@ void tcp_splice_init(struct ctx *c)
>  
>  /**
>   * tcp_splice_timer() - Timer for spliced connections
> - * @c:		Execution context
>   * @conn:	Connection to handle
>   */
> -void tcp_splice_timer(const struct ctx *c, struct tcp_splice_conn *conn)
> +void tcp_splice_timer(struct tcp_splice_conn *conn)
>  {
>  	unsigned sidei;
>  
> -- 
> 2.51.0
> 

-- 
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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