* [PATCH] sockets: Use SOMAXCONN for Unix socket listen()
@ 2021-02-04 22:20 Eric Blake
2021-02-05 9:55 ` Daniel P. Berrangé
2021-02-05 10:01 ` Richard W.M. Jones
0 siblings, 2 replies; 4+ messages in thread
From: Eric Blake @ 2021-02-04 22:20 UTC (permalink / raw)
To: qemu-devel; +Cc: berrange, rjones, qemu-block, Gerd Hoffmann
Our default of a backlog of 1 connection is rather puny, particularly
for scenarios where we expect multiple listeners to connect (such as
qemu-nbd -e X). For Unix sockets, there's no real harm in supporting
a larger backlog, and a definite benefit to the clients: at least on
Linux, a client trying to connect to a Unix socket with a backlog gets
an EAGAIN failure with no way to poll() for when the backlog is no
longer present short of sleeping an arbitrary amount of time before
retrying.
See https://bugzilla.redhat.com/1925045 for a demonstration of where
our low backlog prevents libnbd from connecting as many parallel
clients as it wants.
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
---
util/qemu-sockets.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index 8af0278f15c6..a7573e9f0fda 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -1059,7 +1059,7 @@ int unix_listen(const char *str, Error **errp)
saddr = g_new0(UnixSocketAddress, 1);
saddr->path = g_strdup(str);
- sock = unix_listen_saddr(saddr, 1, errp);
+ sock = unix_listen_saddr(saddr, SOMAXCONN, errp);
qapi_free_UnixSocketAddress(saddr);
return sock;
}
--
2.30.0
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH] sockets: Use SOMAXCONN for Unix socket listen()
2021-02-04 22:20 [PATCH] sockets: Use SOMAXCONN for Unix socket listen() Eric Blake
@ 2021-02-05 9:55 ` Daniel P. Berrangé
2021-02-05 19:04 ` Eric Blake
2021-02-05 10:01 ` Richard W.M. Jones
1 sibling, 1 reply; 4+ messages in thread
From: Daniel P. Berrangé @ 2021-02-05 9:55 UTC (permalink / raw)
To: Eric Blake; +Cc: Gerd Hoffmann, qemu-devel, qemu-block, rjones
On Thu, Feb 04, 2021 at 04:20:18PM -0600, Eric Blake wrote:
> Our default of a backlog of 1 connection is rather puny, particularly
> for scenarios where we expect multiple listeners to connect (such as
> qemu-nbd -e X). For Unix sockets, there's no real harm in supporting
> a larger backlog, and a definite benefit to the clients: at least on
> Linux, a client trying to connect to a Unix socket with a backlog gets
> an EAGAIN failure with no way to poll() for when the backlog is no
> longer present short of sleeping an arbitrary amount of time before
> retrying.
>
> See https://bugzilla.redhat.com/1925045 for a demonstration of where
> our low backlog prevents libnbd from connecting as many parallel
> clients as it wants.
>
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> util/qemu-sockets.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index 8af0278f15c6..a7573e9f0fda 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -1059,7 +1059,7 @@ int unix_listen(const char *str, Error **errp)
>
> saddr = g_new0(UnixSocketAddress, 1);
> saddr->path = g_strdup(str);
> - sock = unix_listen_saddr(saddr, 1, errp);
> + sock = unix_listen_saddr(saddr, SOMAXCONN, errp);
> qapi_free_UnixSocketAddress(saddr);
> return sock;
> }
This method is a legacy back compat function, only used by the QEMU
guest agent, so this can't explain the NBD problems, which use the
QIONetListener class.
IOW, the problem is in the qemu-nbd.c / blockdev-nbd.c code I believe
Regards,
Daniel
--
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^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] sockets: Use SOMAXCONN for Unix socket listen()
2021-02-04 22:20 [PATCH] sockets: Use SOMAXCONN for Unix socket listen() Eric Blake
2021-02-05 9:55 ` Daniel P. Berrangé
@ 2021-02-05 10:01 ` Richard W.M. Jones
1 sibling, 0 replies; 4+ messages in thread
From: Richard W.M. Jones @ 2021-02-05 10:01 UTC (permalink / raw)
To: Eric Blake; +Cc: berrange, qemu-devel, qemu-block, Gerd Hoffmann
On Thu, Feb 04, 2021 at 04:20:18PM -0600, Eric Blake wrote:
> Our default of a backlog of 1 connection is rather puny, particularly
> for scenarios where we expect multiple listeners to connect (such as
> qemu-nbd -e X). For Unix sockets, there's no real harm in supporting
> a larger backlog, and a definite benefit to the clients: at least on
> Linux, a client trying to connect to a Unix socket with a backlog gets
> an EAGAIN failure with no way to poll() for when the backlog is no
> longer present short of sleeping an arbitrary amount of time before
> retrying.
>
> See https://bugzilla.redhat.com/1925045 for a demonstration of where
> our low backlog prevents libnbd from connecting as many parallel
> clients as it wants.
>
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
> util/qemu-sockets.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index 8af0278f15c6..a7573e9f0fda 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -1059,7 +1059,7 @@ int unix_listen(const char *str, Error **errp)
>
> saddr = g_new0(UnixSocketAddress, 1);
> saddr->path = g_strdup(str);
> - sock = unix_listen_saddr(saddr, 1, errp);
> + sock = unix_listen_saddr(saddr, SOMAXCONN, errp);
> qapi_free_UnixSocketAddress(saddr);
> return sock;
> }
Does this fix the problem with qemu-nbd? Not for me - I still seem to
see the old behaviour with this patch applied.
Also looking more generally at util/qemu-sockets.c I think it would be
better if all of those "num" parameters were renamed "backlog".
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines. Supports shell scripting,
bindings from many languages. http://libguestfs.org
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] sockets: Use SOMAXCONN for Unix socket listen()
2021-02-05 9:55 ` Daniel P. Berrangé
@ 2021-02-05 19:04 ` Eric Blake
0 siblings, 0 replies; 4+ messages in thread
From: Eric Blake @ 2021-02-05 19:04 UTC (permalink / raw)
To: Daniel P. Berrangé; +Cc: Gerd Hoffmann, qemu-devel, qemu-block, rjones
On 2/5/21 3:55 AM, Daniel P. Berrangé wrote:
>> +++ b/util/qemu-sockets.c
>> @@ -1059,7 +1059,7 @@ int unix_listen(const char *str, Error **errp)
>>
>> saddr = g_new0(UnixSocketAddress, 1);
>> saddr->path = g_strdup(str);
>> - sock = unix_listen_saddr(saddr, 1, errp);
>> + sock = unix_listen_saddr(saddr, SOMAXCONN, errp);
>> qapi_free_UnixSocketAddress(saddr);
>> return sock;
>> }
>
> This method is a legacy back compat function, only used by the QEMU
> guest agent, so this can't explain the NBD problems, which use the
> QIONetListener class.
>
> IOW, the problem is in the qemu-nbd.c / blockdev-nbd.c code I believe
D'oh. Serves me right for trying to guess the spot using just a grep on
listen() rather than running under gdb with a breakpoint to find the
actual backtrace. v2 posted with a slightly changed subject line, and
this time tested to actually work.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
^ permalink raw reply [flat|nested] 4+ messages in thread
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2021-02-04 22:20 [PATCH] sockets: Use SOMAXCONN for Unix socket listen() Eric Blake
2021-02-05 9:55 ` Daniel P. Berrangé
2021-02-05 19:04 ` Eric Blake
2021-02-05 10:01 ` Richard W.M. Jones
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