From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marc Kleine-Budde Subject: Re: poll broken (for can) Date: Mon, 28 Mar 2011 21:32:50 +0200 Message-ID: <4D90E262.1090201@pengutronix.de> References: <1301321142.9519.10.camel@lukonin-pc> <4D90A7E9.1080804@grandegger.com> <4D90AA8A.9010804@pengutronix.de> <4D90AF67.1080405@grandegger.com> <4D90B3B0.2010401@pengutronix.de> <4D90CB17.4030205@hartkopp.net> Mime-Version: 1.0 Content-Type: multipart/mixed; boundary="===============1113765869952639809==" Cc: Netdev-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, socketcan-users-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org, Wolfgang Grandegger To: Oliver Hartkopp Return-path: In-Reply-To: <4D90CB17.4030205-fJ+pQTUTwRTk1uMJSBkQmQ@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: socketcan-users-bounces-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org Errors-To: socketcan-users-bounces-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org List-Id: netdev.vger.kernel.org This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --===============1113765869952639809== Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enig2D727D7A3683CA563D9B9853" This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enig2D727D7A3683CA563D9B9853 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable On 03/28/2011 07:53 PM, Oliver Hartkopp wrote: > On 28.03.2011 18:13, Marc Kleine-Budde wrote: >> On 03/28/2011 05:55 PM, Wolfgang Grandegger wrote: >>>> BTW: I figured out why poll() wakes you up but the next write will f= ail >>>> with -ENOBUFS again. >>> >>> Ah, I'm curious? I also did realize that poll does burn CPU cycles >>> (instead of waiting). >> >> The poll callback checks if the used memory is less than the half of p= er >> socket snd buffer (IIRC ~60K). See: >> >> datagram_poll (http://lxr.linux.no/linux+v2.6.38/net/core/datagram.c#L= 737) >> sock_writeable (http://lxr.linux.no/linux+v2.6.38/include/net/sock.h#L= 1618) >> >> Because the size of a can frame (+the skb overhead) is much less then >> the ethernet frame (+overhead) the default value for the snd buffer is= >> too big for can. >> >> We get the -ENOBUF from write() if the tx_queue_len (default 10) is >> exceeded. >> >> http://lxr.linux.no/linux+v2.6.38/drivers/net/can/dev.c#L435 >> http://lxr.linux.no/linux+v2.6.38/net/can/af_can.c#L268 >> >=20 > What would be your suggestion? Decreasing the socket send buffer for CA= N by > default? I haven't done any testing.....As far as I understand the code, we can a) increase the default tx_queue_len and/or b) decrease the default snd buffer size. Note: a) is a per device setting whereas b) is a per socket setting. With the current settings the -ENOBUF is triggered if we have X unsend can frames (per device) where X equals the tx_queue_len. This means using 5 applications, it about 2 queued (i.e. unsent) frames per app and device. If we increase the tx_queue_len to a high value (via ifconfig), so that the snd buffer is fully used, before the tx_queue_len is exceeded the write system call will block, (or return -EAGAIN of opened non blocking). At least the last time I've done this. I think solution b) would lead to a similar behavioural change. What do we really want to specify? Something like: queue up to X frames per socket and queue only Y frames per device. Where Y =3D X * n and n is "I don't know yet"? Y is simple, it's the tx_queue_len. But X is more complicated. The can frames have non constant length (i.e. dlc) and I'm not sure that the netdev people say if we misuse the sock_alloc_send_pskb() for our tx-flow-control :) Cheers, Marc --=20 Pengutronix e.K. | Marc Kleine-Budde | Industrial Linux Solutions | Phone: +49-231-2826-924 | Vertretung West/Dortmund | Fax: +49-5121-206917-5555 | Amtsgericht Hildesheim, HRA 2686 | http://www.pengutronix.de | --------------enig2D727D7A3683CA563D9B9853 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk2Q4mYACgkQjTAFq1RaXHNUeQCeN+pL7YvFnwYIo5TWG2Fm2R95 mioAn2Di2Kn+7xCpOiBQBJUe/OgaMi6o =kY7V -----END PGP SIGNATURE----- --------------enig2D727D7A3683CA563D9B9853-- --===============1113765869952639809== Content-Type: text/plain; charset="us-ascii" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline _______________________________________________ Socketcan-users mailing list Socketcan-users-0fE9KPoRgkgATYTw5x5z8w@public.gmane.org https://lists.berlios.de/mailman/listinfo/socketcan-users --===============1113765869952639809==--