From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757153AbbJVK2w (ORCPT ); Thu, 22 Oct 2015 06:28:52 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:56449 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751774AbbJVK2v (ORCPT ); Thu, 22 Oct 2015 06:28:51 -0400 Message-ID: <1445509727.4113.52.camel@infradead.org> Subject: Re: [PATCH] printk: Don't discard earlier unprinted messages to make space From: David Woodhouse To: David Howells , linux-kernel@vger.kernel.org Cc: jack@suse.cz, will.deacon@arm.com, rostedt@goodmis.org, pmladek@suse.cz Date: Thu, 22 Oct 2015 11:28:47 +0100 In-Reply-To: <20151022101650.25995.95155.stgit@warthog.procyon.org.uk> References: <20151022101650.25995.95155.stgit@warthog.procyon.org.uk> Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-WqGNBbVxzawmsVhzsDYy" X-Mailer: Evolution 3.16.5 (3.16.5-3.fc22) Mime-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-WqGNBbVxzawmsVhzsDYy Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2015-10-22 at 11:16 +0100, David Howells wrote: > printk() currently discards earlier messages to make space for new messag= es > arriving. This has the distinct downside that if the kernel starts > churning out messages because of some initial incident, the report of the > initial incident is likely to be lost under a blizzard of: >=20 > ** NNN printk messages dropped ** >=20 > messages from console_unlock(). >=20 > The first message generated (typically an oops) is usually the most > important - the one you want to solve first - so we really want to see > that. But wait... didn't I watch you muttering on IRC about the actual bug you were trying to catch here... and didn't you have a *serial* console hooked up? What broke such that serial console stopped giving you *every* message? Serial console was always *synchronous*. We could do stuff like... printk("Going to do foo...\n"); outb(foo, baz); printk("Did foo and the machine didn't catch fire! Now bar\n"); outb(bar, baz); printk("Done\n"); And with a serial console I could know *precisely* the point at which the machine locked up. And I could enable the silly debugging levels on things like JFFS2 and be sure that with a serial console I could catch *every* printk reliably =E2=80=94 which led to a number of cases where people would reprod= uce a bug with a serial console and debugging, mail me a huge log file, and get a patch back in reply. We *need* to have a mode where serial console is actually *reliable*, and we can know that the message has been sent out the port before the printk() call returns. What happened to it? And how do we fix it? --=20 dwmw2 --=-WqGNBbVxzawmsVhzsDYy Content-Type: application/x-pkcs7-signature; name="smime.p7s" Content-Disposition: attachment; filename="smime.p7s" Content-Transfer-Encoding: base64 MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAQAAoIISjjCCBicw ggUPoAMCAQICAw3vNzANBgkqhkiG9w0BAQUFADCBjDELMAkGA1UEBhMCSUwxFjAUBgNVBAoTDVN0 YXJ0Q29tIEx0ZC4xKzApBgNVBAsTIlNlY3VyZSBEaWdpdGFsIENlcnRpZmljYXRlIFNpZ25pbmcx ODA2BgNVBAMTL1N0YXJ0Q29tIENsYXNzIDEgUHJpbWFyeSBJbnRlcm1lZGlhdGUgQ2xpZW50IENB MB4XDTE1MDUwNTA5NDM0MVoXDTE2MDUwNTA5NTMzNlowQjEcMBoGA1UEAwwTZHdtdzJAaW5mcmFk ZWFkLm9yZzEiMCAGCSqGSIb3DQEJARYTZHdtdzJAaW5mcmFkZWFkLm9yZzCCASIwDQYJKoZIhvcN AQEBBQADggEPADCCAQoCggEBAMkbm9kPbx1j/X4RVyf/pPKSYwelcco69TvnQQbKM8m8xkWjXJI1 jpJ1jMaGUZGFToINMSZi7lZawUozudWbXSKy1SikENSTJHffsdRAIlsp+hR8vWvjsKUry6sEdqPG doa5RY7+N4WRusWZDYW/RRWE6i9EL9qV86CVPYqw22UBOUw4/j/HVGCV6TSB8yE5iEwhk/hUuzRr FZm1MJMR7mCS7BCR8Lr5jFY61lWpBiXNXIxLZCvDc26KR5L5tYX43iUVO3fzES1GRVoYnxxk2tmz 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