From mboxrd@z Thu Jan 1 00:00:00 1970 From: Enke Chen Subject: Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification Date: Wed, 24 Oct 2018 14:56:12 -0700 Message-ID: References: <458c04d8-d189-4a26-729a-bb1d1d751534@cisco.com> <20181023092348.GA14340@redhat.com> <1e68a3ce-32cd-b058-3d1d-36455ceca848@cisco.com> <20181024135212.GF30128@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20181024135212.GF30128@redhat.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Oleg Nesterov Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Peter Zijlstra , Arnd Bergmann , "Eric W. Biederman" , Khalid Aziz , Kate Stewart , Helge Deller , Greg Kroah-Hartman , Al Viro , Andrew Morton , Christian Brauner , Catalin Marinas , Will Deacon , Dave Martin , Mauro Carvalho Chehab , Michal Hocko , Rik van Riel List-Id: linux-arch.vger.kernel.org Hi, Olge: On 10/24/18 6:52 AM, Oleg Nesterov wrote: > On 10/23, Enke Chen wrote: >> >>>> + /* >>>> + * Send the pre-coredump signal to the parent if requested. >>>> + */ >>>> + read_lock(&tasklist_lock); >>>> + notify = do_notify_parent_predump(current); >>>> + read_unlock(&tasklist_lock); >>>> + if (notify) >>>> + cond_resched(); >>> >>> Hmm. I do not understand why do we need cond_resched(). And even if we need it, >>> why we can't call it unconditionally? >> >> Remember the goal is to allow the parent (e.g., a process manager) to take early >> action. The "yield" before doing coredump will help. > > I don't see how can it actually help... > > cond_resched() is nop if CONFIG_PREEMPT or should_resched() == 0. > > and the coredumping thread will certainly need to sleep/wait anyway. I am really surprised by this - cond_resched() is used in many places and it actually does not do anything w/o CONFIG_PREEMPT. Will remove. > >>> And once again, SIGCHLD/SIGUSR do not queue, this means that PR_SET_PREDUMP_SIG >>> is pointless if you have 2 or more children. >> >> Hmm, could you point me to the code where SIGCHLD/SIGUSR is treated differently >> w.r.t. queuing? That does not sound right to me. > > see the legacy_queue() check. Any signal < SIGRTMIN do not queue. IOW, if SIGCHLD > is already pending, then next SIGCHLD is simply ignored. Got it. This means that a distinct signal (in particular a RT signal) would be more preferred. This is what it is done in our application. You earlier suggestion about removing the signal limitation makes a lot sense to me now. Given that a distinct signal is more preferred, I am wondering if I should just remove CLD_PREDUMP from the patch. Thanks. -- Enke From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from rcdn-iport-9.cisco.com ([173.37.86.80]:30733 "EHLO rcdn-iport-9.cisco.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726365AbeJYG0D (ORCPT ); Thu, 25 Oct 2018 02:26:03 -0400 Subject: Re: [PATCH v2] kernel/signal: Signal-based pre-coredump notification References: <458c04d8-d189-4a26-729a-bb1d1d751534@cisco.com> <20181023092348.GA14340@redhat.com> <1e68a3ce-32cd-b058-3d1d-36455ceca848@cisco.com> <20181024135212.GF30128@redhat.com> From: Enke Chen Message-ID: Date: Wed, 24 Oct 2018 14:56:12 -0700 MIME-Version: 1.0 In-Reply-To: <20181024135212.GF30128@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Oleg Nesterov Cc: Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Peter Zijlstra , Arnd Bergmann , "Eric W. Biederman" , Khalid Aziz , Kate Stewart , Helge Deller , Greg Kroah-Hartman , Al Viro , Andrew Morton , Christian Brauner , Catalin Marinas , Will Deacon , Dave Martin , Mauro Carvalho Chehab , Michal Hocko , Rik van Riel , "Kirill A. Shutemov" , Roman Gushchin , Marcos Paulo de Souza , Dominik Brodowski , Cyrill Gorcunov , Yang Shi , Jann Horn , Kees Cook , x86@kernel.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, "Victor Kamensky (kamensky)" , xe-linux-external@cisco.com, Stefan Strogin , Enke Chen Message-ID: <20181024215612._DuFUwSppVZy6XczK1wfE1-LIvqRN7XyBxXBTKACcsA@z> Hi, Olge: On 10/24/18 6:52 AM, Oleg Nesterov wrote: > On 10/23, Enke Chen wrote: >> >>>> + /* >>>> + * Send the pre-coredump signal to the parent if requested. >>>> + */ >>>> + read_lock(&tasklist_lock); >>>> + notify = do_notify_parent_predump(current); >>>> + read_unlock(&tasklist_lock); >>>> + if (notify) >>>> + cond_resched(); >>> >>> Hmm. I do not understand why do we need cond_resched(). And even if we need it, >>> why we can't call it unconditionally? >> >> Remember the goal is to allow the parent (e.g., a process manager) to take early >> action. The "yield" before doing coredump will help. > > I don't see how can it actually help... > > cond_resched() is nop if CONFIG_PREEMPT or should_resched() == 0. > > and the coredumping thread will certainly need to sleep/wait anyway. I am really surprised by this - cond_resched() is used in many places and it actually does not do anything w/o CONFIG_PREEMPT. Will remove. > >>> And once again, SIGCHLD/SIGUSR do not queue, this means that PR_SET_PREDUMP_SIG >>> is pointless if you have 2 or more children. >> >> Hmm, could you point me to the code where SIGCHLD/SIGUSR is treated differently >> w.r.t. queuing? That does not sound right to me. > > see the legacy_queue() check. Any signal < SIGRTMIN do not queue. IOW, if SIGCHLD > is already pending, then next SIGCHLD is simply ignored. Got it. This means that a distinct signal (in particular a RT signal) would be more preferred. This is what it is done in our application. You earlier suggestion about removing the signal limitation makes a lot sense to me now. Given that a distinct signal is more preferred, I am wondering if I should just remove CLD_PREDUMP from the patch. Thanks. -- Enke