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[94.245.46.61]) by smtp.gmail.com with ESMTPSA id n15sm1815043lfi.146.2021.02.13.03.30.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 13 Feb 2021 03:30:32 -0800 (PST) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Sat, 13 Feb 2021 12:30:30 +0100 To: "Paul E. McKenney" Cc: Uladzislau Rezki , Sebastian Andrzej Siewior , LKML , RCU , Michael Ellerman , Andrew Morton , Daniel Axtens , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Peter Zijlstra , Michal Hocko , Thomas Gleixner , "Theodore Y . Ts'o" , Oleksiy Avramchenko Subject: Re: [PATCH 2/2] rcu-tasks: add RCU-tasks self tests Message-ID: <20210213113030.GA1878@pc638.lan> References: <20201209202732.5896-1-urezki@gmail.com> <20201209202732.5896-2-urezki@gmail.com> <20210212192059.wytqwdf4qm4rnq3d@linutronix.de> <20210212211207.GA2046@pc638.lan> <20210212234851.GP2743@paulmck-ThinkPad-P72> <20210213003709.GA27846@paulmck-ThinkPad-P72> <20210213004328.GB27846@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210213004328.GB27846@paulmck-ThinkPad-P72> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Fri, Feb 12, 2021 at 04:43:28PM -0800, Paul E. McKenney wrote: > On Fri, Feb 12, 2021 at 04:37:09PM -0800, Paul E. McKenney wrote: > > On Fri, Feb 12, 2021 at 03:48:51PM -0800, Paul E. McKenney wrote: > > > On Fri, Feb 12, 2021 at 10:12:07PM +0100, Uladzislau Rezki wrote: > > > > On Fri, Feb 12, 2021 at 08:20:59PM +0100, Sebastian Andrzej Siewior wrote: > > > > > On 2020-12-09 21:27:32 [+0100], Uladzislau Rezki (Sony) wrote: > > > > > > Add self tests for checking of RCU-tasks API functionality. > > > > > > It covers: > > > > > > - wait API functions; > > > > > > - invoking/completion call_rcu_tasks*(). > > > > > > > > > > > > Self-tests are run when CONFIG_PROVE_RCU kernel parameter is set. > > > > > > > > > > I just bisected to this commit. By booting with `threadirqs' I end up > > > > > with: > > > > > [ 0.176533] Running RCU-tasks wait API self tests > > > > > > > > > > No stall warning or so. > > > > > It boots again with: > > > > > > > > > > diff --git a/init/main.c b/init/main.c > > > > > --- a/init/main.c > > > > > +++ b/init/main.c > > > > > @@ -1489,6 +1489,7 @@ void __init console_on_rootfs(void) > > > > > fput(file); > > > > > } > > > > > > > > > > +void rcu_tasks_initiate_self_tests(void); > > > > > static noinline void __init kernel_init_freeable(void) > > > > > { > > > > > /* > > > > > @@ -1514,6 +1515,7 @@ static noinline void __init kernel_init_freeable(void) > > > > > > > > > > rcu_init_tasks_generic(); > > > > > do_pre_smp_initcalls(); > > > > > + rcu_tasks_initiate_self_tests(); > > > > > lockup_detector_init(); > > > > > > > > > > smp_init(); > > > > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h > > > > > --- a/kernel/rcu/tasks.h > > > > > +++ b/kernel/rcu/tasks.h > > > > > @@ -1266,7 +1266,7 @@ static void test_rcu_tasks_callback(struct rcu_head *rhp) > > > > > rttd->notrun = true; > > > > > } > > > > > > > > > > -static void rcu_tasks_initiate_self_tests(void) > > > > > +void rcu_tasks_initiate_self_tests(void) > > > > > { > > > > > pr_info("Running RCU-tasks wait API self tests\n"); > > > > > #ifdef CONFIG_TASKS_RCU > > > > > @@ -1322,7 +1322,6 @@ void __init rcu_init_tasks_generic(void) > > > > > #endif > > > > > > > > > > // Run the self-tests. > > > > > - rcu_tasks_initiate_self_tests(); > > > > > } > > > > > > > > > > #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */ > > > > > > > > > > > Signed-off-by: Uladzislau Rezki (Sony) > > > > > > Apologies for the hassle! My testing clearly missed this combination > > > of CONFIG_PROVE_RCU=y and threadirqs=1. :-( > > > > > > But at least I can easily reproduce this hang as follows: > > > > > > tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 2 --configs "TREE03" --kconfig "CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y" --bootargs "threadirqs=1" --trust-make > > > > > > Sadly, I cannot take your patch because that simply papers over the > > > fact that early boot use of synchronize_rcu_tasks() is broken in this > > > particular configuration, which will likely eventually bite others now > > > that init_kprobes() has been moved earlier in boot: > > > > > > 1b04fa990026 ("rcu-tasks: Move RCU-tasks initialization to before early_initcall()") > > > Link: https://lore.kernel.org/rcu/87eekfh80a.fsf@dja-thinkpad.axtens.net/ > > > Fixes: 36dadef23fcc ("kprobes: Init kprobes in early_initcall") > > > > > > > > Sebastian > > > > > > > > > We should be able to use call_rcu_tasks() in the *initcall() callbacks. > > > > The problem is that, ksoftirqd threads are not spawned by the time when > > > > an rcu_init_tasks_generic() is invoked: > > > > > > > > diff --git a/init/main.c b/init/main.c > > > > index c68d784376ca..e6106bb12b2d 100644 > > > > --- a/init/main.c > > > > +++ b/init/main.c > > > > @@ -954,7 +954,6 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void) > > > > rcu_init_nohz(); > > > > init_timers(); > > > > hrtimers_init(); > > > > - softirq_init(); > > > > timekeeping_init(); > > > > > > > > /* > > > > @@ -1512,6 +1511,7 @@ static noinline void __init kernel_init_freeable(void) > > > > > > > > init_mm_internals(); > > > > > > > > + softirq_init(); > > > > rcu_init_tasks_generic(); > > > > do_pre_smp_initcalls(); > > > > lockup_detector_init(); > > > > diff --git a/kernel/softirq.c b/kernel/softirq.c > > > > index 9d71046ea247..cafa55c496d0 100644 > > > > --- a/kernel/softirq.c > > > > +++ b/kernel/softirq.c > > > > @@ -630,6 +630,7 @@ void __init softirq_init(void) > > > > &per_cpu(tasklet_hi_vec, cpu).head; > > > > } > > > > > > > > + spawn_ksoftirqd(); > > > > > > We need a forward reference to allow this to build, but with that added, > > > my test case passes. Good show! > > > > > > > open_softirq(TASKLET_SOFTIRQ, tasklet_action); > > > > open_softirq(HI_SOFTIRQ, tasklet_hi_action); > > > > } > > > > @@ -732,7 +733,6 @@ static __init int spawn_ksoftirqd(void) > > > > > > > > return 0; > > > > } > > > > -early_initcall(spawn_ksoftirqd); > > > > > > > > /* > > > > * [ These __weak aliases are kept in a separate compilation unit, so that > > > > > > > > Any thoughts? > > > > > > One likely problem is that there are almost certainly parts of the kernel > > > that need softirq_init() to stay roughly where it is. So, is it possible > > > to leave softirq_init() where it is, and to arrange for spawn_ksoftirqd() > > > to be invoked just before rcu_init_tasks_generic() is called? > > > > This still seems worth trying (and doing so is next on my list), but just > > And the patch below takes this approach, which also causes the tests to > pass. > > Thoughts? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit f4cd768e341486655c8c196e1f2b48a4463541f3 > Author: Paul E. McKenney > Date: Fri Feb 12 16:41:05 2021 -0800 > > softirq: Don't try waking ksoftirqd before it has been spawned > > If there is heavy softirq activity, the softirq system will attempt > to awaken ksoftirqd and will stop the traditional back-of-interrupt > softirq processing. This is all well and good, but only if the > ksoftirqd kthreads already exist, which is not the case during early > boot, in which case the system hangs. > > One reproducer is as follows: > > tools/testing/selftests/rcutorture/bin/kvm.sh --allcpus --duration 2 --configs "TREE03" --kconfig "CONFIG_DEBUG_LOCK_ALLOC=y CONFIG_PROVE_LOCKING=y" --bootargs "threadirqs=1" --trust-make > > This commit therefore moves the spawning of the ksoftirqd kthreads > earlier in boot. With this change, the above test passes. > > Reported-by: Sebastian Andrzej Siewior > Reported-by: Uladzislau Rezki > Inspired-by: Uladzislau Rezki > Signed-off-by: Paul E. McKenney > > diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h > index bb8ff90..283a02d 100644 > --- a/include/linux/interrupt.h > +++ b/include/linux/interrupt.h > @@ -592,6 +592,8 @@ static inline struct task_struct *this_cpu_ksoftirqd(void) > return this_cpu_read(ksoftirqd); > } > > +int spawn_ksoftirqd(void); > + > /* Tasklets --- multithreaded analogue of BHs. > > This API is deprecated. Please consider using threaded IRQs instead: > diff --git a/init/main.c b/init/main.c > index c68d784..99835bb 100644 > --- a/init/main.c > +++ b/init/main.c > @@ -1512,6 +1512,7 @@ static noinline void __init kernel_init_freeable(void) > > init_mm_internals(); > > + spawn_ksoftirqd(); > rcu_init_tasks_generic(); > do_pre_smp_initcalls(); > lockup_detector_init(); > diff --git a/kernel/softirq.c b/kernel/softirq.c > index 9d71046..45d50d4 100644 > --- a/kernel/softirq.c > +++ b/kernel/softirq.c > @@ -724,7 +724,7 @@ static struct smp_hotplug_thread softirq_threads = { > .thread_comm = "ksoftirqd/%u", > }; > > -static __init int spawn_ksoftirqd(void) > +__init int spawn_ksoftirqd(void) > { > cpuhp_setup_state_nocalls(CPUHP_SOFTIRQ_DEAD, "softirq:dead", NULL, > takeover_tasklets); > @@ -732,7 +732,6 @@ static __init int spawn_ksoftirqd(void) > > return 0; > } > -early_initcall(spawn_ksoftirqd); > > /* > * [ These __weak aliases are kept in a separate compilation unit, so that > I thought about this approach as a first step how to fix it, but then came up with moving the spawn_ksoftirqd(void); into the softirq_init() to make it consolidated at one place and not spread. Then moving the softirq_init() down may cause other drawbacks, like you mentioned if somebody needs it earlier. I agree with your approach. Invoking the spawn_ksoftirqd() before the rcu_init_tasks_generic() makes it safe. At least it prevents other parts to be broken comparing with touching and moving softirq_init(). -- Vlad Rezki