From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id C68FBC43331 for ; Fri, 27 Mar 2020 22:23:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8B4822072F for ; Fri, 27 Mar 2020 22:23:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585347828; bh=SY+0Q13cdiLCyweJl9Y4ZF897cLrmmwILPQHft8+ITw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=L37udMkZkHSv70Rlgyygr18bNPnUYcuTAUpN56Vuqo2UdqDevenaZ99vwlOOD3nYZ mXji4ms0JdUZebDfX7dIHB+6r6bnJTT7hTGveH+t+hhKYBOQbQLhg96B6LdAO3xRkl YlJjoXFwbg5ZiV5sCFkECQ94Pg1b+Xs+8ZhNhma4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727655AbgC0WXs (ORCPT ); Fri, 27 Mar 2020 18:23:48 -0400 Received: from mail.kernel.org ([198.145.29.99]:42296 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726781AbgC0WXs (ORCPT ); Fri, 27 Mar 2020 18:23:48 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 32C6E206DB; Fri, 27 Mar 2020 22:23:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1585347827; bh=SY+0Q13cdiLCyweJl9Y4ZF897cLrmmwILPQHft8+ITw=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=grtF2meVq8NWC3HzSp4Xqd3Hft6LG4JWajJb09mvAUZy4rL6F65Zvg1D04ZfmgKCy S97/kP/BYuljczbbgxht3ZQWTyLXxcJcQLvCuuueGuFjNvMumy1BHe/0mwRLcZl1tP g1u+nKNuKx89PSjL4Dctkmsxv8vYDYcjpF5IT8TY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id EE73D35226D5; Fri, 27 Mar 2020 15:23:46 -0700 (PDT) Date: Fri, 27 Mar 2020 15:23:46 -0700 From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: [PATCH RFC v3 tip/core/rcu 0/34] Prototype RCU usable from idle, exception, offline Message-ID: <20200327222346.GA12082@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200312181618.GA21271@paulmck-ThinkPad-P72> <20200319001024.GA28798@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200319001024.GA28798@paulmck-ThinkPad-P72> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org Hello! This series provides two variants of Tasks RCU, a rude variant inspired by Steven Rostedt's use of schedule_on_each_cpu(), and a tracing variant requested by the BPF folks to be used (for example) to protect BPF programs that unconditionally access userspace memory, and thus might occasionally take a page fault, resulting in a voluntary context switch. The rude variant uses context switches and offline as its quiescent states, so that preempt-disabled regions of code executing on online CPUs form the tasks rude RCU readers. The tracing variant has explicit read-side markers to permit finite grace periods even given in-kernel loops in PREEMPT=n builds. These markers are rcu_read_lock_trace() and rcu_read_unlock_trace(), so that any code not under rcu_read_lock_trace() is a quiescent state. This variant also protects marked code in the idle loop, on exception entry/exit paths, and on the various CPU-hotplug online/offline code paths, thus having protection properties similar to SRCU. However, unlike SRCU, this variant avoids expensive instructions in the read-side primitives, thus having read-side overhead similar to that of preemptible RCU. This difference is important for some BPF programs, according to benchmarking from Alexei Starovoitov: https://lore.kernel.org/lkml/20200310014043.4dbagqbr2wsbuarm@ast-mbp/ There are of course downsides. The grace-period code can send IPIs to CPUs, even when those CPUs are in the idle loop or in nohz_full userspace. However, this version enlists the aid of the context-switch hooks, which eliminates the need for IPIs in context-switch-heavy workloads. It also prohibits sending of IPIs early in the grace period based on a new rcupdate.rcu_task_ipi_delay kernel boot parameter, which provides additional opportunity for the hooks to do their job. Finally, a new TASKS_TRACE_RCU_READ_MB Kconfig option avoids sending IPIs to tasks executing userspace or in the idle loop, at the expense of higher overhead readers during kernel entry/exit code and in the idle loop. It is also necessary to scan the full tasklist, much as for Tasks RCU. There is a single callback queue guarded by a single lock, again, much as for Tasks RCU. If needed, these downsides can be at least partially remedied. Perhaps most important, this variant of RCU does not affect the vanilla flavors, rcu_preempt and rcu_sched. The fact that RCU Tasks Trace readers can operate from idle, offline, and exception entry/exit in no way allows rcu_preempt and rcu_sched readers to also do so. The RCU tasks trace mechanism is based off of RCU tasks rather than SRCU because the latter is more complex and also because the latter uses a CPU-by-CPU approach to tracking quiescent states instead of the task-by-task approach that is needed. It is in theory possible to mash RCU tasks trace into the Tree SRCU implementation, but there will need to be extremely good reasons for doing so. The vanilla RCU mechanism could in theory be used in CONFIG_PREEMPT=y kernels, but fails utterly in CONFIG_PREEMPT=n kernels. Tasks RCU does not work because page faults can result in a voluntary context switch, which prevents it from protecting a BPF program that page faults. The new "rude" variant only protected preempt-disable regions of code, thus also failing to protect BPF programs that page fault. This effort benefited greatly from off-list discussions of BPF requirements with Alexei Starovoitov and Andrii Nakryiko, as well as from numerous on-list discussions, at least some of which are captured in the "Link:" tags on the patches themselves. The patches in this series are as follows, with asterisks indicating significant change from v1: 1*. Add function to sample state of a locked-down task. Added the task_struct argument to the callback function. 2*. Use the above function to add per-task state to RCU CPU stall warnings. This commit was adapted to the updated API. 3. Add rcutorture module parameter to produce non-busy-wait task stalls, thus allowing the above RCU CPU stall change to be exercised. 4. Move Tasks RCU to its own file. 5. Create struct to hold RCU-tasks state information. 6. Reinstate synchronize_rcu_mult(), as there will likely once again be a need to wait on multiple flavors of RCU. 7. Add an rcutorture test for synchronize_rcu_mult(). 8. Refactor RCU-tasks to allow variants to be added. 9*. Add an RCU-tasks rude variant, based on Steven Rostedt's use of schedule_on_each_cpu(). Updated Kconfig default to rely on default default value, updated help text, and updated the header comment. 10. Add torture tests for RCU Tasks Rude. 11. Use unique names for RCU-Tasks kthreads and messages. 12. Further refactor RCU-tasks to allow adding even more variants. 13. Code movement to allow even more Tasks RCU variants. 14*. Add an RCU Tasks Trace to simplify protection of tracing hooks, including BPF. This version fixes even more bugs and adds a URL to an email explaining the memory ordering. It also updates the Kconfig default and updates the help text. Furthermore, it moves a misplaced comment update. Finally, it makes the rcu_read_unlock_trace() function safe for scheduler locks, interrupt handlers, and NMI handlers. 15. Add torture tests for RCU Tasks Trace. 16. Add stall warnings for RCU Tasks Trace. 17. Move #ifdef into tasks.h to ease addition of Kconfig-dependent APIs. 18. Add RCU-tasks-specific information to rcutorture writer stall output, easing debugging of these RCU variants. 19. Make the above rcutorture writer stall output include grace-period state. 20. Cause RCU tasks trace to take advantage of RCU scheduler hooks, thus reducing the number of IPIs. 21. Record grace-period start time for RCU tasks variants for IPI throttling and for debugging. 22. Provide a kernel boot parameter to delay IPIs until a given grace period reaches the specified age, with this age defaulting to half a second, further reducing the number of IPIs. To zero on context-switch-heavy workloads. 23*. Split ->trc_reader_need_end to make room for memory-barrier indication. 24*. Add grace-period and IPI counts to statistics. 25*. Add Kconfig option to mediate smp_mb() vs. IPI. 26*. Avoid IPIing userspace/idle tasks if kernel is so built. 27*. Allow rcu_read_unlock_trace() under scheduler locks. 28*. Disable CPU hotplug across RCU tasks trace scans. This enables detection of idle tasks for offline CPUs. 29*. Handle the running-offline idle-task special case. 30*. Make RCU tasks trace also wait for idle tasks. 31*. Add rcu_dynticks_zero_in_eqs() effectiveness statistics. 32*. Add count for idle tasks on offline CPUs. 33*. Add TRACE02 scenario enabling RCU Tasks Trace IPIs. The existing TRACE01 scenario avoids IPIs to userspace and idle CPUs. 34*. Add IPI failure count to statistics. These new versions of Tasks RCU now pass heavy rcutorture testing, and should thus be fine for experimental use. The original Tree RCU went upstream with less testing than this has seen, but then again those were simpler times. ;-) Changes since v2: o Leveraged idle entry/exit hooks to reduce IPIing of idle and userspace tasks. o Switch to read-side memory barriers during idle and userspace execution in kernels built for real-time or battery-powered use, mediated by a new TASKS_TRACE_RCU_READ_MB Kconfig option. Also add an rcutorture test scenario for this option. o Adjust rcutorture to better test the IPI path. (Seeing zero IPIs might be satisfying to me personally, but it is a lousy test strategy!) o Added more information to stall warnings and rcutorture end-of-test printout. o Make rcu_read_unlock_trace() usable when invoked with scheduler locks held. o Make rcu_read_unlock_trace() usable in interrupt and NMI handlers. o Fix handling of idle tasks, including those "running" on offline CPUs. o Fixed a number of other bugs found during testing and responded to review feedback. Changes since v1: o Updated this cover letter to provide more detail, including on roads not taken. o Updated commit logs based on feedback from v1. o Updated the function providing a consistent view of the specified non-running task's state to invoke the specified function even if the task is currently running. This will be necessary to safely eliminate IPIs for long-term idle and userspace execution. The function may also now return false to transmit a failure indication to the caller, for example, if the function cannot handle being invoked on a running CPU. The function is now passed the relevant task_struct pointer as well as the specified argument. Changes were of course made to use the new API. o Leveraged context-switch hooks to avoid unnecessary IPIs. o Held off IPIs for the first half second (by default) of each grace period to give the context-switch hooks a better chance to do their job. o Lots of testing. o Fixed a number of bugs and responded to v2 feedback. Todo: o Even more testing. o If all goes well, post a non-RFC series. Thanx, Paul ------------------------------------------------------------------------ Documentation/admin-guide/kernel-parameters.txt | 12 include/linux/rcupdate.h | 48 include/linux/rcupdate_trace.h | 98 include/linux/rcupdate_wait.h | 19 include/linux/rcutiny.h | 2 include/linux/sched.h | 12 include/linux/wait.h | 2 init/init_task.c | 5 kernel/fork.c | 5 kernel/rcu/Kconfig | 50 kernel/rcu/Kconfig.debug | 4 kernel/rcu/rcu.h | 5 kernel/rcu/rcutorture.c | 99 kernel/rcu/tasks.h | 2089 +++++++++--- kernel/rcu/tree.c | 24 kernel/rcu/tree.h | 2 kernel/rcu/tree_plugin.h | 24 kernel/rcu/tree_stall.h | 40 kernel/rcu/update.c | 375 -- kernel/sched/core.c | 48 tools/testing/selftests/rcutorture/configs/rcu/CFLIST | 3 tools/testing/selftests/rcutorture/configs/rcu/RUDE01 | 10 tools/testing/selftests/rcutorture/configs/rcu/RUDE01.boot | 1 tools/testing/selftests/rcutorture/configs/rcu/TRACE01 | 11 tools/testing/selftests/rcutorture/configs/rcu/TRACE01.boot | 1 tools/testing/selftests/rcutorture/configs/rcu/TRACE02 | 11 tools/testing/selftests/rcutorture/configs/rcu/TRACE02.boot | 1 27 files changed, 2140 insertions(+), 861 deletions(-)