On 29/11/2021 19:07, Paul E. McKenney wrote: > On Mon, Nov 29, 2021 at 06:50:01PM +0000, Guillaume Tucker wrote: >> Hi Paul, >> >> Please see the bisection report below about a new kernel alert >> message. >> >> Reports aren't automatically sent to the public while we're >> trialing new bisection features on kernelci.org but this one >> looks valid. >> >> Some more details can be found here: >> >> https://linux.kernelci.org/test/case/id/61a4f02ad5a9e3a5f218f6f6/ >> >> The bisection was run on QEMU but it seems to appear on all >> platforms as it's not an architecture-specific change. >> Presumably, none of the platforms in KernelCI is setting a value >> for rcu_task_enqueue_lim at the moment. >> >> Alert messages normally highlight potential problems, so ideally >> they would need to be resolved. Should the message in this case >> really be a pr_alert(), and if so what would be the recommended >> way to avoid it? > > It is to be unconditionally printed, so I am queueing a fixup that > downgrades it to pr_info(). Does that work for you? Yes, that will certainly make the message fall under the radar of KernelCI. Thanks! Guillaume >> GitHub: https://github.com/kernelci/kernelci-project/issues/78 >> >> On 26/11/2021 21:15, kernelci.org bot wrote: >>> * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * >>> * This automated bisection report was sent to you on the basis * >>> * that you may be involved with the breaking commit it has * >>> * found. No manual investigation has been done to verify it, * >>> * and the root cause of the problem may be somewhere else. * >>> * * >>> * If you do send a fix, please include this trailer: * >>> * Reported-by: "kernelci.org bot" <bot@xxxxxxxxxxxx> * >>> * * >>> * Hope this helps! * >>> * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * >>> >>> next/master bisection: baseline.dmesg.alert on qemu_arm64-virt-gicv3 >>> >>> Summary: >>> Start: f30a24ed97b40 Add linux-next specific files for 20211126 >>> Plain log: https://storage.kernelci.org/next/master/next-20211126/arm64/defconfig+CONFIG_ARM64_64K_PAGES=y/clang-13/lab-collabora/baseline-qemu_arm64-virt-gicv3.txt >>> HTML log: https://storage.kernelci.org/next/master/next-20211126/arm64/defconfig+CONFIG_ARM64_64K_PAGES=y/clang-13/lab-collabora/baseline-qemu_arm64-virt-gicv3.html >>> Result: dc9166f1253df rcu-tasks: Use more callback queues if contention encountered >>> >>> Checks: >>> revert: PASS >>> verify: PASS >>> >>> Parameters: >>> Tree: next >>> URL: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git >>> Branch: master >>> Target: qemu_arm64-virt-gicv3 >>> CPU arch: arm64 >>> Lab: lab-collabora >>> Compiler: clang-13 >>> Config: defconfig+CONFIG_ARM64_64K_PAGES=y >>> Test case: baseline.dmesg.alert >>> >>> Breaking commit found: >>> >>> ------------------------------------------------------------------------------- >>> commit dc9166f1253df20e01b0132ece22efaeb5b0a7e0 >>> Author: Paul E. McKenney <paulmck@xxxxxxxxxx> >>> Date: Wed Nov 24 15:12:15 2021 -0800 >>> >>> rcu-tasks: Use more callback queues if contention encountered >>> >>> The rcupdate.rcu_task_enqueue_lim module parameter allows system >>> administrators to tune the number of callback queues used by the RCU >>> Tasks flavors. However if callback storms are infrequent, it would >>> be better to operate with a single queue on a given system unless and >>> until that system actually needed more queues. Systems not needing >>> more queues can then avoid the overhead of checking the extra queues >>> and especially avoid the overhead of fanning workqueue handlers out to >>> all CPUs to invoke callbacks. >>> >>> This commit therefore switches to using all the CPUs' callback queues if >>> call_rcu_tasks_generic() encounters too much lock contention. The amount >>> of lock contention to tolerate defaults to 100 contended lock acquisitions >>> per jiffy, and can be adjusted using the new rcupdate.rcu_task_contend_lim >>> module parameter. >>> >>> Such switching is undertaken only if the rcupdate.rcu_task_enqueue_lim >>> module parameter is negative, which is its default value (-1). >>> This allows savvy systems administrators to set the number of queues >>> to some known good value and to not have to worry about the kernel doing >>> any second guessing. >>> >>> Reported-by: Martin Lau <kafai@xxxxxx> >>> Cc: Neeraj Upadhyay <neeraj.iitr10@xxxxxxxxx> >>> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> >>> >>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt >>> index f1f3761fe42c8..fc5e20afaeefa 100644 >>> --- a/Documentation/admin-guide/kernel-parameters.txt >>> +++ b/Documentation/admin-guide/kernel-parameters.txt >>> @@ -4813,6 +4813,14 @@ >>> period to instead use normal non-expedited >>> grace-period processing. >>> >>> + rcupdate.rcu_task_contend_lim= [KNL] >>> + Set the minimum number of callback-queuing-time >>> + lock-contention events per jiffy required to >>> + cause the RCU Tasks flavors to switch to per-CPU >>> + callback queuing. This switching only occurs >>> + when rcupdate.rcu_task_enqueue_lim is set to >>> + the default value of -1. >>> + >>> rcupdate.rcu_task_enqueue_lim= [KNL] >>> Set the number of callback queues to use for the >>> RCU Tasks family of RCU flavors. The default >>> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h >>> index 0cb20e5b3e856..7da3b500fb901 100644 >>> --- a/kernel/rcu/tasks.h >>> +++ b/kernel/rcu/tasks.h >>> @@ -142,6 +142,10 @@ module_param(rcu_task_stall_timeout, int, 0644); >>> static int rcu_task_enqueue_lim __read_mostly = -1; >>> module_param(rcu_task_enqueue_lim, int, 0444); >>> >>> +static bool rcu_task_cb_adjust; >>> +static int rcu_task_contend_lim __read_mostly = 100; >>> +module_param(rcu_task_contend_lim, int, 0444); >>> + >>> /* RCU tasks grace-period state for debugging. */ >>> #define RTGS_INIT 0 >>> #define RTGS_WAIT_WAIT_CBS 1 >>> @@ -207,8 +211,11 @@ static void cblist_init_generic(struct rcu_tasks *rtp) >>> int lim; >>> >>> raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags); >>> - if (rcu_task_enqueue_lim < 0) >>> - rcu_task_enqueue_lim = nr_cpu_ids; >>> + if (rcu_task_enqueue_lim < 0) { >>> + rcu_task_enqueue_lim = 1; >>> + rcu_task_cb_adjust = true; >>> + pr_alert("%s: Setting adjustable number of callback queues.\n", __func__); >>> + } >>> else if (rcu_task_enqueue_lim == 0) >>> rcu_task_enqueue_lim = 1; >>> lim = rcu_task_enqueue_lim; >>> @@ -251,6 +258,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func, >>> { >>> unsigned long flags; >>> unsigned long j; >>> + bool needadjust = false; >>> bool needwake; >>> struct rcu_tasks_percpu *rtpcp; >>> >>> @@ -266,7 +274,9 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func, >>> rtpcp->rtp_jiffies = j; >>> rtpcp->rtp_n_lock_retries = 0; >>> } >>> - rtpcp->rtp_n_lock_retries++; >>> + if (rcu_task_cb_adjust && ++rtpcp->rtp_n_lock_retries > rcu_task_contend_lim && >>> + READ_ONCE(rtp->percpu_enqueue_lim) != nr_cpu_ids) >>> + needadjust = true; // Defer adjustment to avoid deadlock. >>> } >>> if (!rcu_segcblist_is_enabled(&rtpcp->cblist)) { >>> raw_spin_unlock_rcu_node(rtpcp); // irqs remain disabled. >>> @@ -276,6 +286,15 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func, >>> needwake = rcu_segcblist_empty(&rtpcp->cblist); >>> rcu_segcblist_enqueue(&rtpcp->cblist, rhp); >>> raw_spin_unlock_irqrestore_rcu_node(rtpcp, flags); >>> + if (unlikely(needadjust)) { >>> + raw_spin_lock_irqsave(&rtp->cbs_gbl_lock, flags); >>> + if (rtp->percpu_enqueue_lim != nr_cpu_ids) { >>> + WRITE_ONCE(rtp->percpu_enqueue_shift, ilog2(nr_cpu_ids)); >>> + smp_store_release(&rtp->percpu_enqueue_lim, nr_cpu_ids); >>> + pr_alert("Switching %s to per-CPU callback queuing.\n", rtp->name); >>> + } >>> + raw_spin_unlock_irqrestore(&rtp->cbs_gbl_lock, flags); >>> + } >>> /* We can't create the thread unless interrupts are enabled. */ >>> if (needwake && READ_ONCE(rtp->kthread_ptr)) { >>> irq_work_queue(&rtpcp->rtp_irq_work); >>> ------------------------------------------------------------------------------- >>> >>> >>> Git bisection log: >>> >>> ------------------------------------------------------------------------------- >>> git bisect start >>> # good: [8ced7ca3570333998ad2088d5a6275701970e28e] Merge tag 'block-5.16-2021-11-25' of git://git.kernel.dk/linux-block >>> git bisect good 8ced7ca3570333998ad2088d5a6275701970e28e >>> # bad: [f30a24ed97b401416118756fa35fbe5d28f999e3] Add linux-next specific files for 20211126 >>> git bisect bad f30a24ed97b401416118756fa35fbe5d28f999e3 >>> # good: [d7359ce68f20102beeec57e760b746e1f76ab65c] Merge branch 'mtd/next' of git://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git >>> git bisect good d7359ce68f20102beeec57e760b746e1f76ab65c >>> # good: [6eb0c6fb36d50e66ca0b686ec0eebd7eddff811b] Merge branch 'next' of git://git.kernel.org/pub/scm/linux/kernel/git/ulfh/mmc.git >>> git bisect good 6eb0c6fb36d50e66ca0b686ec0eebd7eddff811b >>> # bad: [e62c6fa2d7d055d657c00734314d6abc664d28b5] Merge branch 'staging-next' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git >>> git bisect bad e62c6fa2d7d055d657c00734314d6abc664d28b5 >>> # good: [0ebaaa3b9daaf0341c75a66b8b33399e23e40a8f] Merge branch 'edac-for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/ras/ras.git >>> git bisect good 0ebaaa3b9daaf0341c75a66b8b33399e23e40a8f >>> # bad: [556fc834ce0be2f0dbfd8ae2bfb3840326d12cb9] Merge branch 'for-next' of git://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86.git >>> git bisect bad 556fc834ce0be2f0dbfd8ae2bfb3840326d12cb9 >>> # good: [ad3b4071d577f2779c706a22956b6d167710459b] refscale: Simplify the errexit checkpoint >>> git bisect good ad3b4071d577f2779c706a22956b6d167710459b >>> # good: [24f474cf45326225f613b58b191600311109695b] rcutorture: Cause TREE02 and TREE10 scenarios to do more callback flooding >>> git bisect good 24f474cf45326225f613b58b191600311109695b >>> # bad: [731f2830b96ce040e52b227346087e0dc3a6f5d0] Merge branch 'linux-next' of git://git.kernel.org/pub/scm/linux/kernel/git/xen/tip.git >>> git bisect bad 731f2830b96ce040e52b227346087e0dc3a6f5d0 >>> # bad: [3a528960b4b5d4069e4b066a1d9278862e902abc] Merge branch 'rcu/next' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git >>> git bisect bad 3a528960b4b5d4069e4b066a1d9278862e902abc >>> # bad: [a106e93308b6c60944c9021fc2cad377639584a2] rcu/nocb: Remove rdp from nocb list when de-offloaded >>> git bisect bad a106e93308b6c60944c9021fc2cad377639584a2 >>> # good: [a3d09240cbfd98525dbd7038a4ec1afb4bc896a2] rcu-tasks: Avoid raw-spinlocked wakeups from call_rcu_tasks_generic() >>> git bisect good a3d09240cbfd98525dbd7038a4ec1afb4bc896a2 >>> # bad: [4e83e8074d198b5849926d4a7d921063b24c0250] rcutorture: Test RCU Tasks lock-contention detection >>> git bisect bad 4e83e8074d198b5849926d4a7d921063b24c0250 >>> # bad: [dc9166f1253df20e01b0132ece22efaeb5b0a7e0] rcu-tasks: Use more callback queues if contention encountered >>> git bisect bad dc9166f1253df20e01b0132ece22efaeb5b0a7e0 >>> # first bad commit: [dc9166f1253df20e01b0132ece22efaeb5b0a7e0] rcu-tasks: Use more callback queues if contention encountered >>> ------------------------------------------------------------------------------- >>> >>
