Messages in this thread | | | From | Schspa Shi <> | Subject | Re: [PATCH] workqueue: Use active mask for new worker when pool is DISASSOCIATED | Date | Sun, 07 Aug 2022 21:52:27 +0800 |
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Lai Jiangshan <jiangshanlai@gmail.com> writes:
> On Mon, Aug 1, 2022 at 1:08 PM Schspa Shi <schspa@gmail.com> wrote: >> >> >> Lai Jiangshan <jiangshanlai@gmail.com> writes: >> >> > On Sat, Jul 30, 2022 at 12:19 PM Schspa Shi <schspa@gmail.com> wrote: >> >> >> >> >> >> Peter Zijlstra <peterz@infradead.org> writes: >> >> >> >> > On Wed, Jul 13, 2022 at 05:52:58PM +0800, Lai Jiangshan wrote: >> >> >> >> >> >> >> >> >> CC Peter. >> >> >> Peter has changed the CPU binding code in workqueue.c. >> >> > >> >> > [ 1622.829091] WARNING: CPU: 3 PID: 31 at kernel/sched/core.c:7756 sched_cpu_dying+0x74/0x204 >> >> > [ 1622.829374] CPU: 3 PID: 31 Comm: migration/3 Tainted: P O 5.10.59-rt52 #2 >> >> > ^^^^^^^^^^^^^^^^^^^^^ >> >> > >> >> > I think we can ignore this as being some ancient kernel. Please try >> >> > something recent. >> >> >> >> Hi peter: >> >> >> >> I spent a few days writing a test case and reproduced the problem on >> >> kernel 5.19. I think it's time for us to review the V3 patch for a fix. >> >> >> >> The V3 patch is at >> >> https://lore.kernel.org/all/20220714031645.28004-1-schspa@gmail.com/ >> >> Please help to review it. >> > >> > Because of the implementation of is_cpu_allowed(), >> > I am still suspicious about how the newly created worker can be woken >> > up in the dying cpu since it has no KTHREAD_IS_PER_CPU set. >> > >> >> I think the comments of the V3 patch have explained the problem, the >> newly created worker runs on the dying CPU because we bind it to the >> dying CPU via kthread_bind_mask or set_cpus_allowed_ptr. >> > > wake_up_process() will modify the task's cpumask and select > a fallback cpu. The schedule code does it for all kthreads that > happen to have a single CPU affinity which is lost. >
There are two exceptions I can think of here: 1. For tasks that are prohibited from migration, the system uses cpu_online_mask, and will not select the fallback cpu. 2. When the scheduling code judges cpu_dying, there is no means of synchronization. At this time, cpu_dying_mask is an unstable variable. The scheduler does not guarantee that the above migration will be possible.
In the case of PREEMPT_RT, spin_lock will prohibit migration, which also causes this problem to be easily reproduced in the RT version. In view of some of the above limitations, I think we should not rely on the implementation of the above scheduler, but handle this when creating threads.
>> Please refer to the following scenarios. >> >> CPU0 CPU1 >> ------------------------------------------------------------------ >> sched_cpu_deactivate(cpu_active_mask clear) >> workqueue_offline_cpu(work pool POOL_DISASSOCIATED) >> -- all worker will migrate to another cpu -- >> worker_thread >> -- will create new worker if >> pool->worklist is not empty >> create_worker() >> -- new kworker will bound to CPU0 >> (pool->attrs->cpumask will be mask of CPU0). >> kworker/0:x will running on rq >> >> sched_cpu_dying >> if (rq->nr_running != 1 || rq_has_pinned_tasks(rq)) >> WARN(true, "Dying CPU not properly vacated!"); >> ---------OOPS------------- >> >> When CPUHP_HRTIMERS_PREPARE < CPU0's state < CPUHP_AP_WORKQUEUE_ONLINE, >> the CPU0 still can schedule a new work, which will add a new work to >> pool->worklist of a dying CPU's work pool. >> >> > Is it a dying cpu when onlining a CPU fails? I think balance_push >> > is not set in this case when it fails at some point during online. >> > But I haven't looked into the details and linked the clues. >> > >> >> Not this case, there is on onlining CPU fails log in my test log. >> >> > >> >> >> >> Test branch as: >> >> https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tag/?h=v5.19-rc8-rt8 >> >> >> > >> > Can it only be triggered in the rt kernel? >> > >> >> I only test on rt kernel, because of I work on RT kernel now, But I >> think non-rt kernel have the issues too. >> >> >> I think this code is new enough to demonstrate that the problem persists. >> >> >> >> The log as fellowing: >> >> >> >> [ 3103.198684] ------------[ cut here ]------------ >> >> [ 3103.198684] Dying CPU not properly vacated! >> >> [ 3103.198684] WARNING: CPU: 1 PID: 23 at kernel/sched/core.c:9575 sched_cpu_dying.cold+0xc/0xc3 >> >> [ 3103.198684] Modules linked in: work_test(O) >> > >> > Could you give me the code of this module? >> >> The module's source code is at >> https://github.com/schspa/code_snippet/blob/master/kernel_module/workqueue_test/work-test.c >> Kernel defconfig is at >> https://github.com/schspa/ktest/blob/main/configs/linux_rt_defconfig >> > > How long will it take to reproduce the bug? > I have tested for a short time on the non-rt kernel, no thing > has happened yet. > >> I add a hook to the entry of worker_thread, which add extra 50ms delay >> to make it reproduced faster. >> >> -- >> BRs >> Schspa Shi
-- BRs Schspa Shi
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