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([2601:188:c102:b180:1f8b:71d0:77b1:1f6e]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4ed3536a24bsm15383671cf.35.2025.10.31.12.29.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 31 Oct 2025 12:29:30 -0700 (PDT) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: <3a15d293-43b5-453d-9ba7-9b145aaba492@redhat.com> Date: Fri, 31 Oct 2025 15:29:28 -0400 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 12/33] sched/isolation: Convert housekeeping cpumasks to rcu pointers To: Frederic Weisbecker , Waiman Long Cc: Chen Ridong , LKML , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Andrew Morton , Bjorn Helgaas , Catalin Marinas , Danilo Krummrich , "David S . Miller" , Eric Dumazet , Gabriele Monaco , Greg Kroah-Hartman , Ingo Molnar , Jakub Kicinski , Jens Axboe , Johannes Weiner , Lai Jiangshan , Marco Crivellari , Michal Hocko , Muchun Song , Paolo Abeni , Peter Zijlstra , Phil Auld , "Rafael J . Wysocki" , Roman Gushchin , Shakeel Butt , Simon Horman , Tejun Heo , Thomas Gleixner , Vlastimil Babka , Will Deacon , cgroups@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-block@vger.kernel.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, netdev@vger.kernel.org References: <20251013203146.10162-1-frederic@kernel.org> <20251013203146.10162-13-frederic@kernel.org> <510b0185-51d6-44e6-8c39-dfc4c1721e03@redhat.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 10/31/25 12:17 PM, Frederic Weisbecker wrote: > Le Tue, Oct 21, 2025 at 12:03:05AM -0400, Waiman Long a écrit : >> On 10/20/25 9:46 PM, Chen Ridong wrote: >>> On 2025/10/14 4:31, Frederic Weisbecker wrote: >>>> HK_TYPE_DOMAIN's cpumask will soon be made modifyable by cpuset. >>>> A synchronization mechanism is then needed to synchronize the updates >>>> with the housekeeping cpumask readers. >>>> >>>> Turn the housekeeping cpumasks into RCU pointers. Once a housekeeping >>>> cpumask will be modified, the update side will wait for an RCU grace >>>> period and propagate the change to interested subsystem when deemed >>>> necessary. >>>> >>>> Signed-off-by: Frederic Weisbecker >>>> --- >>>> kernel/sched/isolation.c | 58 +++++++++++++++++++++++++--------------- >>>> kernel/sched/sched.h | 1 + >>>> 2 files changed, 37 insertions(+), 22 deletions(-) >>>> >>>> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c >>>> index 8690fb705089..b46c20b5437f 100644 >>>> --- a/kernel/sched/isolation.c >>>> +++ b/kernel/sched/isolation.c >>>> @@ -21,7 +21,7 @@ DEFINE_STATIC_KEY_FALSE(housekeeping_overridden); >>>> EXPORT_SYMBOL_GPL(housekeeping_overridden); >>>> struct housekeeping { >>>> - cpumask_var_t cpumasks[HK_TYPE_MAX]; >>>> + struct cpumask __rcu *cpumasks[HK_TYPE_MAX]; >>>> unsigned long flags; >>>> }; >>>> @@ -33,17 +33,28 @@ bool housekeeping_enabled(enum hk_type type) >>>> } >>>> EXPORT_SYMBOL_GPL(housekeeping_enabled); >>>> +const struct cpumask *housekeeping_cpumask(enum hk_type type) >>>> +{ >>>> + if (static_branch_unlikely(&housekeeping_overridden)) { >>>> + if (housekeeping.flags & BIT(type)) { >>>> + return rcu_dereference_check(housekeeping.cpumasks[type], 1); >>>> + } >>>> + } >>>> + return cpu_possible_mask; >>>> +} >>>> +EXPORT_SYMBOL_GPL(housekeeping_cpumask); >>>> + >>>> int housekeeping_any_cpu(enum hk_type type) >>>> { >>>> int cpu; >>>> if (static_branch_unlikely(&housekeeping_overridden)) { >>>> if (housekeeping.flags & BIT(type)) { >>>> - cpu = sched_numa_find_closest(housekeeping.cpumasks[type], smp_processor_id()); >>>> + cpu = sched_numa_find_closest(housekeeping_cpumask(type), smp_processor_id()); >>>> if (cpu < nr_cpu_ids) >>>> return cpu; >>>> - cpu = cpumask_any_and_distribute(housekeeping.cpumasks[type], cpu_online_mask); >>>> + cpu = cpumask_any_and_distribute(housekeeping_cpumask(type), cpu_online_mask); >>>> if (likely(cpu < nr_cpu_ids)) >>>> return cpu; >>>> /* >>>> @@ -59,28 +70,18 @@ int housekeeping_any_cpu(enum hk_type type) >>>> } >>>> EXPORT_SYMBOL_GPL(housekeeping_any_cpu); >>>> -const struct cpumask *housekeeping_cpumask(enum hk_type type) >>>> -{ >>>> - if (static_branch_unlikely(&housekeeping_overridden)) >>>> - if (housekeeping.flags & BIT(type)) >>>> - return housekeeping.cpumasks[type]; >>>> - return cpu_possible_mask; >>>> -} >>>> -EXPORT_SYMBOL_GPL(housekeeping_cpumask); >>>> - >>>> void housekeeping_affine(struct task_struct *t, enum hk_type type) >>>> { >>>> if (static_branch_unlikely(&housekeeping_overridden)) >>>> if (housekeeping.flags & BIT(type)) >>>> - set_cpus_allowed_ptr(t, housekeeping.cpumasks[type]); >>>> + set_cpus_allowed_ptr(t, housekeeping_cpumask(type)); >>>> } >>>> EXPORT_SYMBOL_GPL(housekeeping_affine); >>>> bool housekeeping_test_cpu(int cpu, enum hk_type type) >>>> { >>>> - if (static_branch_unlikely(&housekeeping_overridden)) >>>> - if (housekeeping.flags & BIT(type)) >>>> - return cpumask_test_cpu(cpu, housekeeping.cpumasks[type]); >>>> + if (housekeeping.flags & BIT(type)) >>>> + return cpumask_test_cpu(cpu, housekeeping_cpumask(type)); >>>> return true; >>>> } >>>> EXPORT_SYMBOL_GPL(housekeeping_test_cpu); >>>> @@ -96,20 +97,33 @@ void __init housekeeping_init(void) >>>> if (housekeeping.flags & HK_FLAG_KERNEL_NOISE) >>>> sched_tick_offload_init(); >>>> - >>>> + /* >>>> + * Realloc with a proper allocator so that any cpumask update >>>> + * can indifferently free the old version with kfree(). >>>> + */ >>>> for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX) { >>>> + struct cpumask *omask, *nmask = kmalloc(cpumask_size(), GFP_KERNEL); >>>> + >>>> + if (WARN_ON_ONCE(!nmask)) >>>> + return; >>>> + >>>> + omask = rcu_dereference(housekeeping.cpumasks[type]); >>>> + >>>> /* We need at least one CPU to handle housekeeping work */ >>>> - WARN_ON_ONCE(cpumask_empty(housekeeping.cpumasks[type])); >>>> + WARN_ON_ONCE(cpumask_empty(omask)); >>>> + cpumask_copy(nmask, omask); >>>> + RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask); >>>> + memblock_free(omask, cpumask_size()); >>>> } >>>> } >>>> static void __init housekeeping_setup_type(enum hk_type type, >>>> cpumask_var_t housekeeping_staging) >>>> { >>>> + struct cpumask *mask = memblock_alloc_or_panic(cpumask_size(), SMP_CACHE_BYTES); >>>> - alloc_bootmem_cpumask_var(&housekeeping.cpumasks[type]); >>>> - cpumask_copy(housekeeping.cpumasks[type], >>>> - housekeeping_staging); >>>> + cpumask_copy(mask, housekeeping_staging); >>>> + RCU_INIT_POINTER(housekeeping.cpumasks[type], mask); >>>> } >>>> static int __init housekeeping_setup(char *str, unsigned long flags) >>>> @@ -162,7 +176,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags) >>>> for_each_set_bit(type, &iter_flags, HK_TYPE_MAX) { >>>> if (!cpumask_equal(housekeeping_staging, >>>> - housekeeping.cpumasks[type])) { >>>> + housekeeping_cpumask(type))) { >>>> pr_warn("Housekeeping: nohz_full= must match isolcpus=\n"); >>>> goto free_housekeeping_staging; >>>> } >>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >>>> index 1f5d07067f60..0c0ef8999fd6 100644 >>>> --- a/kernel/sched/sched.h >>>> +++ b/kernel/sched/sched.h >>>> @@ -42,6 +42,7 @@ >>>> #include >>>> #include >>>> #include >>>> +#include >>>> #include >>>> #include >>>> #include >>> A warning was detected: >>> >>> ============================= >>> WARNING: suspicious RCU usage >>> 6.17.0-next-20251009-00033-g4444da88969b #808 Not tainted >>> ----------------------------- >>> kernel/sched/isolation.c:60 suspicious rcu_dereference_check() usage! >>> >>> other info that might help us debug this: >>> >>> >>> rcu_scheduler_active = 2, debug_locks = 1 >>> 1 lock held by swapper/0/1: >>> #0: ffff888100600ce0 (&type->i_mutex_dir_key#3){++++}-{4:4}, at: walk_compone >>> >>> stack backtrace: >>> CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-next-20251009-00033-g4 >>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239 >>> Call Trace: >>> >>> dump_stack_lvl+0x68/0xa0 >>> lockdep_rcu_suspicious+0x148/0x1b0 >>> housekeeping_cpumask+0xaa/0xb0 >>> housekeeping_test_cpu+0x25/0x40 >>> find_get_block_common+0x41/0x3e0 >>> bdev_getblk+0x28/0xa0 >>> ext4_getblk+0xba/0x2d0 >>> ext4_bread_batch+0x56/0x170 >>> __ext4_find_entry+0x17c/0x410 >>> ? lock_release+0xc6/0x290 >>> ext4_lookup+0x7a/0x1d0 >>> __lookup_slow+0xf9/0x1b0 >>> walk_component+0xe0/0x150 >>> link_path_walk+0x201/0x3e0 >>> path_openat+0xb1/0xb30 >>> ? stack_depot_save_flags+0x41e/0xa00 >>> do_filp_open+0xbc/0x170 >>> ? _raw_spin_unlock_irqrestore+0x2c/0x50 >>> ? __create_object+0x59/0x80 >>> ? trace_kmem_cache_alloc+0x1d/0xa0 >>> ? vprintk_emit+0x2b2/0x360 >>> do_open_execat+0x56/0x100 >>> alloc_bprm+0x1a/0x200 >>> ? __pfx_kernel_init+0x10/0x10 >>> kernel_execve+0x4b/0x160 >>> kernel_init+0xe5/0x1c0 >>> ret_from_fork+0x185/0x1d0 >>> ? __pfx_kernel_init+0x10/0x10 >>> ret_from_fork_asm+0x1a/0x30 >>> >>> random: crng init done >>> >> It is because bh_lru_install() of fs/buffer.c calls cpu_is_isolated() >> without holding a rcu_read_lock. Will need to add a rcu_read_lock() there. > But this is called within bh_lru_lock() which should have either disabled > IRQs or preemption off. I would expect rcu_dereference_check() to automatically > verify those implied RCU read-side critical sections. > > Let's see, lockdep_assert_in_rcu_reader() checks preemptible(), which is: > > #define preemptible() (preempt_count() == 0 && !irqs_disabled()) > > Ah but if !CONFIG_PREEMPT_COUNT: > > #define preemptible() 0 > > Chen did you have !CONFIG_PREEMPT_COUNT ? > > Probably lockdep_assert_in_rcu_reader() should be fixed accordingly and consider > preemption always disabled whenever !CONFIG_PREEMPT_COUNT. Let me check that... Yes, !CONFIG_PREEMPT_COUNT could be the problem here. I thought it was missing rcu_read_lock(), but I didn't really check the code to ensure that. Cheers, Longman