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McKenney" Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Minchan Kim , Matthew Wilcox , Mel Gorman , Nicolas Saenz Julienne , Juri Lelli , Thomas Gleixner , Sebastian Andrzej Siewior Subject: Re: [patch v3] mm: lru_cache_disable: replace work queue synchronization with synchronize_rcu Message-ID: References: <20220303170323.82d8424d214fcb3a32155952@linux-foundation.org> <20220304014930.GJ4285@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220304014930.GJ4285@paulmck-ThinkPad-P17-Gen-1> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: cfmn345i1mm5j6p6wtac99qye34oku7j Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=PT09Qy9T; spf=none (imf10.hostedemail.com: domain of mtosatti@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=mtosatti@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Queue-Id: 694CDC002C X-HE-Tag: 1646406726-218742 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Mar 03, 2022 at 05:49:30PM -0800, Paul E. McKenney wrote: > On Thu, Mar 03, 2022 at 05:03:23PM -0800, Andrew Morton wrote: > > (Question for paulmck below, please) > > > > On Tue, 22 Feb 2022 13:07:35 -0300 Marcelo Tosatti wrote: > > > > > > > > On systems that run FIFO:1 applications that busy loop > > > on isolated CPUs, executing tasks on such CPUs under > > > lower priority is undesired (since that will either > > > hang the system, or cause longer interruption to the > > > FIFO task due to execution of lower priority task > > > with very small sched slices). > > > > > > Commit d479960e44f27e0e52ba31b21740b703c538027c ("mm: disable LRU > > > pagevec during the migration temporarily") relies on > > > queueing work items on all online CPUs to ensure visibility > > > of lru_disable_count. > > > > > > However, its possible to use synchronize_rcu which will provide the same > > > guarantees (see comment this patch modifies on lru_cache_disable). > > > > > > Fixes: > > > > > > [ 1873.243925] INFO: task kworker/u160:0:9 blocked for more than 622 seconds. > > > [ 1873.243927] Tainted: G I --------- --- 5.14.0-31.rt21.31.el9.x86_64 #1 > > > [ 1873.243929] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > > > [ 1873.243929] task:kworker/u160:0 state:D stack: 0 pid: 9 ppid: 2 flags:0x00004000 > > > [ 1873.243932] Workqueue: cpuset_migrate_mm cpuset_migrate_mm_workfn > > > [ 1873.243936] Call Trace: > > > [ 1873.243938] __schedule+0x21b/0x5b0 > > > [ 1873.243941] schedule+0x43/0xe0 > > > [ 1873.243943] schedule_timeout+0x14d/0x190 > > > [ 1873.243946] ? resched_curr+0x20/0xe0 > > > [ 1873.243953] ? __prepare_to_swait+0x4b/0x70 > > > [ 1873.243958] wait_for_completion+0x84/0xe0 > > > [ 1873.243962] __flush_work.isra.0+0x146/0x200 > > > [ 1873.243966] ? flush_workqueue_prep_pwqs+0x130/0x130 > > > [ 1873.243971] __lru_add_drain_all+0x158/0x1f0 > > > [ 1873.243978] do_migrate_pages+0x3d/0x2d0 > > > [ 1873.243985] ? pick_next_task_fair+0x39/0x3b0 > > > [ 1873.243989] ? put_prev_task_fair+0x1e/0x30 > > > [ 1873.243992] ? pick_next_task+0xb30/0xbd0 > > > [ 1873.243995] ? __tick_nohz_task_switch+0x1e/0x70 > > > [ 1873.244000] ? raw_spin_rq_unlock+0x18/0x60 > > > [ 1873.244002] ? finish_task_switch.isra.0+0xc1/0x2d0 > > > [ 1873.244005] ? __switch_to+0x12f/0x510 > > > [ 1873.244013] cpuset_migrate_mm_workfn+0x22/0x40 > > > [ 1873.244016] process_one_work+0x1e0/0x410 > > > [ 1873.244019] worker_thread+0x50/0x3b0 > > > [ 1873.244022] ? process_one_work+0x410/0x410 > > > [ 1873.244024] kthread+0x173/0x190 > > > [ 1873.244027] ? set_kthread_struct+0x40/0x40 > > > [ 1873.244031] ret_from_fork+0x1f/0x30 > > > > > > ... > > > > > > --- a/mm/swap.c > > > +++ b/mm/swap.c > > > @@ -831,8 +831,7 @@ inline void __lru_add_drain_all(bool force_all_cpus) > > > for_each_online_cpu(cpu) { > > > struct work_struct *work = &per_cpu(lru_add_drain_work, cpu); > > > > > > - if (force_all_cpus || > > > - pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) || > > > + if (pagevec_count(&per_cpu(lru_pvecs.lru_add, cpu)) || > > > data_race(pagevec_count(&per_cpu(lru_rotate.pvec, cpu))) || > > > pagevec_count(&per_cpu(lru_pvecs.lru_deactivate_file, cpu)) || > > > pagevec_count(&per_cpu(lru_pvecs.lru_deactivate, cpu)) || > > > > This change appears to be "don't queue work on CPUs which don't have > > any work to do". Correct? This isn't changelogged? Its replaced by synchronize_rcu, and its mentioned in the changelog: "However, its possible to use synchronize_rcu which will provide the same guarantees (see comment this patch modifies on lru_cache_disable)." Will resend -v4 with a more verbose changelog. > > > @@ -876,14 +875,19 @@ atomic_t lru_disable_count = ATOMIC_INIT(0); > > > void lru_cache_disable(void) > > > { > > > atomic_inc(&lru_disable_count); > > > + synchronize_rcu(); > > > #ifdef CONFIG_SMP > > > /* > > > - * lru_add_drain_all in the force mode will schedule draining on > > > - * all online CPUs so any calls of lru_cache_disabled wrapped by > > > - * local_lock or preemption disabled would be ordered by that. > > > - * The atomic operation doesn't need to have stronger ordering > > > - * requirements because that is enforced by the scheduling > > > - * guarantees. > > > + * synchronize_rcu() waits for preemption disabled > > > + * and RCU read side critical sections. > > > + * For the users of lru_disable_count: > > > + * > > > + * preempt_disable, local_irq_disable [bh_lru_lock()] > > > + * rcu_read_lock [rt_spin_lock CONFIG_PREEMPT_RT] > > > + * preempt_disable [local_lock !CONFIG_PREEMPT_RT] > > > + * > > > + * so any calls of lru_cache_disabled wrapped by local_lock or > > > + * preemption disabled would be ordered by that. > > > */ > > > __lru_add_drain_all(true); > > > #else > > > > Does this also work with CONFIG_TINY_RCU? > > > > This seems abusive of synchronize_rcu(). None of this code uses RCU, > > but it so happens that synchronize_rcu() happily provides the desired > > effects. Changes in RCU's happy side-effects might break this. > > Perhaps a formal API function which does whatever-you-want-it-to-do > > would be better. > > I don't claim to understand the full lru_cache_disable() use case, but > since v5.1 synchronize_rcu() is guaranteed to wait on preempt_disable() > regions of code. In contrast, back in the old days, you had to use > synchronize_sched() to wait on preempt_disable() regions, even in > CONFIG_PREEMPT=y kernels. So if the comment is accurate, it is OK. OK, will add an additional comment regarding v5.1. > Just be careful what you backport past v5.1... > > > And... I really don't understand the fix. What is it about > > synchronize_rcu() which guarantees that a work function which is queued > > on CPU N will now get executed even if CPU N is spinning in SCHED_FIFO > > userspace? > > I don't understand this part, either. All CPUs should see lru_disable_count (and therefore not add pages to per-CPU LRU pvecs, otherwise the page migration bug fixed by d479960e44f27e0e52ba31b21740b703c538027c can occur. To do this, the commit above ("mm: disable LRU pagevec during the migration temporarily") relies on queueing work items on all online CPUs to ensure visibility of lru_disable_count: */ +void lru_cache_disable(void) +{ + atomic_inc(&lru_disable_count); +#ifdef CONFIG_SMP + /* + * lru_add_drain_all in the force mode will schedule draining on + * all online CPUs so any calls of lru_cache_disabled wrapped by + * local_lock or preemption disabled would be ordered by that. + * The atomic operation doesn't need to have stronger ordering + * requirements because that is enforeced by the scheduling + * guarantees. + */ + __lru_add_drain_all(true); +#else CPU-0 CPU-1 local_lock(&lru_pvecs.lock); pvec = this_cpu_ptr(&lru_pvecs.lru_deactivate_file); add page to per-CPU LRU pvec if atomic_read(lru_disable_count) != 0 flush per-CPU LRU pvec atomic_inc(lru_disable_count) local_unlock(&lru_pvec.lock) lru_add_drain_all(force_all_cpus=true) However queueing the work items disturbs isolated CPUs. To avoid it, its possible to use synchronize_rcu instead: CPU-0 CPU-1 local_lock(&lru_pvecs.lock); pvec = this_cpu_ptr(&lru_pvecs.lru_deactivate_file); add page to per-CPU LRU pvec if atomic_read(lru_disable_count) != 0 flush per-CPU LRU pvec atomic_inc(lru_disable_count) local_unlock(&lru_pvec.lock) synchronize_rcu() Which will wait for all preemption (or IRQ disabled) sections to complete, therefore ensuring visibilily of lru_disable_count.