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Date: Fri, 4 Mar 2022 08:02:39 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Marcelo Tosatti <mtosatti@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Minchan Kim <minchan@...nel.org>,
Matthew Wilcox <willy@...radead.org>,
Mel Gorman <mgorman@...hsingularity.net>,
Nicolas Saenz Julienne <nsaenzju@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch v3] mm: lru_cache_disable: replace work queue
synchronization with synchronize_rcu
On Fri, Mar 04, 2022 at 12:08:46PM -0300, Marcelo Tosatti wrote:
> 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 <mtosatti@...hat.com> 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.
And if someone does need to backport to a kernel version with old-style
limited synchronize_rcu() semantics, you can do this:
synchronize_rcu_mult(call_rcu_mult, call_rcu_sched);
This will wait concurrently for an RCU and and RCU-sched grace period.
If you want the full-up semantics, you can do this to wait on all
three flavors, also including RCU-bh:
synchronize_rcu_mult(call_rcu_mult, call_rcu_sched, call_rcu_bh);
> > 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.
OK, that does sound plausible. (But please note that I am not familiar
with that code.)
Thanx, Paul
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