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Date: Fri, 4 Mar 2022 12:08:46 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
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 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.
> 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.
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