From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: Petr Mladek <pmladek@suse.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
"Luis Claudio R. Goncalves" <lgoncalv@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Boqun Feng <boqun.feng@gmail.com>, Ingo Molnar <mingo@redhat.com>,
John Ogness <john.ogness@linutronix.de>,
Mel Gorman <mgorman@techsingularity.net>,
Michal Hocko <mhocko@suse.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Waiman Long <longman@redhat.com>, Will Deacon <will@kernel.org>
Subject: Re: [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin_nested()
Date: Wed, 28 Jun 2023 21:14:16 +0900 [thread overview]
Message-ID: <a1c559b7-335e-5401-d167-301c5b1cd312@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <ZJmkPuqpW-wQAyNz@alley>
On 2023/06/26 23:44, Petr Mladek wrote:
> On Mon 2023-06-26 10:12:54, Sebastian Andrzej Siewior wrote:
>> On 2023-06-24 15:54:12 [+0900], Tetsuo Handa wrote:
>>> Why not to do the same on the end side?
>>>
>>> static inline void do_write_seqcount_end(seqcount_t *s)
>>> {
>>> - seqcount_release(&s->dep_map, _RET_IP_);
>>> do_raw_write_seqcount_end(s);
>>> + seqcount_release(&s->dep_map, _RET_IP_);
>>> }
>>
>> I don't have a compelling argument for doing it. It is probably better
>> to release the lock from lockdep's point of view and then really release
>> it (so it can't be acquired before it is released).
>
> If this is true then we should not change the ordering on the _begin
> side either. I mean that we should call the lockdep code only
> after the lock is taken. Anyway, both sides should be symmetric.
>
> That said, lockdep is about chains of locks and not about timing.
> We must not call lockdep annotation when the lock is still available
> for a nested context. So the ordering is probably important only when
> the lock might be taken from both normal and interrupt context.
>
> Anyway, please do not do this change only because of printk().
> IMHO, the current ordering is more logical and the printk() problem
> should be solved another way.
Then, since [PATCH 1/2] cannot be applied, [PATCH 2/2] is automatically
rejected.
I found
/*
* Locking a pcp requires a PCP lookup followed by a spinlock. To avoid
* a migration causing the wrong PCP to be locked and remote memory being
* potentially allocated, pin the task to the CPU for the lookup+lock.
* preempt_disable is used on !RT because it is faster than migrate_disable.
* migrate_disable is used on RT because otherwise RT spinlock usage is
* interfered with and a high priority task cannot preempt the allocator.
*/
#ifndef CONFIG_PREEMPT_RT
#define pcpu_task_pin() preempt_disable()
#define pcpu_task_unpin() preempt_enable()
#else
#define pcpu_task_pin() migrate_disable()
#define pcpu_task_unpin() migrate_enable()
#endif
in mm/page_alloc.c . Thus, I think that calling migrate_disable() if CONFIG_PREEMPT_RT=y
and calling local_irq_save() if CONFIG_PREEMPT_RT=n (i.e. Alternative 3) will work.
But thinking again, since CONFIG_PREEMPT_RT=y uses special printk() approach where messages
are printed from a dedicated kernel thread, do we need to call printk_deferred_enter() if
CONFIG_PREEMPT_RT=y ? That is, isn't the fix as straightforward as below?
----------------------------------------
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 47421bedc12b..a2a3bfa69a12 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5805,6 +5805,7 @@ static void __build_all_zonelists(void *data)
int nid;
int __maybe_unused cpu;
pg_data_t *self = data;
+#ifndef CONFIG_PREEMPT_RT
unsigned long flags;
/*
@@ -5820,6 +5821,7 @@ static void __build_all_zonelists(void *data)
* calling kmalloc(GFP_ATOMIC | __GFP_NOWARN) with port->lock held.
*/
printk_deferred_enter();
+#endif
write_seqlock(&zonelist_update_seq);
#ifdef CONFIG_NUMA
@@ -5858,8 +5860,10 @@ static void __build_all_zonelists(void *data)
}
write_sequnlock(&zonelist_update_seq);
+#ifndef CONFIG_PREEMPT_RT
printk_deferred_exit();
local_irq_restore(flags);
+#endif
}
static noinline void __init
----------------------------------------
next prev parent reply other threads:[~2023-06-28 12:14 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-23 17:12 [PATCH v2 0/2] seqlock,mm: lockdep annotation + write_seqlock_irqsave() Sebastian Andrzej Siewior
2023-06-23 17:12 ` [PATCH v2 1/2] seqlock: Do the lockdep annotation before locking in do_write_seqcount_begin_nested() Sebastian Andrzej Siewior
2023-06-24 6:54 ` Tetsuo Handa
2023-06-26 8:12 ` Sebastian Andrzej Siewior
2023-06-26 9:25 ` Tetsuo Handa
2023-06-26 10:48 ` Peter Zijlstra
2023-06-26 11:26 ` Tetsuo Handa
2023-06-26 11:35 ` Michal Hocko
2023-06-26 12:27 ` Tetsuo Handa
2023-06-26 13:16 ` Michal Hocko
2023-06-26 12:46 ` Sebastian Andrzej Siewior
2023-06-26 13:13 ` Sebastian Andrzej Siewior
2023-06-26 14:44 ` Petr Mladek
2023-06-28 12:14 ` Tetsuo Handa [this message]
2023-07-27 15:10 ` Sebastian Andrzej Siewior
2023-07-29 5:31 ` Tetsuo Handa
2023-07-29 11:05 ` Tetsuo Handa
2023-07-31 14:25 ` Michal Hocko
2023-08-03 13:18 ` Tetsuo Handa
2023-08-03 14:49 ` Michal Hocko
2023-08-04 13:27 ` Tetsuo Handa
2023-08-07 8:20 ` Michal Hocko
2023-06-26 12:56 ` Mel Gorman
2023-06-23 17:12 ` [PATCH v2 2/2] mm/page_alloc: Use write_seqlock_irqsave() instead write_seqlock() + local_irq_save() Sebastian Andrzej Siewior
2023-06-23 18:17 ` Michal Hocko
2023-06-23 20:15 ` [PATCH v3 " Sebastian Andrzej Siewior
2023-06-26 7:56 ` David Hildenbrand
2023-06-26 13:14 ` Mel Gorman
2023-06-28 13:56 ` Michal Hocko
2023-06-25 2:27 ` [PATCH v2 0/2] seqlock,mm: lockdep annotation + write_seqlock_irqsave() Tetsuo Handa
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