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From: Uladzislau Rezki <urezki@gmail.com>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
	dipankar@in.ibm.com, akpm@linux-foundation.org,
	mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
	tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
	dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
	oleg@redhat.com
Subject: Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT
Date: Mon, 6 Jul 2020 21:55:57 +0200
Message-ID: <20200706195557.GA24082@pc636> (raw)
In-Reply-To: <20200706194232.GA233429@google.com>

On Mon, Jul 06, 2020 at 03:42:32PM -0400, Joel Fernandes wrote:
> On Thu, Jul 02, 2020 at 09:45:06PM +0200, Uladzislau Rezki wrote:
> > On Thu, Jul 02, 2020 at 04:12:16PM +0200, Sebastian Andrzej Siewior wrote:
> > > On 2020-06-30 11:35:34 [-0700], Paul E. McKenney wrote:
> > > > > This is not going to work together with the "wait context validator"
> > > > > (CONFIG_PROVE_RAW_LOCK_NESTING). As of -rc3 it should complain about
> > > > > printk() which is why it is still disabled by default.
> > > > 
> > > > Fixing that should be "interesting".  In particular, RCU CPU stall
> > > > warnings rely on the raw spin lock to reduce false positives due
> > > > to race conditions.  Some thought will be required here.
> > > 
> > > I don't get this part. Can you explain/give me an example where to look
> > > at?
> > > 
> > > > > So assume that this is fixed and enabled then on !PREEMPT_RT it will
> > > > > complain that you have a raw_spinlock_t acquired (the one from patch
> > > > > 02/17) and attempt to acquire a spinlock_t in the memory allocator.
> > > > 
> > > > Given that the slab allocator doesn't acquire any locks until it gets
> > > > a fair way in, wouldn't it make sense to allow a "shallow" allocation
> > > > while a raw spinlock is held?  This would require yet another GFP_ flag,
> > > > but that won't make all that much of a difference in the total.  ;-)
> > > 
> > > That would be one way of dealing with. But we could go back to
> > > spinlock_t and keep the memory allocation even for RT as is. I don't see
> > > a downside of this. And we would worry about kfree_rcu() from real
> > > IRQ-off region once we get to it.
> > > 
> 
> Sorry for my late reply as the day job and family demanded a lot last week...
> 
> > Another way of fixing it is just dropping the lock letting the page
> > allocator to do an allocation without our "upper/local" lock. I did a
> > proposal like that once upon a time, so maybe it is a time to highlight
> > it again:
> > <snip>
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 21c2fa5bd8c3..249f10a89bb9 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3278,9 +3278,11 @@ static void kfree_rcu_monitor(struct work_struct *work)
> >  }
> > 
> >  static inline bool
> > -kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> > +kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
> > +       void *ptr, unsigned long *flags)
> >  {
> >         struct kvfree_rcu_bulk_data *bnode;
> > +       struct kfree_rcu_cpu *tmp;
> >         int idx;
> > 
> >         if (unlikely(!krcp->initialized))
> > @@ -3306,6 +3308,9 @@ kvfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, void *ptr)
> >                         if (IS_ENABLED(CONFIG_PREEMPT_RT))
> >                                 return false;
> > 
> > +                       migrate_disable();
> > +                       krc_this_cpu_unlock(krcp, *flags);
> 
> If I remember, the issue here is that migrate_disable is not implemented on a
> non-RT kernel due to issues with starvation.
> 
It is implemented. Please have a look linux/preempt.h for regular kernel.

--
Vlad Rezki

  reply index

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-24 20:12 [PATCH tip/core/rcu 0/17] kfree_rcu updates for v5.9 Paul E. McKenney
2020-06-24 20:12 ` [PATCH tip/core/rcu 01/17] rcu: Fix a kernel-doc warnings for "count" paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 02/17] rcu/tree: Keep kfree_rcu() awake during lock contention paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT paulmck
2020-06-30 16:45   ` Sebastian Andrzej Siewior
2020-06-30 18:35     ` Paul E. McKenney
2020-07-02 14:12       ` Sebastian Andrzej Siewior
2020-07-02 16:48         ` Paul E. McKenney
2020-07-02 20:19           ` Sebastian Andrzej Siewior
2020-07-06 21:06             ` Paul E. McKenney
2020-07-07 17:34               ` Uladzislau Rezki
2020-07-07 18:45                 ` Joel Fernandes
2020-07-08 18:48                   ` Uladzislau Rezki
2020-07-02 19:45         ` Uladzislau Rezki
2020-07-06 19:42           ` Joel Fernandes
2020-07-06 19:55             ` Uladzislau Rezki [this message]
2020-07-06 20:29               ` Joel Fernandes
2020-07-07  9:27                 ` Sebastian Andrzej Siewior
2020-07-15 13:38     ` Uladzislau Rezki
2020-07-15 14:16       ` Sebastian Andrzej Siewior
2020-07-15 14:20         ` Uladzislau Rezki
2020-06-24 20:12 ` [PATCH tip/core/rcu 04/17] rcu/tree: Repeat the monitor if any free channel is busy paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 05/17] rcu/tree: Make debug_objects logic independent of rcu_head paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 06/17] rcu/tree: Simplify KFREE_BULK_MAX_ENTR macro paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 07/17] rcu/tree: Move kfree_rcu_cpu locking/unlocking to separate functions paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 08/17] rcu/tree: Use static initializer for krc.lock paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 09/17] rcu/tree: cache specified number of objects paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 10/17] rcu/tree: Maintain separate array for vmalloc ptrs paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 11/17] rcu/tiny: support vmalloc in tiny-RCU paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 12/17] rcu: Rename *_kfree_callback/*_kfree_rcu_offset/kfree_call_* paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 13/17] mm/list_lru.c: Rename kvfree_rcu() to local variant paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 14/17] rcu: Introduce 2 arg kvfree_rcu() interface paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 15/17] rcu: Support reclaim for head-less object paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 16/17] rcu: Introduce single argument kvfree_rcu() interface paulmck
2020-06-24 20:12 ` [PATCH tip/core/rcu 17/17] lib/test_vmalloc.c: Add test cases for kvfree_rcu() paulmck

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