From: peterz@infradead.org
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Michal Hocko <mhocko@suse.com>,
Uladzislau Rezki <urezki@gmail.com>,
LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>,
Matthew Wilcox <willy@infradead.org>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Joel Fernandes <joel@joelfernandes.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag
Date: Fri, 14 Aug 2020 18:19:04 +0200 [thread overview]
Message-ID: <20200814161904.GD2674@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200814141425.GM4295@paulmck-ThinkPad-P72>
On Fri, Aug 14, 2020 at 07:14:25AM -0700, Paul E. McKenney wrote:
> Doing this to kfree_rcu() is the first step. We will also be doing this
> to call_rcu(), which has some long-standing invocations from various
> raw contexts, including hardirq handler.
Most hardirq handler are not raw on RT due to threaded interrupts.
Lockdep knows about this.
> > > 4) As kfree_rcu() can be used from any context except NMI and RT
> > > relies on it we ran into a circular dependency problem.
> >
> > Well, which actual usage sites are under a raw spinlock? Most of the
> > ones I could find are not.
>
> There are some on their way in, but this same optimization will be applied
> to call_rcu(), and there are no shortage of such call sites in that case.
> And these call sites have been around for a very long time.
Luckily there is lockdep to help you find the ones that need converting
to raw_call_rcu() :-)
> > > Clearly the simplest solution but not Pauls favourite and
> > > unfortunately he has a good reason.
> >
> > Which isn't actually stated anywhere I suppose ?
>
> Several times, but why not one more? ;-)
>
> In CONFIG_PREEMPT_NONE=y kernels, which are heavily used, and for which
> the proposed kfree_rcu() and later call_rcu() optimizations are quite
> important, there is no way to tell at runtime whether or you are in
> atomic raw context.
CONFIG_PREEMPT_NONE has nothing what so ever to do with any of this.
> > > > 2. Adding a GFP_ flag.
> > >
> > > Michal does not like the restriction for !RT kernels and tries to
> > > avoid the introduction of a new allocation mode.
> >
> > Like above, I tend to be with Michal on this, just wrap the actual
> > allocation in CONFIG_PREEMPT_RT, the code needs to handle a NULL pointer
> > there anyway.
>
> That disables the optimization in the CONFIG_PREEMPT_NONE=y case,
> where it really is needed.
No, it disables it for CONFIG_PREEMPT_RT.
> I would be OK with either. In CONFIG_PREEMPT_NONE=n kernels, the
> kfree_rcu() code could use preemptible() to determine whether it was safe
> to invoke the allocator. The code in kfree_rcu() might look like this:
>
> mem = NULL;
> if (IS_ENABLED(CONFIG_PREEMPT_NONE) || preemptible())
> mem = __get_free_page(...);
>
> Is your point is that the usual mistakes would then be caught by the
> usual testing on CONFIG_PREEMPT_NONE=n kernels?
mem = NULL;
#if !defined(CONFIG_PREEMPT_RT) && !defined(CONFIG_PROVE_LOCKING)
mem = __get_free_page(...)
#endif
if (!mem)
But I _really_ would much rather have raw_kfree_rcu() and raw_call_rcu()
variants for the few places that actually need it.
> > > These are not seperate of each other as #3 requires #4. The most
> > > horrible solution IMO from a technical POV as it proliferates
> > > inconsistency for no good reaosn.
> > >
> > > Aside of that it'd be solving a problem which does not exist simply
> > > because kfree_rcu() does not depend on it and we really don't want to
> > > set precedence and encourage the (ab)use of this in any way.
> >
> > My preferred solution is 1, if you want to use an allocator, you simply
> > don't get to play under raw_spinlock_t. And from a quick grep, most
> > kfree_rcu() users are not under raw_spinlock_t context.
>
> There is at least one on its way in, but more to the point, we will
> need to apply this same optimization to call_rcu(), which is used in
There is no need, call_rcu() works perfectly fine today, thank you.
You might want to, but that's an entirely different thing.
> raw atomic context, including from hardirq handlers.
Threaded IRQs. There really is very little code that is 'raw' on RT.
next prev parent reply other threads:[~2020-08-14 16:19 UTC|newest]
Thread overview: 111+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-09 20:43 [RFC-PATCH 0/2] __GFP_NO_LOCKS Uladzislau Rezki (Sony)
2020-08-09 20:43 ` [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag Uladzislau Rezki (Sony)
2020-08-10 12:31 ` Michal Hocko
2020-08-10 16:07 ` Uladzislau Rezki
2020-08-10 19:25 ` Michal Hocko
2020-08-11 8:19 ` Michal Hocko
2020-08-11 9:37 ` Uladzislau Rezki
2020-08-11 9:42 ` Uladzislau Rezki
2020-08-11 10:28 ` Michal Hocko
2020-08-11 10:45 ` Uladzislau Rezki
2020-08-11 10:26 ` Michal Hocko
2020-08-11 11:33 ` Uladzislau Rezki
2020-08-11 9:18 ` Uladzislau Rezki
2020-08-11 10:21 ` Michal Hocko
2020-08-11 11:10 ` Uladzislau Rezki
2020-08-11 14:44 ` Thomas Gleixner
2020-08-11 15:22 ` Thomas Gleixner
2020-08-12 11:38 ` Thomas Gleixner
2020-08-12 12:01 ` Uladzislau Rezki
2020-08-13 7:18 ` Michal Hocko
2020-08-11 15:33 ` Paul E. McKenney
2020-08-11 15:43 ` Thomas Gleixner
2020-08-11 15:56 ` Sebastian Andrzej Siewior
2020-08-11 16:02 ` Paul E. McKenney
2020-08-11 16:19 ` Paul E. McKenney
2020-08-11 19:39 ` Thomas Gleixner
2020-08-11 21:09 ` Paul E. McKenney
2020-08-12 0:13 ` Thomas Gleixner
2020-08-12 4:29 ` Paul E. McKenney
2020-08-12 8:32 ` Thomas Gleixner
2020-08-12 13:30 ` Paul E. McKenney
2020-08-13 7:50 ` Michal Hocko
2020-08-13 9:58 ` Uladzislau Rezki
2020-08-13 11:15 ` Michal Hocko
2020-08-13 13:27 ` Thomas Gleixner
2020-08-13 13:45 ` Michal Hocko
2020-08-13 14:32 ` Matthew Wilcox
2020-08-13 16:14 ` Thomas Gleixner
2020-08-13 16:22 ` Matthew Wilcox
2020-08-13 13:22 ` Thomas Gleixner
2020-08-13 13:33 ` Michal Hocko
2020-08-13 14:34 ` Thomas Gleixner
2020-08-13 14:53 ` Michal Hocko
2020-08-13 15:41 ` Paul E. McKenney
2020-08-13 15:54 ` Michal Hocko
2020-08-13 16:04 ` Paul E. McKenney
2020-08-13 16:13 ` Michal Hocko
2020-08-13 16:29 ` Paul E. McKenney
2020-08-13 17:12 ` Michal Hocko
2020-08-13 17:27 ` Paul E. McKenney
2020-08-13 18:31 ` peterz
2020-08-13 19:13 ` Michal Hocko
2020-08-13 16:20 ` Uladzislau Rezki
2020-08-13 16:36 ` Michal Hocko
2020-08-14 11:54 ` Uladzislau Rezki
2020-08-13 17:09 ` Thomas Gleixner
2020-08-13 17:22 ` Michal Hocko
2020-08-14 7:17 ` Michal Hocko
2020-08-14 12:15 ` Uladzislau Rezki
2020-08-14 12:48 ` Michal Hocko
2020-08-14 13:34 ` Paul E. McKenney
2020-08-14 14:06 ` Michal Hocko
2020-08-14 18:01 ` Paul E. McKenney
2020-08-14 23:14 ` Thomas Gleixner
2020-08-14 23:41 ` Paul E. McKenney
2020-08-15 0:43 ` Thomas Gleixner
2020-08-15 3:01 ` Paul E. McKenney
2020-08-15 8:27 ` Peter Zijlstra
2020-08-15 13:03 ` Paul E. McKenney
2020-08-15 8:42 ` Peter Zijlstra
2020-08-15 14:18 ` Paul E. McKenney
2020-08-15 14:23 ` Paul E. McKenney
2020-08-17 8:47 ` Michal Hocko
2020-08-13 18:26 ` peterz
2020-08-13 18:52 ` Paul E. McKenney
2020-08-13 22:06 ` peterz
2020-08-13 23:23 ` Paul E. McKenney
2020-08-13 23:59 ` Thomas Gleixner
2020-08-14 8:30 ` Peter Zijlstra
2020-08-14 10:23 ` peterz
2020-08-14 15:26 ` Paul E. McKenney
2020-08-14 14:14 ` Paul E. McKenney
2020-08-14 16:11 ` Paul E. McKenney
2020-08-14 17:49 ` Peter Zijlstra
2020-08-14 18:02 ` Paul E. McKenney
2020-08-14 19:33 ` Thomas Gleixner
2020-08-14 20:41 ` Paul E. McKenney
2020-08-14 21:52 ` Peter Zijlstra
2020-08-14 23:27 ` Paul E. McKenney
2020-08-14 23:40 ` Thomas Gleixner
2020-08-16 22:56 ` Uladzislau Rezki
2020-08-17 8:28 ` Michal Hocko
2020-08-17 10:36 ` Uladzislau Rezki
2020-08-17 22:28 ` Paul E. McKenney
2020-08-18 7:43 ` Michal Hocko
2020-08-18 13:53 ` Paul E. McKenney
2020-08-18 14:43 ` Thomas Gleixner
2020-08-18 16:13 ` Paul E. McKenney
2020-08-18 16:55 ` Thomas Gleixner
2020-08-18 17:13 ` Paul E. McKenney
2020-08-18 23:26 ` Thomas Gleixner
2020-08-19 23:07 ` Paul E. McKenney
2020-08-18 15:02 ` Michal Hocko
2020-08-18 15:45 ` Uladzislau Rezki
2020-08-18 16:18 ` Paul E. McKenney
2020-08-14 16:19 ` peterz [this message]
2020-08-14 18:15 ` Paul E. McKenney
2020-08-13 13:29 ` Uladzislau Rezki
2020-08-13 13:41 ` Michal Hocko
2020-08-13 14:22 ` Uladzislau Rezki
2020-08-09 20:43 ` [PATCH 2/2] rcu/tree: use " Uladzislau Rezki (Sony)
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