From: Joel Fernandes <joel@joelfernandes.org>
To: Michal Hocko <mhocko@suse.com>
Cc: Uladzislau Rezki <urezki@gmail.com>,
Mel Gorman <mgorman@techsingularity.net>,
"Paul E. McKenney" <paulmck@kernel.org>,
LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Vlastimil Babka <vbabka@suse.cz>,
Thomas Gleixner <tglx@linutronix.de>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>,
Mel Gorman <mgorman@suse.de>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Date: Wed, 30 Sep 2020 13:03:29 -0400 [thread overview]
Message-ID: <CAEXW_YSLhXR=7Asa19v7z2Aj=AqDiehHQLych3B7S+qxgPEWzQ@mail.gmail.com> (raw)
In-Reply-To: <20200930164822.GX2277@dhcp22.suse.cz>
On Wed, Sep 30, 2020 at 12:48 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 30-09-20 11:25:17, Joel Fernandes wrote:
> > On Fri, Sep 25, 2020 at 05:47:41PM +0200, Michal Hocko wrote:
> > > On Fri 25-09-20 17:31:29, Uladzislau Rezki wrote:
> > > > > > > >
> > > > > > > > All good points!
> > > > > > > >
> > > > > > > > On the other hand, duplicating a portion of the allocator functionality
> > > > > > > > within RCU increases the amount of reserved memory, and needlessly most
> > > > > > > > of the time.
> > > > > > > >
> > > > > > >
> > > > > > > But it's very similar to what mempools are for.
> > > > > > >
> > > > > > As for dynamic caching or mempools. It requires extra logic on top of RCU
> > > > > > to move things forward and it might be not efficient way. As a side
> > > > > > effect, maintaining of the bulk arrays in the separate worker thread
> > > > > > will introduce other drawbacks:
> > > > >
> > > > > This is true but it is also true that it is RCU to require this special
> > > > > logic and we can expect that we might need to fine tune this logic
> > > > > depending on the RCU usage. We definitely do not want to tune the
> > > > > generic page allocator for a very specific usecase, do we?
> > > > >
> > > > I look at it in scope of GFP_ATOMIC/GFP_NOWAIT issues, i.e. inability
> > > > to provide a memory service for contexts which are not allowed to
> > > > sleep, RCU is part of them. Both flags used to provide such ability
> > > > before but not anymore.
> > > >
> > > > Do you agree with it?
> > >
> > > Yes this sucks. But this is something that we likely really want to live
> > > with. We have to explicitly _document_ that really atomic contexts in RT
> > > cannot use the allocator. From the past discussions we've had this is
> > > likely the most reasonable way forward because we do not really want to
> > > encourage anybody to do something like that and there should be ways
> > > around that. The same is btw. true also for !RT. The allocator is not
> > > NMI safe and while we should be able to make it compatible I am not
> > > convinced we really want to.
> > >
> > > Would something like this be helpful wrt documentation?
> > >
> > > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > > index 67a0774e080b..9fcd47606493 100644
> > > --- a/include/linux/gfp.h
> > > +++ b/include/linux/gfp.h
> > > @@ -238,7 +238,9 @@ struct vm_area_struct;
> > > * %__GFP_FOO flags as necessary.
> > > *
> > > * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
> > > - * watermark is applied to allow access to "atomic reserves"
> > > + * watermark is applied to allow access to "atomic reserves".
> > > + * The current implementation doesn't support NMI and other non-preemptive context
> > > + * (e.g. raw_spin_lock).
> >
> > I think documenting is useful.
> >
> > Could it be more explicit in what the issue is? Something like:
> >
> > * Even with GFP_ATOMIC, calls to the allocator can sleep on PREEMPT_RT
> > systems. Therefore, the current low-level allocator implementation does not
> > support being called from special contexts that are atomic on RT - such as
> > NMI and raw_spin_lock. Due to these constraints and considering calling code
> > usually has no control over the PREEMPT_RT configuration, callers of the
> > allocator should avoid calling the allocator from these cotnexts even in
> > non-RT systems.
>
> I do not mind documenting RT specific behavior but as mentioned in other
> reply, this should likely go via RT tree for now. There is likely more
> to clarify about atomicity for PREEMPT_RT.
I am sorry, I did not understand what you meant by something missing
in Linus Tree. CONFIG_PREEMPT_RT is there.
Could you clarify? Also the CONFIG_PREEMPT_RT is the only thing
driving this requirement for GFP_ATOMIC right? Or are there non-RT
reasons why GFP_ATOMIC allocation cannot be done from
NMI/raw_spin_lock ?
Thanks,
- Joel
next prev parent reply other threads:[~2020-09-30 17:04 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-18 19:48 [PATCH 0/4] kvfree_rcu() and _LOCK_NESTING/_PREEMPT_RT Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [PATCH 1/4] rcu/tree: Add a work to allocate pages from regular context Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func Uladzislau Rezki (Sony)
2020-09-21 7:47 ` Michal Hocko
2020-09-21 15:45 ` Paul E. McKenney
2020-09-21 16:03 ` Michal Hocko
2020-09-21 19:48 ` Uladzislau Rezki
2020-09-22 7:50 ` Michal Hocko
2020-09-22 13:12 ` Uladzislau Rezki
2020-09-22 15:35 ` Michal Hocko
2020-09-23 10:37 ` Mel Gorman
2020-09-23 15:41 ` Paul E. McKenney
2020-09-23 23:22 ` Mel Gorman
2020-09-24 8:16 ` Uladzislau Rezki
2020-09-24 11:16 ` Peter Zijlstra
2020-09-24 15:16 ` Uladzislau Rezki
2020-09-24 11:19 ` Peter Zijlstra
2020-09-24 15:21 ` Uladzislau Rezki
2020-09-25 8:15 ` Peter Zijlstra
2020-09-25 10:25 ` Uladzislau Rezki
2020-09-24 15:38 ` Paul E. McKenney
2020-09-25 8:26 ` Peter Zijlstra
2020-09-26 14:37 ` Paul E. McKenney
2020-09-25 8:05 ` Michal Hocko
2020-09-25 15:31 ` Uladzislau Rezki
2020-09-25 15:47 ` Michal Hocko
2020-09-29 16:25 ` Uladzislau Rezki
2020-09-30 9:27 ` Michal Hocko
2020-09-30 12:35 ` Uladzislau Rezki
2020-09-30 12:44 ` Michal Hocko
2020-09-30 13:39 ` Uladzislau Rezki
2020-09-30 16:46 ` Michal Hocko
2020-09-30 20:36 ` Uladzislau Rezki
2020-09-30 15:25 ` Joel Fernandes
2020-09-30 16:48 ` Michal Hocko
2020-09-30 17:03 ` Joel Fernandes [this message]
2020-09-30 17:22 ` Michal Hocko
2020-09-30 17:48 ` Joel Fernandes
2020-09-25 16:17 ` Mel Gorman
2020-09-25 17:57 ` Uladzislau Rezki
2020-09-22 15:49 ` Paul E. McKenney
2020-09-22 3:35 ` Paul E. McKenney
2020-09-22 8:03 ` Michal Hocko
2020-09-22 15:46 ` Paul E. McKenney
2020-09-23 11:27 ` Uladzislau Rezki
2020-09-29 10:15 ` Vlastimil Babka
2020-09-29 22:07 ` Uladzislau Rezki
2020-09-30 10:35 ` Michal Hocko
2020-10-01 19:32 ` Uladzislau Rezki
2020-09-30 14:39 ` Vlastimil Babka
2020-09-30 15:37 ` Joel Fernandes
2020-10-01 19:26 ` Uladzislau Rezki
2020-10-02 7:11 ` Michal Hocko
2020-10-02 8:50 ` Mel Gorman
2020-10-02 9:05 ` Michal Hocko
2020-10-05 15:08 ` Uladzislau Rezki
2020-10-05 15:41 ` Michal Hocko
2020-10-06 22:25 ` Uladzislau Rezki
2020-10-07 10:02 ` Michal Hocko
2020-10-07 11:02 ` Uladzislau Rezki
2020-10-02 9:07 ` Peter Zijlstra
2020-10-02 9:45 ` Mel Gorman
2020-10-02 9:58 ` Peter Zijlstra
2020-10-02 10:19 ` Mel Gorman
2020-10-02 14:41 ` Paul E. McKenney
2020-10-06 10:03 ` Mel Gorman
2020-10-06 15:41 ` Paul E. McKenney
2020-10-05 13:58 ` Uladzislau Rezki
2020-10-02 8:06 ` Mel Gorman
2020-10-05 14:12 ` Uladzislau Rezki
2020-09-18 19:48 ` [PATCH 3/4] rcu/tree: use " Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [PATCH 4/4] rcu/tree: Use schedule_delayed_work() instead of WQ_HIGHPRI queue Uladzislau Rezki (Sony)
2020-09-20 15:06 ` Paul E. McKenney
2020-09-21 13:27 ` Uladzislau Rezki
2020-09-18 22:15 ` [PATCH 0/4] kvfree_rcu() and _LOCK_NESTING/_PREEMPT_RT Paul E. McKenney
2020-09-30 15:52 ` Joel Fernandes
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