From: "Paul E. McKenney" <paulmck@kernel.org>
To: Michal Hocko <mhocko@suse.com>
Cc: Uladzislau Rezki <urezki@gmail.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
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: Tue, 18 Aug 2020 09:18:46 -0700 [thread overview]
Message-ID: <20200818161846.GF27891@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200818150232.GQ28270@dhcp22.suse.cz>
On Tue, Aug 18, 2020 at 05:02:32PM +0200, Michal Hocko wrote:
> On Tue 18-08-20 06:53:27, Paul E. McKenney wrote:
> > On Tue, Aug 18, 2020 at 09:43:44AM +0200, Michal Hocko wrote:
> > > On Mon 17-08-20 15:28:03, Paul E. McKenney wrote:
> > > > On Mon, Aug 17, 2020 at 10:28:49AM +0200, Michal Hocko wrote:
> > > > > On Mon 17-08-20 00:56:55, Uladzislau Rezki wrote:
> > > >
> > > > [ . . . ]
> > > >
> > > > > > wget ftp://vps418301.ovh.net/incoming/1000000_kmalloc_kfree_rcu_proc_percpu_pagelist_fractio_is_8.png
> > > > >
> > > > > 1/8 of the memory in pcp lists is quite large and likely not something
> > > > > used very often.
> > > > >
> > > > > Both these numbers just make me think that a dedicated pool of page
> > > > > pre-allocated for RCU specifically might be a better solution. I still
> > > > > haven't read through that branch of the email thread though so there
> > > > > might be some pretty convincing argments to not do that.
> > > >
> > > > To avoid the problematic corner cases, we would need way more dedicated
> > > > memory than is reasonable, as in well over one hundred pages per CPU.
> > > > Sure, we could choose a smaller number, but then we are failing to defend
> > > > against flooding, even on systems that have more than enough free memory
> > > > to be able to do so. It would be better to live within what is available,
> > > > taking the performance/robustness hit only if there isn't enough.
> > >
> > > Thomas had a good point that it doesn't really make much sense to
> > > optimize for flooders because that just makes them more effective.
> >
> > The point is not to make the flooders go faster, but rather for the
> > system to be robust in the face of flooders. Robust as in harder for
> > a flooder to OOM the system.
>
> Do we see this to be a practical problem? I am really confused because
> the initial argument was revolving around an optimization now you are
> suggesting that this is actually system stability measure. And I fail to
> see how allowing an easy way to deplete pcp caches completely solves
> any of that. Please do realize that if allow that then every user who
> relies on pcp caches will have to take a slow(er) path and that will
> have performance consequences. The pool is a global and a scarce
> resource. That's why I've suggested a dedicated preallocated pool and
> use it instead of draining global pcp caches.
Both the optimization and the robustness are important.
The problem with this thing is that I have to start describing it
somewhere, and I have not yet come up with a description of the whole
thing that isn't TL;DR.
> > And reducing the number of post-grace-period cache misses makes it
> > easier for the callback-invocation-time memory freeing to keep up with
> > the flooder, thus avoiding (or at least delaying) the OOM.
> >
> > > > My current belief is that we need a combination of (1) either the
> > > > GFP_NOLOCK flag or Peter Zijlstra's patch and
> > >
> > > I must have missed the patch?
> >
> > If I am keeping track, this one:
> >
> > https://lore.kernel.org/lkml/20200814215206.GL3982@worktop.programming.kicks-ass.net/
>
> OK, I have certainly noticed that one but didn't react but my response
> would be similar to the dedicated gfp flag. This is less of a hack than
> __GFP_NOLOCK but it still exposes very internal parts of the allocator
> and I find that a quite problematic from the future maintenance of the
> allocator. The risk of an easy depletion of the pcp pool is also there
> of course.
I had to ask. ;-)
Thanx, Paul
next prev parent reply other threads:[~2020-08-18 16:18 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 [this message]
2020-08-14 16:19 ` peterz
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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