From: Michal Hocko <mhocko@suse.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: "Uladzislau Rezki (Sony)" <urezki@gmail.com>,
LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
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>,
Joel Fernandes <joel@joelfernandes.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Date: Mon, 21 Sep 2020 18:03:18 +0200 [thread overview]
Message-ID: <20200921160318.GO12990@dhcp22.suse.cz> (raw)
In-Reply-To: <20200921154558.GD29330@paulmck-ThinkPad-P72>
On Mon 21-09-20 08:45:58, Paul E. McKenney wrote:
> On Mon, Sep 21, 2020 at 09:47:16AM +0200, Michal Hocko wrote:
> > On Fri 18-09-20 21:48:15, Uladzislau Rezki (Sony) wrote:
> > [...]
> > > Proposal
> > > ========
> > > Introduce a lock-free function that obtain a page from the per-cpu-lists
> > > on current CPU. It returns NULL rather than acquiring any non-raw spinlock.
> >
> > I was not happy about this solution when we have discussed this
> > last time and I have to say I am still not happy. This is exposing
> > an internal allocator optimization and allows a hard to estimate
> > consumption of pcp free pages. IIUC this run on pcp cache can be
> > controled directly from userspace (close(open) loop IIRC) which makes it
> > even bigger no-no.
>
> Yes, I do well remember that you are unhappy with this approach.
> Unfortunately, thus far, there is no solution that makes all developers
> happy. You might be glad to hear that we are also looking into other
> solutions, each of which makes some other developers unhappy. So we
> are at least not picking on you alone. :-/
No worries I do not feel like a whipping boy here. But do expect me to
argue against the approach. I would also appreciate it if there was some
more information on other attempts, why they have failed. E.g. why
pre-allocation is not an option that works well enough in most
reasonable workloads. I would also appreciate some more thoughts why we
need to optimize for heavy abusers of RCU (like close(open) extremes).
> > I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@nanos.tec.linutronix.de
> > that this optimization is not aiming at reasonable workloads. Really, go
> > with pre-allocated buffer and fallback to whatever slow path you have
> > already. Exposing more internals of the allocator is not going to do any
> > good for long term maintainability.
>
> I suggest that you carefully re-read the thread following that email.
I clearly remember Thomas not being particularly happy that you optimize
for a corner case. I do not remember there being a consensus that this
is the right approach. There was some consensus that this is better than
a gfp flag. Still quite bad though if you ask me.
> Given a choice between making users unhappy and making developers
> unhappy, I will side with the users each and every time.
Well, let me rephrase. It is not only about me (as a developer) being
unhappy but also all the side effects this would have for users when
performance of their favorite workload declines for no apparent reason
just because pcp caches are depleted by an unrelated process.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-09-21 16:03 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 [this message]
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
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200921160318.GO12990@dhcp22.suse.cz \
--to=mhocko@suse.com \
--cc=akpm@linux-foundation.org \
--cc=bigeasy@linutronix.de \
--cc=joel@joelfernandes.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=oleksiy.avramchenko@sonymobile.com \
--cc=paulmck@kernel.org \
--cc=peterz@infradead.org \
--cc=rcu@vger.kernel.org \
--cc=tglx@linutronix.de \
--cc=tytso@mit.edu \
--cc=urezki@gmail.com \
--cc=vbabka@suse.cz \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).