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From: Jan Glauber <jglauber@marvell.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	Jan Glauber <jglauber@cavium.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] lockref: Limit number of cmpxchg loop retries
Date: Fri, 7 Jun 2019 07:27:01 +0000
Message-ID: <20190607072652.GA5522@hc> (raw)
In-Reply-To: <20190606102803.GA15499@hc>

On Thu, Jun 06, 2019 at 10:28:12AM +0000, Jan Glauber wrote:
> On Thu, Jun 06, 2019 at 10:41:54AM +0100, Will Deacon wrote:
> > On Thu, Jun 06, 2019 at 08:03:27AM +0000, Jan Glauber wrote:
> > > On Wed, Jun 05, 2019 at 01:16:46PM -0700, Linus Torvalds wrote:
> > > > On Wed, Jun 5, 2019 at 6:49 AM Jan Glauber <jglauber@cavium.com> wrote:
> > > > >
> > > > > Add an upper bound to the loop to force the fallback to spinlocks
> > > > > after some time. A retry value of 100 should not impact any hardware
> > > > > that does not have this issue.
> > > > >
> > > > > With the retry limit the performance of an open-close testcase
> > > > > improved between 60-70% on ThunderX2.
> > > > 
> > > > Btw, did you do any kind of performance analysis across different
> > > > retry limit values?
> > > 
> > > I tried 15/50/100/200/500, results were largely identical up to 100.
> > > For SMT=4 a higher retry value might be better, but unless we can add a
> > > sysctl value 100 looked like a good compromise to me.
> > 
> > Perhaps I'm just getting confused pre-morning-coffee, but I thought the
> > original complaint (and the reason for this patch even existing) was that
> > when many CPUs were hammering the lockref then performance tanked? In which
> > case, increasing the threshold as the number of CPUs increases seems
> > counter-intuitive to me because it suggests that the larger the system,
> > the harder we should try to make the cmpxchg work.
> 
> For SMT=4 the top hit I see is queued_spin_lock_slowpath(). Maybe this is more
> costly with more threads, so trying harder to use lockref-cmpxchg makes
> the microbenchmark faster in that case?

To clarify, with 224 threads & CPUs queued_spin_lock_slowpath is the top hit
even without a retry limit in lockref. This could be unrelated to the lockref
fallback, it looks like it's coming from the spinlock in:
	do_sys_open -> get_unused_fd_flags -> __alloc_fd

--Jan

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Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-29 14:52 [RFC] Disable lockref on arm64 Jan Glauber
2019-05-01 16:01 ` Will Deacon
2019-05-02  8:38   ` Jan Glauber
2019-05-01 16:41 ` Linus Torvalds
2019-05-02  8:27   ` Jan Glauber
2019-05-02 16:12     ` Linus Torvalds
2019-05-02 23:19       ` Jayachandran Chandrasekharan Nair
2019-05-03 19:40         ` Linus Torvalds
2019-05-06  6:13           ` [EXT] " Jayachandran Chandrasekharan Nair
2019-05-06 17:13             ` Linus Torvalds
2019-05-06 18:10             ` Will Deacon
2019-05-18  4:24               ` Jayachandran Chandrasekharan Nair
2019-05-18 10:00                 ` Ard Biesheuvel
2019-05-22 16:04                   ` Will Deacon
2019-06-12  4:10                     ` Jayachandran Chandrasekharan Nair
2019-06-12  9:31                       ` Will Deacon
2019-06-14  7:09                         ` Jayachandran Chandrasekharan Nair
2019-06-14  9:58                           ` Will Deacon
2019-06-14 10:24                             ` Ard Biesheuvel
2019-06-14 10:38                               ` Will Deacon
2019-06-15  4:21                                 ` Kees Cook
2019-06-15  8:47                                   ` Ard Biesheuvel
2019-06-15 13:59                                     ` Kees Cook
2019-06-15 14:18                                       ` Ard Biesheuvel
2019-06-16 21:31                                         ` Kees Cook
2019-06-17 11:33                                           ` Ard Biesheuvel
2019-06-17 17:26                                             ` Will Deacon
2019-06-17 20:07                                               ` Jayachandran Chandrasekharan Nair
2019-06-18  5:41                                               ` Kees Cook
2019-06-13  9:53                       ` Hanjun Guo
2019-06-05 13:48   ` [PATCH] lockref: Limit number of cmpxchg loop retries Jan Glauber
2019-06-05 20:16     ` Linus Torvalds
2019-06-06  8:03       ` Jan Glauber
2019-06-06  9:41         ` Will Deacon
2019-06-06 10:28           ` Jan Glauber
2019-06-07  7:27             ` Jan Glauber [this message]
2019-06-07 20:14               ` Linus Torvalds

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