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From:   Jan Glauber <jglauber@marvell.com>
To:     Will Deacon <will.deacon@arm.com>
CC:     Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Glauber <jglauber@cavium.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        "Jayachandran Chandrasekharan Nair" <jnair@marvell.com>
Subject: Re: [PATCH] lockref: Limit number of cmpxchg loop retries
Thread-Topic: [PATCH] lockref: Limit number of cmpxchg loop retries
Thread-Index: AQHVHFKKV4nY33iz10+fZHwc2kb4SKaPzBsA
Date:   Fri, 7 Jun 2019 07:27:01 +0000
Message-ID: <20190607072652.GA5522@hc>
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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> wr=
ote:
> > > > >
> > > > > 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 hardw=
are
> > > > > that does not have this issue.
> > > > >
> > > > > With the retry limit the performance of an open-close testcase
> > > > > improved between 60-70% on ThunderX2.
> > > >=20
> > > > Btw, did you do any kind of performance analysis across different
> > > > retry limit values?
> > >=20
> > > I tried 15/50/100/200/500, results were largely identical up to 100.
> > > For SMT=3D4 a higher retry value might be better, but unless we can a=
dd a
> > > sysctl value 100 looked like a good compromise to me.
> >=20
> > Perhaps I'm just getting confused pre-morning-coffee, but I thought the
> > original complaint (and the reason for this patch even existing) was th=
at
> > when many CPUs were hammering the lockref then performance tanked? In w=
hich
> > 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.
>=20
> For SMT=3D4 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 hi=
t
even without a retry limit in lockref. This could be unrelated to the lockr=
ef
fallback, it looks like it's coming from the spinlock in:
	do_sys_open -> get_unused_fd_flags -> __alloc_fd

--Jan