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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Jan Glauber <jglauber@marvell.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	Jan Glauber <jglauber@cavium.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] lockref: Limit number of cmpxchg loop retries
Date: Fri, 7 Jun 2019 13:14:08 -0700
Message-ID: <CAHk-=wjwgOUP8ihvxcDmP4dr6gjSv-y9sJ4MzXM5NQFe0B7-pg@mail.gmail.com> (raw)
In-Reply-To: <20190607072652.GA5522@hc>

On Fri, Jun 7, 2019 at 12:27 AM Jan Glauber <jglauber@marvell.com> wrote:
>
> 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

At some point I stop worrying about microbenchmarks just because it's
easy to hit some locking paths in them, without it necessarily being
relevant in real loads.

But I'll apply the lockref patch because I think the "limit cmpxchg
loop" is conceptually a valid model, and while I think the "hitting
the same dentry lockref over and over again" is likely also an
artifact of a microbenchmark, I could at least imagine that it happens
with some common dentries (root, cwd) in some situations.

                   Linus

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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
2019-06-07 20:14               ` Linus Torvalds [this message]

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