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From: Al Viro <>
To: Shaokun Zhang <>
	Yuqi Jin <>,
	kernel test robot <>,
	Will Deacon <>,
	Mark Rutland <>,
	Peter Zijlstra <>,
	Boqun Feng <>
Subject: Re: [NAK] Re: [PATCH] fs: Optimized fget to improve performance
Date: Mon, 31 Aug 2020 04:21:27 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Mon, Aug 31, 2020 at 09:43:31AM +0800, Shaokun Zhang wrote:

> How about this? We try to replace atomic_cmpxchg with atomic_add to improve
> performance. The atomic_add does not check the current f_count value.
> Therefore, the number of online CPUs is reserved to prevent multi-core
> competition.

No.  Really, really - no.  Not unless you can guarantee that process on another
CPU won't lose its timeslice, ending up with more than one increment happening on
the same CPU - done by different processes scheduled there, one after another.

If you have some change of atomic_long_add_unless(), do it there.  And get it
past the arm64 folks.  get_file_rcu() is nothing special in that respect *AND*
it has to cope with any architecture out there.

BTW, keep in mind that there's such thing as a KVM - race windows are much
wider there, since a thread representing a guest CPU might lose its timeslice
whenever the host feels like that.  At which point you get a single instruction
on a guest CPU taking longer than many thousands of instructions on another
CPU of the same guest.

AFAIK, arm64 does support KVM with SMP guests.

  reply	other threads:[~2020-08-31  3:21 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-27 10:19 Shaokun Zhang
2020-08-27 12:30 ` Matthew Wilcox
2020-08-27 13:07   ` David Laight
2020-08-27 14:28 ` [NAK] " Al Viro
2020-08-28 11:04   ` Will Deacon
2020-08-31  1:43   ` Shaokun Zhang
2020-08-31  3:21     ` Al Viro [this message]
2020-09-01  9:29       ` David Laight

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