RE: [NAK] Re: [PATCH] fs: Optimized fget to improve performance

[David Laight]

From: Al Viro
> Sent: 31 August 2020 04:21
> 
> 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.

The same thing can happen if a hardware interrupt occurs.
Not only the delays for the interrupt itself, but all the softint
processing that happens as well.
That can take a long time - even milliseconds.

	David

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