Re: [PATCH v2] locking/osq_lock: Avoid false sharing in optimistic_spin_node

[Zeng Heng]

在 2023/12/22 20:40, David Laight 写道:
> From: Zeng Heng
> > Sent: 22 December 2023 12:11
> >
> > Using the UnixBench test suite, we clearly find that osq_lock() cause
> > extremely high overheads with perf tool in the File Copy items:
> >
> > Overhead  Shared Object            Symbol
> >    94.25%  [kernel]                 [k] osq_lock
> >     0.74%  [kernel]                 [k] rwsem_spin_on_owner
> >     0.32%  [kernel]                 [k] filemap_get_read_batch
> >
> > In response to this, we conducted an analysis and made some gains:
> >
> > In the prologue of osq_lock(), it set `cpu` member of percpu struct
> > optimistic_spin_node with the local cpu id, after that the value of the
> > percpu struct would never change in fact. Based on that, we can regard
> > the `cpu` member as a constant variable.
> ...
> > @@ -9,7 +11,13 @@
> >   struct optimistic_spin_node {
> >   	struct optimistic_spin_node *next, *prev;
> >   	int locked; /* 1 if lock acquired */
> > -	int cpu; /* encoded CPU # + 1 value */
> > +
> > +	CACHELINE_PADDING(_pad1_);
> > +	/*
> > +	 * Stores an encoded CPU # + 1 value.
> > +	 * Only read by other cpus, so split into different cache lines.
> > +	 */
> > +	int cpu;
> >   };
> Isn't this structure embedded in every mutex and rwsem (etc)?
> So that is a significant bloat especially on systems with
> large cache lines.
>
> Did you try just moving the initialisation of the per-cpu 'node'
> below the first fast-path (uncontended) test in osq_lock()?
>
> OTOH if you really have multiple cpu spinning on the same rwsem
> perhaps the test and/or filemap code are really at fault!
>
> 	David

Hi，

The File Copy items of UnixBench testsuite are using 1 read file and 1 
write file

for file read/write/copy test. In multi-parallel scenario, that would 
lead to high

file lock contention.

That is just a performance test suite and has nothing to do with whether 
the user

program design is correct or not.


B.R.,

Zeng Heng