linux-kernel - Re: [PATCH 1/6] kill-the-BKL/reiserfs: release write lock on fs

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Message-ID: <20090501132825.GE6011@nowhere>
Date:	Fri, 1 May 2009 15:28:25 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Jeff Mahoney <jeffm@...e.com>,
	ReiserFS Development List <reiserfs-devel@...r.kernel.org>,
	Chris Mason <chris.mason@...cle.com>,
	Alexander Beregalov <a.beregalov@...il.com>,
	Alessio Igor Bogani <abogani@...ware.it>,
	Jonathan Corbet <corbet@....net>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 1/6] kill-the-BKL/reiserfs: release write lock on
	fs_changed()

On Fri, May 01, 2009 at 08:31:12AM +0200, Andi Kleen wrote:
> Frederic Weisbecker <fweisbec@...il.com> writes:
> >
> > diff --git a/include/linux/reiserfs_fs.h b/include/linux/reiserfs_fs.h
> > index 6587b4e..397d281 100644
> > --- a/include/linux/reiserfs_fs.h
> > +++ b/include/linux/reiserfs_fs.h
> > @@ -1302,7 +1302,13 @@ static inline loff_t max_reiserfs_offset(struct inode *inode)
> >  #define get_generation(s) atomic_read (&fs_generation(s))
> >  #define FILESYSTEM_CHANGED_TB(tb)  (get_generation((tb)->tb_sb) != (tb)->fs_gen)
> >  #define __fs_changed(gen,s) (gen != get_generation (s))
> > -#define fs_changed(gen,s) ({cond_resched(); __fs_changed(gen, s);})
> > +#define fs_changed(gen,s)		\
> > +({					\
> > +	reiserfs_write_unlock(s);	\
> > +	cond_resched();			\
> > +	reiserfs_write_lock(s);		\
> 
> Did you try writing that 
> 
>     if (need_resched()) {               \
> 	reiserfs_write_unlock(s);	\
> 	cond_resched();			\  (or schedule(), but cond_resched does a loop)
> 	reiserfs_write_lock(s);		\
>     }				
> 
> ? That might give better performance under load because users will be better
> batched and you don't release the lock unnecessarily in the unloaded case.



Good catch!
And I guess this pattern matches most of the cond_resched()
all over the code (the only condition is that we must already hold
the write lock).

I will merge your idea and Ingo's one, write a
reiserfs_cond_resched() to have a helper which
factorizes this pattern.

Thanks.


 
> -Andi
> 
> -- 
> ak@...ux.intel.com -- Speaking for myself only.

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