Re: [PATCH 1/6] kill-the-BKL/reiserfs: release write lock onfs_changed()

[Chris Mason]

On Fri, 2009-05-01 at 15:28 +0200, Frederic Weisbecker wrote:
> On Fri, May 01, 2009 at 08:31:12AM +0200, Andi Kleen wrote:
> > Frederic Weisbecker <fweisbec@xxxxxxxxx> 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.

The pattern you'll find goes like this:

lock_kernel()
do some work
do something that might schedule
run fs_changed(), fixup as required.

In your setup it is translating to:

reiserfs_write_lock(s)
do some work
reiserfs_write_unlock(s)

do something that might schedule

reiserfs_write_lock(s)
if (need_resched()) {
    reiserfs_write_unlock(s)
    cond_resched()
    reiserfs_write_lock(s)
}

if (__fs_changed()) fixup as required

You'll also find that item_moved is similar to __fs_changed() but more
fine grained.

One easy optimization is to make an fs_changed_relock()

static inline int fs_changed_relock(gen, s) {
	cond_resched();
	reiserfs_write_lock(s)
	return __fs_changed(gen, s)
}

Another cause of scheduling is going to be reiserfs_prepare_for_journal.
This function gets called before we modify a metadata buffer and it
waits for IO to finish.

Not sure if your patch series already found it, but if you change this:

int reiserfs_prepare_for_journal(struct super_block *sb,
                                 struct buffer_head *bh, int wait)
{
        PROC_INFO_INC(sb, journal.prepare);

        if (!trylock_buffer(bh)) {
                if (!wait)
                        return 0;
                lock_buffer(bh);
        }

Into:

	if (!trylock_buffer(bh)) {
		if (!wait)
			return 0;
		reiserfs_write_unlock(s);
		wait_on_buffer(bh);
		reiserfs_write_lock(s);
		lock_buffer(bh);
	}

You'll catch a big cause of waiting for the disk with the lock held.

-chris


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