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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