On Fri, May 01, 2009 at 09:44:16AM -0400, Chris Mason wrote:
> 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)
> }
Nice idea!
Does it means I can also replace the item_moved() calls to __fs_changed()?
They seem to not work the same way, I guess I should provide two different
helpers, depending on the check.
>
> 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.
Again, good catch. I will try that.
I should also check the different other lock_buffer() which
indeed might sleep.
Thanks!
> -chris
>
>
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