Re: [PATCH RESEND] fs: Move bh_cachep to the __read_mostly section

[Vlad Zolotarov <vlad@scalemp.com>]

On Thursday 07 June 2012 17:07:21 Andrew Morton wrote:
> On Mon, 28 May 2012 14:58:42 +0300
> 
> Vlad Zolotarov <vlad@scalemp.com> wrote:
> > From: Shai Fultheim <shai@scalemp.com>
> > 
> > bh_cachep is only written to once on initialization, so move it to the
> > __read_mostly section.
> > 
> > Signed-off-by: Shai Fultheim <shai@scalemp.com>
> > Signed-off-by: Vlad Zolotarov <vlad@scalemp.com>
> > ---
> > 
> >  fs/buffer.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index ad5938c..838a9cf 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -3152,7 +3152,7 @@ SYSCALL_DEFINE2(bdflush, int, func, long, data)
> > 
> >  /*
> >  
> >   * Buffer-head allocation
> >   */
> > 
> > -static struct kmem_cache *bh_cachep;
> > +static struct kmem_cache *bh_cachep __read_mostly;
> 
> hm, I thought I replied to this earlier, but I can't find that email.
> 
> Yes, bh_cachep is read-mostly.  In fact it's write-once.  But the same
> is true of all kmem_cache*'s.  I don't see any particular reason for
> singling out bh_cachep.
> 
> 
> Alas, I don't see a smart way of addressing this.  It's either a
> patchset which adds __read_mostly to all kmem_cache*'s, or a patchset
> which converts all the definitions to use some nasty macro which
> inserts the __read_mostly.

Well, it may be done. However my ability to properly check it is limited as I 
have only a certain number of systems to check it on. I can create the patch, 
test it in our labs and post it on this mailing list with request to test it 
on other platforms (like non-x86 platforms). However we may also hit the 
problem u describe below if do so...

> 
> And I still have theoretical concerns with __read_mostly.  As we
> further sort the storage into read-mostly and write-often sections, the
> density of writes in the write-mostly section increases.  IOW, removing
> the read-mostly padding *increase* cross-CPU traffic in the write-often
> scction.  IOW2, leaving the read-mostly stuff where it is provides
> beneficial padding to the write-often fields.  I don't think it has
> been shown that there will be net gains.

Great explanation! The above actually nicely concludes (maybe u haven't 
actually meant that ;)) why defining write-mostly section(s) is pointless. ;)

This is a main topic of this (http://markmail.org/thread/wl4lnjluroqxgabf) 
thread between me and Ingo.

However there is a clear motivation to define a read-mostly section(s) just 
the same way there is a motivation to put constants separately from non-
constant variables which I don't think anybody argues about. ;)

On the other hand, generally speaking, if we "complete the task" and put ALL 
read-mostly variables into a separate section all the variables that would be 
left will actually represent the write-mostly section, which we would prefer 
not to have (according to u). Yet we are still far from it today... ;)

Unfortunately, we can't consider all types of bad C-code then we define 
something like "const" or "__read_mostly". We do our best. And if someone 
haven't defined a per-CPU write-mostly variable in order to prevent heavy 
cross-CPU traffic in his/her code (like in your example above) we can only try 
to fix this code. But I don't think that the existence of such code shell 
imply that the whole idea of __read_mostly section is actually bad or useless. 
It's this bad C-code that should be fixed and IMHO padding the variables with 
constants is not the proper way to fix it...

That's why I think it could be dangerous to go ahead and patch all variables 
of a certain sort (like kmem_cache*'s) with __read_mostly as we may mess the 
things up in some places (like in your example above) where there should be 
done a deeper code analysis than just pattern matching. 

So, getting back to the first section of my reply, do u still think we want to 
patch all kmem_cache*'s with __read_mostly this time or we would prefer this 
to be done incrementally in order to have better regression-ability?

Pls., comment.

thanks in advance,
vlad