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

From: Vlad Zolotarov
Date: Sun Jun 10 2012 - 05:37:06 EST


On Thursday 07 June 2012 17:07:21 Andrew Morton wrote:
> On Mon, 28 May 2012 14:58:42 +0300
>
> Vlad Zolotarov <vlad@xxxxxxxxxxx> wrote:
> > From: Shai Fultheim <shai@xxxxxxxxxxx>
> >
> > bh_cachep is only written to once on initialization, so move it to the
> > __read_mostly section.
> >
> > Signed-off-by: Shai Fultheim <shai@xxxxxxxxxxx>
> > Signed-off-by: Vlad Zolotarov <vlad@xxxxxxxxxxx>
> > ---
> >
> > 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


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