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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
