On Sun, Sep 01, 2013 at 03:16:24PM -0700, Linus Torvalds wrote: > Does not seem to matter. Still 66% mntput_no_expire, 31% path_init. > And that lg_local_lock() takes 5-6% of CPU, pretty much all of which > is that single xadd instruction that implements the spinlock. > > This is on /tmp, which is tmpfs. But I don't see how any of that could > matter. "mntput()" does an unconditional call to mntput_no_expire(), > and mntput_no_expire() does that br_read_lock() unconditionally too. > > Note that I'm talking about that "cheap" *read* lock being expensive. > It's the local one, not the global one. So it's not what Waiman saw > with the global lock. This is a local per-cpu thing. > > That read-lock is supposed to be very cheap - it's just a per-cpu > spinlock. But it ends up being very expensive for some reason. I'm not > quite sure why - I don't see any lg_global_lock() calls at all, so... > > I wonder if there is some false sharing going on. But I don't see that > either, this is the percpu offset map afaik: > > 000000000000f560 d files_lglock_lock > 000000000000f564 d nr_dentry > 000000000000f568 d last_ino > 000000000000f56c d nr_unused > 000000000000f570 d nr_inodes > 000000000000f574 d vfsmount_lock_lock > 000000000000f580 d bh_accounting > > and I don't see anything there that would get cross-cpu accesses, so > there shouldn't be any cacheline bouncing. That's the whole point of > percpu variables, after all.
Hell knows... Are you sure you don't see br_write_lock() at all? I don't see anything else that would cause cross-cpu traffic with that layout... -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
