Messages in this thread | | | Date | Fri, 28 Nov 2008 23:43:18 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH 6/6] fs: Introduce kern_mount_special() to mount special vfs |
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Eric Dumazet a écrit : > Al Viro a écrit : >> On Thu, Nov 27, 2008 at 12:32:59AM +0100, Eric Dumazet wrote: >>> This function arms a flag (MNT_SPECIAL) on the vfs, to avoid >>> refcounting on permanent system vfs. >>> Use this function for sockets, pipes, anonymous fds. >> >> IMO that's pushing it past the point of usefulness; unless you can show >> that this really gives considerable win on pipes et.al. *AND* that it >> doesn't hurt other loads... > > Well, if this is the last cache line that might be shared, then yes, > numbers can talk. > But coming from 10 to 1 instead of 0 is OK I guess > >> >> dput() part: again, I want to see what happens on other loads; it's >> probably >> fine (and win is certainly more than from mntput() change), but... The >> thing is, atomic_dec_and_lock() in there is often done on dentries with >> d_count > 1 and that's fairly cheap (and doesn't involve contention on >> dcache_lock on sane targets). >> >> FWIW, unless there's a really good reason to do alpha >> atomic_dec_and_lock() >> in a special way, I'd try to compare with > >> if (atomic_add_unless(&dentry->d_count, -1, 1)) >> return; > > I dont know, but *reading* d_count before trying to write it is expensive > on modern cpus. Oprofile clearly show that on Intel Core2. > > Then, *testing* the flag before doing the atomic_something() has the same > problem. Or we should put flag in a different cache line. > > I am lazy (time for a sleep here), maybe we are smart here and use a > trick like that already ? > > atomic_t atomic_read_with_write_intent(atomic_t *v) > { > int val = 0; > /* > * No LOCK prefix here, we only give a write intent hint to cpu > */ > asm volatile("xaddl %0, %1" > : "+r" (val), "+m" (v->counter) > : : "memory"); > return val; > }
Forget it, its wrong... I really need to sleep :)
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