lkml.org 
[lkml]   [2008]   [Nov]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 6/6] fs: Introduce kern_mount_special() to mount special vfs
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;
}



> if (your flag)
> sod off to special
> spin_lock(&dcache_lock);
> if (atomic_dec_and_test(&dentry->d_count)) {
> spin_unlock(&dcache_lock);
> return;
> }
> the rest as usual
>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2008-11-28 23:41    [W:0.217 / U:0.400 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site