Subject: Re: [Bug 106241] New: shutdown(3)/close(3) behaviour is incorrect for sockets in accept(3)

From: Al Viro <[email-masked]>
Date: Wed, 28 Oct 2015 12:35:44 +0000

[Linus and Dave added, Solaris and NetBSD folks dropped from Cc]

On Tue, Oct 27, 2015 at 05:13:56PM -0700, Eric Dumazet wrote:
> On Tue, 2015-10-27 at 23:17 +0000, Al Viro wrote:
> 
> > 	* [Linux-specific aside] our __alloc_fd() can degrade quite badly
> > with some use patterns.  The cacheline pingpong in the bitmap is probably
> > inevitable, unless we accept considerably heavier memory footprint,
> > but we also have a case when alloc_fd() takes O(n) and it's _not_ hard
> > to trigger - close(3);open(...); will have the next open() after that
> > scanning the entire in-use bitmap.  I think I see a way to improve it
> > without slowing the normal case down, but I'll need to experiment a
> > bit before I post patches.  Anybody with examples of real-world loads
> > that make our descriptor allocator to degrade is very welcome to post
> > the reproducers...
> 
> Well, I do have real-world loads, but quite hard to setup in a lab :(
> 
> Note that we also hit the 'struct cred'->usage refcount for every
> open()/close()/sock_alloc(), and simply moving uid/gid out of the first
> cache line really helps, as current_fsuid() and current_fsgid() no
> longer forces a pingpong.
> 
> I moved seldom used fields on the first cache line, so that overall
> memory usage did not change (192 bytes on 64 bit arches)

[snip]

Makes sense, but there's a funny thing about that refcount - the part
coming from ->f_cred is the most frequently changed *and* almost all
places using ->f_cred are just looking at its fields and do not manipulate
its refcount.  The only exception (do_process_acct()) is easy to eliminate
just by storing a separate reference to the current creds of acct(2) caller
and using it instead of looking at ->f_cred.  What's more, the place where we
grab what will be ->f_cred is guaranteed to have a non-f_cred reference *and*
most of the time such a reference is there for dropping ->f_cred (in
file_free()/file_free_rcu()).

With that change in kernel/acct.c done, we could do the following:
	a) split the cred refcount into the normal and percpu parts and
add a spinlock in there.
	b) have put_cred() do this:
		if (atomic_dec_and_test(&cred->usage)) {
			this_cpu_add(&cred->f_cred_usage, 1);
			call_rcu(&cred->rcu, put_f_cred_rcu);
		}
	c) have get_empty_filp() increment current_cred ->f_cred_usage with
this_cpu_add()
	d) have file_free() do
		percpu_counter_dec(&nr_files);
		rcu_read_lock();
		if (likely(atomic_read(&f->f_cred->usage))) {
			this_cpu_add(&f->f_cred->f_cred_usage, -1);
			rcu_read_unlock();
			call_rcu(&f->f_u.fu_rcuhead, file_free_rcu_light);
		} else {
			rcu_read_unlock();
			call_rcu(&f->f_u.fu_rcuhead, file_free_rcu);
		}
file_free_rcu() being
static void file_free_rcu(struct rcu_head *head)
{
        struct file *f = container_of(head, struct file, f_u.fu_rcuhead);
        put_f_cred(&f->f_cred->rcu);
        kmem_cache_free(filp_cachep, f);
}
and file_free_rcu_light() - the same sans put_f_cred();

with put_f_cred() doing
	spin_lock cred->lock
	this_cpu_add(&cred->f_cred_usage, -1);
	find the sum of cred->f_cred_usage
	spin_unlock cred->lock
	if the sum has not reached 0
		return
	current put_cred_rcu(cred)

IOW, let's try to get rid of cross-cpu stores in ->f_cred grabbing and
(most of) ->f_cred dropping.

Note that there are two paths leading to put_f_cred() in the above - via
call_rcu() on &cred->rcu and from file_free_rcu() called via call_rcu() on
&f->f_u.fu_rcuhead.  Both are RCU-delayed and they can happen in parallel -
different rcu_head are used.

atomic_read() check in file_free() might give false positives if it comes
just before put_cred() on another CPU kills the last non-f_cred reference.
It's not a problem, since put_f_cred() from that put_cred() won't be
executed until we drop rcu_read_lock(), so we can safely decrement the
cred->f_cred_usage without cred->lock here (and we are guaranteed that we won't
be dropping the last of that - the same put_cred() would've incremented
->f_cred_usage).

Does anybody see problems with that approach?  I'm going to grab some sleep
(only a couple of hours so far tonight ;-/), will cook an incremental to Eric's
field-reordering patch when I get up...