change filp_close() to use __fput_sync() ? (Was: [PATCH] task_work: remove fifo ordering guarantee)

[Oleg Nesterov <oleg@redhat.com>]

On 08/29, Eric Dumazet wrote:
>
> On Sat, 2015-08-29 at 14:49 +0200, Oleg Nesterov wrote:
> > On 08/28, Eric Dumazet wrote:
> > >
> > > From: Eric Dumazet <edumazet@google.com>
> > >
> > > In commit f341861fb0b ("task_work: add a scheduling point in
> > > task_work_run()") I fixed a latency problem adding a cond_resched()
> > > call.
> > >
> > > Later, commit ac3d0da8f329 added yet another loop to reverse a list,
> > > bringing back the latency spike :
> > >
> > > I've seen in some cases this loop taking 275 ms, if for example a
> > > process with 2,000,000 files is killed.
> > >
> > > We could add yet another cond_resched() in the reverse loop,
> >
> > Can't we do this?
>
> Well, I stated in the changelog we could. Obviously we can.
>
> Adding 275 ms of pure overhead to perform this list reversal for files
> to be closed is quite unfortunate.

Well, if the first loop takes 275 ms, then probably the next one which
actually does a lot of __fput's takes much, much more time, so perhaps
these 275 ms are not very noticable. Ignoring the latency problem.

But of course, this is not good, I agree. Please see below.

> > Fifo just looks more sane to me.
>
> Well, files are closed in a random order. These are the main user of
> this stuff.

This is the most "heavy" user. But task_works is the generic API.

> Now we also could question why we needed commit
> 4a9d4b024a3102fc083c925c242d98ac27b1c5f6 ("switch fput to task_work_add
> ") since it seems quite an overhead at task exit with 10^6 of files to
> close.

How about the patch below? I didn't try to test it yet, but since
filp_close() does ->flush() I think __fput_sync() should be safe here.

Al, what do you think?

Oleg.


--- x/fs/file_table.c
+++ x/fs/file_table.c
@@ -292,11 +292,8 @@ void fput(struct file *file)
  */
 void __fput_sync(struct file *file)
 {
-	if (atomic_long_dec_and_test(&file->f_count)) {
-		struct task_struct *task = current;
-		BUG_ON(!(task->flags & PF_KTHREAD));
+	if (atomic_long_dec_and_test(&file->f_count))
 		__fput(file);
-	}
 }
 
 EXPORT_SYMBOL(fput);
--- x/fs/open.c
+++ x/fs/open.c
@@ -1074,7 +1074,7 @@ int filp_close(struct file *filp, fl_owner_t id)
 		dnotify_flush(filp, id);
 		locks_remove_posix(filp, id);
 	}
-	fput(filp);
+	__fput_sync(filp);
 	return retval;
 }