Re: [patch] voluntary-preempt-2.6.8.1-P9

[Ingo Molnar <mingo@elte.hu>]

* Lee Revell <rlrevell@joe-job.com> wrote:

> >   http://redhat.com/~mingo/voluntary-preempt/voluntary-preempt-2.6.8.1-P9
> 
> Hmm, it seems that those strange ~1ms latencies are back.  This was
> triggered by mounting an NTFS volume:
> 
> http://krustophenia.net/testresults.php?dataset=2.6.8.1-P9#/var/www/2.6.8.1-P9/trace5.txt

this is genuine NTFS overhead.

the 1 msec events you get because the timer fires once every 1 msec, but
this is not irregular. There is a single NTFS function running, which is
profiled nicely by the timer IRQ:

 00010001 0.372ms (+0.372ms): do_IRQ (load_and_init_upcase)
 ...
 00010001 1.374ms (+0.981ms): do_IRQ (load_and_init_upcase)
 ...
 00010001 2.376ms (+0.978ms): do_IRQ (load_and_init_upcase)
 ...
 00000001 2.615ms (+0.216ms): vfree (load_and_init_upcase)

to be able to debug such latencies is one reason why i changed the
do_IRQ() trace-entry to show the interrupted function's name. (it
wouldnt normally, mcount() doesnt reach across IRQ frames.)

load_and_init_upcase() is called by ntfs_fill_super() which is called by
the mount code, which runs under lock_kernel(). It seems NTFS does not
rely on the BKL - could you try the patch below, does it solve the
latency?

	Ingo

--- linux/fs/ntfs/super.c.orig
+++ linux/fs/ntfs/super.c
@@ -2288,6 +2288,8 @@ static int ntfs_fill_super(struct super_
 	vol->fmask = 0177;
 	vol->dmask = 0077;
 
+	unlock_kernel();
+
 	/* Important to get the mount options dealt with now. */
 	if (!parse_options(vol, (char*)opt))
 		goto err_out_now;
@@ -2424,6 +2426,7 @@ static int ntfs_fill_super(struct super_
 		}
 		up(&ntfs_lock);
 		sb->s_export_op = &ntfs_export_ops;
+		lock_kernel();
 		return 0;
 	}
 	ntfs_error(sb, "Failed to allocate root directory.");
@@ -2527,6 +2530,7 @@ iput_tmp_ino_err_out_now:
 	}
 	/* Errors at this stage are irrelevant. */
 err_out_now:
+	lock_kernel();
 	sb->s_fs_info = NULL;
 	kfree(vol);
 	ntfs_debug("Failed, returning -EINVAL.");