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Use power-of-2 sized buffers to reduce overhead

Now that the ktrace_enter() code is using atomics,
the non-power-of-2 buffer sizes - which require modulus
operations to get the index - are showing up as using
substantial CPU in the profiles.

Force the buffer sizes to be rounded up to the nearest
power of two and use masking rather than modulus operations
to convert the index counter to the buffer index. This
reduces ktrace_enter overhead to 8% of a CPU time, and
again almost halves the trace intensive test runtime.
Merge of xfs-linux-melb:xfs-kern:30538a by kenmcd.

  Use power-of-2 buffer sizes so we can use more efficient
  masking operations to get the buffer index.

/*
 * Copyright (c) 2000-2003,2005 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it would be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
#ifndef __XFS_SUPPORT_KTRACE_H__
#define __XFS_SUPPORT_KTRACE_H__

/*
 * Trace buffer entry structure.
 */
typedef struct ktrace_entry {
	void	*val[16];
} ktrace_entry_t;

/*
 * Trace buffer header structure.
 */
typedef struct ktrace {
	int		kt_nentries;	/* number of entries in trace buf */
	atomic_t	kt_index;	/* current index in entries */
	unsigned int	kt_index_mask;
	int		kt_rollover;
	ktrace_entry_t	*kt_entries;	/* buffer of entries */
} ktrace_t;

/*
 * Trace buffer snapshot structure.
 */
typedef struct ktrace_snap {
	int		ks_start;	/* kt_index at time of snap */
	int		ks_index;	/* current index */
} ktrace_snap_t;


#ifdef CONFIG_XFS_TRACE

extern void ktrace_init(int zentries);
extern void ktrace_uninit(void);

extern ktrace_t *ktrace_alloc(int, unsigned int __nocast);
extern void ktrace_free(ktrace_t *);

extern void ktrace_enter(
	ktrace_t	*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*,
	void		*);

extern ktrace_entry_t   *ktrace_first(ktrace_t *, ktrace_snap_t *);
extern int              ktrace_nentries(ktrace_t *);
extern ktrace_entry_t   *ktrace_next(ktrace_t *, ktrace_snap_t *);
extern ktrace_entry_t   *ktrace_skip(ktrace_t *, int, ktrace_snap_t *);

#else
#define ktrace_init(x)	do { } while (0)
#define ktrace_uninit()	do { } while (0)
#endif	/* CONFIG_XFS_TRACE */

#endif	/* __XFS_SUPPORT_KTRACE_H__ */