[rfc] larger batches for crc32c

Commit Message

Dave Chinner Nov. 4, 2016, 12:12 a.m. UTC

On Fri, Oct 28, 2016 at 03:17:47AM +1100, Nicholas Piggin wrote:
> Hi guys,
> 
> We're seeing crc32c_le show up in xfs log checksumming on a MySQL benchmark
> on powerpc. I could reproduce similar overheads with dbench as well.
> 
> 1.11%  mysqld           [kernel.vmlinux]            [k] __crc32c_le
>         |
>         ---__crc32c_le
>            |          
>             --1.11%--chksum_update
>                       |          
>                        --1.11%--crypto_shash_update
>                                  crc32c
>                                  xlog_cksum
>                                  xlog_sync
>                                  _xfs_log_force_lsn
>                                  xfs_file_fsync
>                                  vfs_fsync_range
>                                  do_fsync
>                                  sys_fsync
>                                  system_call
>                                  0x17738
>                                  0x17704
>                                  os_file_flush_func
>                                  fil_flush
> 
> As a rule, it helps the crc implementation if it can operate on as large a
> chunk as possible (alignment, startup overhead, etc). So I did a quick hack
> at getting XFS checksumming to feed crc32c() with larger chunks, by setting
> the existing crc to 0 before running over the entire buffer. Together with
> some small work on the powerpc crc implementation, crc drops below 0.1%.
> 
> I don't know if something like this would be acceptable? It's not pretty,
> but I didn't see an easier way.

Here's an alternative, slightly cleaner patch that optimises the CRC
update side but leaves the verify side as it is. I've not yet
decided exactly what is cleanest for the xlog_cksum() call in log
recovery, but that won't change the performance of the code.  Can
you give this a run through, Nick?

Cheers,

Dave.

Patch

diff --git a/fs/xfs/libxfs/xfs_cksum.h b/fs/xfs/libxfs/xfs_cksum.h
index fad1676ad8cd..a416c7cb23ea 100644
--- a/fs/xfs/libxfs/xfs_cksum.h
+++ b/fs/xfs/libxfs/xfs_cksum.h
@@ -6,10 +6,11 @@ 
 /*
  * Calculate the intermediate checksum for a buffer that has the CRC field
  * inside it.  The offset of the 32bit crc fields is passed as the
- * cksum_offset parameter.
+ * cksum_offset parameter. We do not modify the buffer during verification,
+ * hence we have to split the CRC calculation across the cksum_offset.
  */
 static inline __uint32_t
-xfs_start_cksum(char *buffer, size_t length, unsigned long cksum_offset)
+xfs_start_cksum_safe(char *buffer, size_t length, unsigned long cksum_offset)
 {
 	__uint32_t zero = 0;
 	__uint32_t crc;
@@ -26,6 +27,20 @@  xfs_start_cksum(char *buffer, size_t length, unsigned long cksum_offset)
 }
 
 /*
+ * Fast CRC method where the buffer is modified. Callers must have exclusive
+ * access to the buffer while the calculation takes place.
+ */
+static inline __uint32_t
+xfs_start_cksum_update(char *buffer, size_t length, unsigned long cksum_offset)
+{
+	/* zero the CRC field */
+	*(__le32 *)(buffer + cksum_offset) = 0;
+
+	/* single pass CRC calculation for the entire buffer */
+	return crc32c(XFS_CRC_SEED, buffer, length);
+}
+
+/*
  * Convert the intermediate checksum to the final ondisk format.
  *
  * The CRC32c calculation uses LE format even on BE machines, but returns the
@@ -40,11 +55,14 @@  xfs_end_cksum(__uint32_t crc)
 
 /*
  * Helper to generate the checksum for a buffer.
+ *
+ * This modifies the buffer temporarily - callers must have exclusive
+ * access to the buffer while the calculation takes place.
  */
 static inline void
 xfs_update_cksum(char *buffer, size_t length, unsigned long cksum_offset)
 {
-	__uint32_t crc = xfs_start_cksum(buffer, length, cksum_offset);
+	__uint32_t crc = xfs_start_cksum_update(buffer, length, cksum_offset);
 
 	*(__le32 *)(buffer + cksum_offset) = xfs_end_cksum(crc);
 }
@@ -55,7 +73,7 @@  xfs_update_cksum(char *buffer, size_t length, unsigned long cksum_offset)
 static inline int
 xfs_verify_cksum(char *buffer, size_t length, unsigned long cksum_offset)
 {
-	__uint32_t crc = xfs_start_cksum(buffer, length, cksum_offset);
+	__uint32_t crc = xfs_start_cksum_safe(buffer, length, cksum_offset);
 
 	return *(__le32 *)(buffer + cksum_offset) == xfs_end_cksum(crc);
 }
diff --git a/fs/xfs/libxfs/xfs_inode_buf.c b/fs/xfs/libxfs/xfs_inode_buf.c
index 134424fac434..d3aa50eb998f 100644
--- a/fs/xfs/libxfs/xfs_inode_buf.c
+++ b/fs/xfs/libxfs/xfs_inode_buf.c
@@ -436,7 +436,7 @@  xfs_dinode_calc_crc(
 		return;
 
 	ASSERT(xfs_sb_version_hascrc(&mp->m_sb));
-	crc = xfs_start_cksum((char *)dip, mp->m_sb.sb_inodesize,
+	crc = xfs_start_cksum_update((char *)dip, mp->m_sb.sb_inodesize,
 			      XFS_DINODE_CRC_OFF);
 	dip->di_crc = xfs_end_cksum(crc);
 }
diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 3b74fa011bb1..3ebe444eb60f 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1668,7 +1668,7 @@  xlog_cksum(
 	__uint32_t		crc;
 
 	/* first generate the crc for the record header ... */
-	crc = xfs_start_cksum((char *)rhead,
+	crc = xfs_start_cksum_update((char *)rhead,
 			      sizeof(struct xlog_rec_header),
 			      offsetof(struct xlog_rec_header, h_crc));
 
diff --git a/fs/xfs/xfs_log_recover.c b/fs/xfs/xfs_log_recover.c
index 9b3d7c76915d..56b7a2f6aaf2 100644
--- a/fs/xfs/xfs_log_recover.c
+++ b/fs/xfs/xfs_log_recover.c
@@ -5113,19 +5113,21 @@  xlog_recover_process(
 	struct list_head	*buffer_list)
 {
 	int			error;
+	__le32			old_crc = rhead->h_crc;
 	__le32			crc;
 
+
 	crc = xlog_cksum(log, rhead, dp, be32_to_cpu(rhead->h_len));
 
 	/*
 	 * Nothing else to do if this is a CRC verification pass. Just return
 	 * if this a record with a non-zero crc. Unfortunately, mkfs always
-	 * sets h_crc to 0 so we must consider this valid even on v5 supers.
+	 * sets old_crc to 0 so we must consider this valid even on v5 supers.
 	 * Otherwise, return EFSBADCRC on failure so the callers up the stack
 	 * know precisely what failed.
 	 */
 	if (pass == XLOG_RECOVER_CRCPASS) {
-		if (rhead->h_crc && crc != rhead->h_crc)
+		if (old_crc && crc != old_crc)
 			return -EFSBADCRC;
 		return 0;
 	}
@@ -5136,11 +5138,11 @@  xlog_recover_process(
 	 * zero CRC check prevents warnings from being emitted when upgrading
 	 * the kernel from one that does not add CRCs by default.
 	 */
-	if (crc != rhead->h_crc) {
-		if (rhead->h_crc || xfs_sb_version_hascrc(&log->l_mp->m_sb)) {
+	if (crc != old_crc) {
+		if (old_crc || xfs_sb_version_hascrc(&log->l_mp->m_sb)) {
 			xfs_alert(log->l_mp,
 		"log record CRC mismatch: found 0x%x, expected 0x%x.",
-					le32_to_cpu(rhead->h_crc),
+					le32_to_cpu(old_crc),
 					le32_to_cpu(crc));
 			xfs_hex_dump(dp, 32);
 		}