Squashfs: add multi-threaded decompression using percpu variable

Add a multi-threaded decompression implementation which uses
percpu variables.

Using percpu variables has advantages and disadvantages over
implementations which do not use percpu variables.

Advantages:
* the nature of percpu variables ensures decompression is
load-balanced across the multiple cores.
* simplicity.

Disadvantages: it limits decompression to one thread per core.

Signed-off-by: Phillip Lougher <[e-mail]>

fs/squashfs/Kconfig

···
 
 	  If unsure, say N.
 
+choice
+	prompt "Decompressor parallelisation options"
+	depends on SQUASHFS
+	help
+	  Squashfs now supports three parallelisation options for
+	  decompression.  Each one exhibits various trade-offs between
+	  decompression performance and CPU and memory usage.
+
+	  If in doubt, select "Single threaded compression"
+
+config SQUASHFS_DECOMP_SINGLE
+	bool "Single threaded compression"
+	help
+	  Traditionally Squashfs has used single-threaded decompression.
+	  Only one block (data or metadata) can be decompressed at any
+	  one time.  This limits CPU and memory usage to a minimum.
+
+config SQUASHFS_DECOMP_MULTI
+	bool "Use multiple decompressors for parallel I/O"
+	help
+	  By default Squashfs uses a single decompressor but it gives
+	  poor performance on parallel I/O workloads when using multiple CPU
+	  machines due to waiting on decompressor availability.
+
+	  If you have a parallel I/O workload and your system has enough memory,
+	  using this option may improve overall I/O performance.
+
+	  This decompressor implementation uses up to two parallel
+	  decompressors per core.  It dynamically allocates decompressors
+	  on a demand basis.
+
+config SQUASHFS_DECOMP_MULTI_PERCPU
+	bool "Use percpu multiple decompressors for parallel I/O"
+	help
+	  By default Squashfs uses a single decompressor but it gives
+	  poor performance on parallel I/O workloads when using multiple CPU
+	  machines due to waiting on decompressor availability.
+
+	  This decompressor implementation uses a maximum of one
+	  decompressor per core.  It uses percpu variables to ensure
+	  decompression is load-balanced across the cores.
+
+endchoice
+
 config SQUASHFS_XATTR
 	bool "Squashfs XATTR support"
 	depends on SQUASHFS
···
 
 	  LZO is not the standard compression used in Squashfs and so most
 	  file systems will be readable without selecting this option.
-
-	  If unsure, say N.
-
-config SQUASHFS_MULTI_DECOMPRESSOR
-	bool "Use multiple decompressors for handling parallel I/O"
-	depends on SQUASHFS
-	help
-	  By default Squashfs uses a single decompressor but it gives
-	  poor performance on parallel I/O workloads when using multiple CPU
-	  machines due to waiting on decompressor availability.
-
-	  If you have a parallel I/O workload and your system has enough memory,
-	  using this option may improve overall I/O performance.
 
 	  If unsure, say N.
 

fs/squashfs/Makefile

···
 obj-$(CONFIG_SQUASHFS) += squashfs.o
 squashfs-y += block.o cache.o dir.o export.o file.o fragment.o id.o inode.o
 squashfs-y += namei.o super.o symlink.o decompressor.o
-
+squashfs-$(CONFIG_SQUASHFS_DECOMP_SINGLE) += decompressor_single.o
+squashfs-$(CONFIG_SQUASHFS_DECOMP_MULTI) += decompressor_multi.o
+squashfs-$(CONFIG_SQUASHFS_DECOMP_MULTI_PERCPU) += decompressor_multi_percpu.o
 squashfs-$(CONFIG_SQUASHFS_XATTR) += xattr.o xattr_id.o
 squashfs-$(CONFIG_SQUASHFS_LZO) += lzo_wrapper.o
 squashfs-$(CONFIG_SQUASHFS_XZ) += xz_wrapper.o
 squashfs-$(CONFIG_SQUASHFS_ZLIB) += zlib_wrapper.o
-
-ifdef CONFIG_SQUASHFS_MULTI_DECOMPRESSOR
-	squashfs-y		+= decompressor_multi.o
-else
-	squashfs-y		+= decompressor_single.o
-endif

fs/squashfs/decompressor_multi_percpu.c

···
+/*
+ * Copyright (c) 2013
+ * Phillip Lougher <[e-mail]>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2. See
+ * the COPYING file in the top-level directory.
+ */
+
+#include <linux/types.h>
+#include <linux/slab.h>
+#include <linux/percpu.h>
+#include <linux/buffer_head.h>
+
+#include "squashfs_fs.h"
+#include "squashfs_fs_sb.h"
+#include "decompressor.h"
+#include "squashfs.h"
+
+/*
+ * This file implements multi-threaded decompression using percpu
+ * variables, one thread per cpu core.
+ */
+
+struct squashfs_stream {
+	void		*stream;
+};
+
+void *squashfs_decompressor_create(struct squashfs_sb_info *msblk,
+						void *comp_opts)
+{
+	struct squashfs_stream *stream;
+	struct squashfs_stream __percpu *percpu;
+	int err, cpu;
+
+	percpu = alloc_percpu(struct squashfs_stream);
+	if (percpu == NULL)
+		return ERR_PTR(-ENOMEM);
+
+	for_each_possible_cpu(cpu) {
+		stream = per_cpu_ptr(percpu, cpu);
+		stream->stream = msblk->decompressor->init(msblk, comp_opts);
+		if (IS_ERR(stream->stream)) {
+			err = PTR_ERR(stream->stream);
+			goto out;
+		}
+	}
+
+	kfree(comp_opts);
+	return (__force void *) percpu;
+
+out:
+	for_each_possible_cpu(cpu) {
+		stream = per_cpu_ptr(percpu, cpu);
+		if (!IS_ERR_OR_NULL(stream->stream))
+			msblk->decompressor->free(stream->stream);
+	}
+	free_percpu(percpu);
+	return ERR_PTR(err);
+}
+
+void squashfs_decompressor_destroy(struct squashfs_sb_info *msblk)
+{
+	struct squashfs_stream __percpu *percpu =
+			(struct squashfs_stream __percpu *) msblk->stream;
+	struct squashfs_stream *stream;
+	int cpu;
+
+	if (msblk->stream) {
+		for_each_possible_cpu(cpu) {
+			stream = per_cpu_ptr(percpu, cpu);
+			msblk->decompressor->free(stream->stream);
+		}
+		free_percpu(percpu);
+	}
+}
+
+int squashfs_decompress(struct squashfs_sb_info *msblk,
+	void **buffer, struct buffer_head **bh, int b, int offset, int length,
+	int srclength, int pages)
+{
+	struct squashfs_stream __percpu *percpu =
+			(struct squashfs_stream __percpu *) msblk->stream;
+	struct squashfs_stream *stream = get_cpu_ptr(percpu);
+	int res = msblk->decompressor->decompress(msblk, stream->stream, buffer,
+		bh, b, offset, length, srclength, pages);
+	put_cpu_ptr(stream);
+
+	if (res < 0)
+		ERROR("%s decompression failed, data probably corrupt\n",
+			msblk->decompressor->name);
+
+	return res;
+}
+
+int squashfs_max_decompressors(void)
+{
+	return num_possible_cpus();
+}