alloc_tag: add ioctl to /proc/allocinfo

Patch series "alloc_tag: introduce IOCTL-based filtering for MAP", v8.

Currently, memory allocation profiling data is primarily exposed through
/proc/allocinfo.  While useful for manual inspection, this text-based
interface poses challenges for production monitoring and large-scale
analysis:

1. Userspace must parse large amounts of text to extract specific
fields.
2. To find specific tags, userspace must read the entire dataset,
requiring many context switches and high data copying.
3. The kernel currently aggregates per-CPU counters for every allocation
size, even those the user intends to filter out immediately.

This series introduces a new IOCTL-based binary interface for allocinfo
that supports kernel-side filtering.  By allowing the user to specify a
filter mask, we significantly reduce the work performed in-kernel and the
amount of data transferred to userspace.  The IOCTL mechanism was chosen
for allocinfo to address the per-CPU counter aggregation bottleneck.  A
traditional read() operation must report the total allocation count and
sizes for every code tag in the system.  Doing so requires iterating
across all CPUs to sum their per-CPU counters for thousands of tags, which
introduces substantial runtime overhead.

The IOCTL interface allows userspace to push selective filtering criteria
directly into the kernel before the per-CPU counter aggregation.  The
kernel aggregates per-CPU counters only for a small subset of tags that
match the filter.  This results in significant performance improvement.

Beyond fast filtered retrieval, the IOCTL foundation allows introducing a
context capture mechanism in the future to capture the context for
specific allocations.

Performance measurements were conducted on an Intel Xeon Platinum 8481C
(224 CPUs) with caches dropped before each run.

The IOCTL mechanism shows a ~20x performance improvement for
filtered queries. The kernel avoids the expensive per-CPU counter
aggregation (alloc_tag_read) for any tags that fail the initial string
or location filters.

Scenario 1: Specific File Filtering (arch/x86/events/rapl.c)
1. Traditional (cat /proc/allocinfo | grep): 22ms (sys)
2. IOCTL Interface: 1ms (sys)

Scenario 2: Compound Filtering (Filename + Size)
1. Traditional: (cat ... | grep | awk): 21ms (sys)
2. IOCTL Interface: 1ms (sys)

Scenario 3: Size-Based Filtering (min_size = 1MB)
1. Traditional: (cat ... | awk): 21ms (sys)
2. IOCTL Interface: 14ms (sys)


This patch (of 6):

Add the following ioctl commands for /proc/allocinfo file:

ALLOCINFO_IOC_CONTENT_ID - gets content identifier which can be used to
check whether the file content has changed specifically due to module
load/unload.  Every time a module is loaded / unloaded, the returned value
will be different.  By comparing the identifier value at the beginning and
at the end of the content retrieval operation, users can validate
retrieved information for consistency.

ALLOCINFO_IOC_GET_AT - gets the record at the specified position.  This is
the position of a record in /proc/allocinfo.

ALLOCINFO_IOC_GET_NEXT - gets the record next to the last retrieved one. 
If no records were previously retrieved, returns the first record.

Note, function file and module names often have the same prefixes,
therefore when filtering for them, we compare the last 64 characters to
minimize the chances of name collisions.

Link: https://lore.kernel.org/cover.1783532853.git.abhishekbapat@google.com
Link: https://lore.kernel.org/15596de2607ef13e7c77c6d74763f4ae992ec475.1783532853.git.abhishekbapat@google.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Abhishek Bapat <abhishekbapat@google.com>
Acked-by: Hao Ge <hao.ge@linux.dev>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Sourav Panda <souravpanda@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 65 insertions, 0 deletions

diff --git a/include/uapi/linux/alloc_tag.h b/include/uapi/linux/alloc_tag.h
new file mode 100644
index 000000000000..ee6a023cbaf4
--- /dev/null
+++ b/include/uapi/linux/alloc_tag.h
@@ -0,0 +1,65 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+/*
+ * alloc_tag IOCTL API definition
+ *
+ * Copyright (C) 2026 Google, LLC.  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 version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#ifndef _UAPI_ALLOC_TAG_H
+#define _UAPI_ALLOC_TAG_H
+
+#include <linux/types.h>
+
+/*
+ * Function, file and module names often have the same prefixes, therefore
+ * when filtering by these criteria, we compare the last 64 characters to
+ * minimize the chances of name collisions
+ */
+#define ALLOCINFO_STR_SIZE	64
+
+struct allocinfo_content_id {
+	__u64 id;
+};
+
+struct allocinfo_tag {
+	/* Longer names are trimmed */
+	char modname[ALLOCINFO_STR_SIZE];
+	char function[ALLOCINFO_STR_SIZE];
+	char filename[ALLOCINFO_STR_SIZE];
+	__u64 lineno;
+};
+
+/* The alignment ensures 32-bit compatible interfaces are not broken */
+struct allocinfo_counter {
+	__u64 bytes;
+	__u64 calls;
+	__u8 accurate;
+} __attribute__((aligned(8)));
+
+struct allocinfo_tag_data {
+	struct allocinfo_tag tag;
+	struct allocinfo_counter counter;
+};
+
+struct allocinfo_get_at {
+	__u64 pos;	/* input */
+	struct allocinfo_tag_data data;
+};
+
+#define _ALLOCINFO_IOC_CONTENT_ID	0
+#define _ALLOCINFO_IOC_GET_AT		1
+#define _ALLOCINFO_IOC_GET_NEXT		2
+
+#define ALLOCINFO_IOC_BASE		0xA6
+#define ALLOCINFO_IOC_CONTENT_ID	_IOR(ALLOCINFO_IOC_BASE, _ALLOCINFO_IOC_CONTENT_ID,	\
+					     struct allocinfo_content_id)
+#define ALLOCINFO_IOC_GET_AT		_IOWR(ALLOCINFO_IOC_BASE, _ALLOCINFO_IOC_GET_AT,	\
+					      struct allocinfo_get_at)
+#define ALLOCINFO_IOC_GET_NEXT		_IOR(ALLOCINFO_IOC_BASE, _ALLOCINFO_IOC_GET_NEXT,	\
+					     struct allocinfo_tag_data)
+
+#endif /* _UAPI_ALLOC_TAG_H */