This memory pool is created when the lttng-tracer module is loaded. It
allocates 4 buffers of 4k on each CPU. These buffers are designed to
allow tracepoint probes to temporarily store data that does not fit on
the stack (during the code_pre and code_post phases). The memory is
freed when the lttng-tracer module is unloaded.

This removes the need for dynamic allocation during the execution of
tracepoint probes, which does not behave well on PREEMPT_RT kernel, even
if the GFP_ATOMIC and GFP_NOWAIT flags are set.

Signed-off-by: Julien Desfossez <jdesfos...@efficios.com>
---
 Makefile                             |   3 +-
 lttng-abi.c                          |   9 ++
 probes/lttng-tracepoint-event-impl.h |   1 +
 tp-mempool.c                         | 175 +++++++++++++++++++++++++++++++++++
 tp-mempool.h                         |  48 ++++++++++
 5 files changed, 235 insertions(+), 1 deletion(-)
 create mode 100644 tp-mempool.c
 create mode 100644 tp-mempool.h

diff --git a/Makefile b/Makefile
index 2cd2df0..78f6661 100644
--- a/Makefile
+++ b/Makefile
@@ -59,7 +59,8 @@ ifneq ($(KERNELRELEASE),)
                        lttng-filter.o lttng-filter-interpreter.o \
                        lttng-filter-specialize.o \
                        lttng-filter-validator.o \
-                       probes/lttng-probe-user.o
+                       probes/lttng-probe-user.o \
+                       tp-mempool.o
 
   ifneq ($(CONFIG_HAVE_SYSCALL_TRACEPOINTS),)
     lttng-tracer-objs += lttng-syscalls.o
diff --git a/lttng-abi.c b/lttng-abi.c
index d202b72..9c29612 100644
--- a/lttng-abi.c
+++ b/lttng-abi.c
@@ -56,6 +56,7 @@
 #include <lttng-abi-old.h>
 #include <lttng-events.h>
 #include <lttng-tracer.h>
+#include <tp-mempool.h>
 #include <lib/ringbuffer/frontend_types.h>
 
 /*
@@ -1771,6 +1772,12 @@ int __init lttng_abi_init(void)
 
        wrapper_vmalloc_sync_all();
        lttng_clock_ref();
+
+       ret = tp_mempool_init();
+       if (ret) {
+               goto error;
+       }
+
        lttng_proc_dentry = proc_create_data("lttng", S_IRUSR | S_IWUSR, NULL,
                                        &lttng_fops, NULL);
 
@@ -1784,6 +1791,7 @@ int __init lttng_abi_init(void)
 
 error:
        lttng_clock_unref();
+       tp_mempool_destroy();
        return ret;
 }
 
@@ -1793,4 +1801,5 @@ void lttng_abi_exit(void)
        lttng_clock_unref();
        if (lttng_proc_dentry)
                remove_proc_entry("lttng", NULL);
+       tp_mempool_destroy();
 }
diff --git a/probes/lttng-tracepoint-event-impl.h 
b/probes/lttng-tracepoint-event-impl.h
index 61f1c2d..2a8fe58 100644
--- a/probes/lttng-tracepoint-event-impl.h
+++ b/probes/lttng-tracepoint-event-impl.h
@@ -34,6 +34,7 @@
 #include <wrapper/rcu.h>
 #include <lttng-events.h>
 #include <lttng-tracer-core.h>
+#include <tp-mempool.h>
 
 #define __LTTNG_NULL_STRING    "(null)"
 
diff --git a/tp-mempool.c b/tp-mempool.c
new file mode 100644
index 0000000..38df26f
--- /dev/null
+++ b/tp-mempool.c
@@ -0,0 +1,175 @@
+/*
+ * tp-mempool.c
+ *
+ * Copyright (C) 2018 Julien Desfossez <jdesfos...@efficios.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; only
+ * version 2.1 of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <linux/slab.h>
+#include <linux/percpu.h>
+
+#include <tp-mempool.h>
+
+#define BUF_SIZE 4 * 1024
+#define NR_BUF_PER_CPU 4
+
+struct tp_buf_entry {
+       int cpu; /* To make sure we return the entry to the right pool. */
+       char buf[BUF_SIZE];
+       struct list_head list;
+};
+
+struct per_cpu_buf {
+       struct list_head free_list; /* Free struct tp_buf_entry. */
+       struct list_head allocated_list; /* Allocated struct tp_buf_entry. */
+};
+
+static struct per_cpu_buf __percpu *pool = NULL; /* Per-cpu buffer. */
+
+int tp_mempool_init(void)
+{
+       int ret, cpu;
+
+       /* The pool is only supposed to be allocated once. */
+       if (pool) {
+               WARN_ON_ONCE(1);
+               ret = -1;
+               goto end;
+       }
+
+       pool = alloc_percpu(struct per_cpu_buf);
+       if (!pool) {
+               ret = -ENOMEM;
+               goto end;
+       }
+
+       for_each_possible_cpu(cpu) {
+               int i;
+               struct per_cpu_buf *cpu_buf = per_cpu_ptr(pool, cpu);
+
+               INIT_LIST_HEAD(&cpu_buf->free_list);
+               INIT_LIST_HEAD(&cpu_buf->allocated_list);
+
+               for (i = 0; i < NR_BUF_PER_CPU; i++) {
+                       struct tp_buf_entry *entry;
+
+                       entry = kzalloc(sizeof(struct tp_buf_entry),
+                                       GFP_KERNEL);
+                       if (!entry) {
+                               ret = -ENOMEM;
+                               goto error_free_pool;
+                       }
+                       entry->cpu = cpu;
+                       list_add_tail(&entry->list, &cpu_buf->free_list);
+               }
+       }
+
+       ret = 0;
+       goto end;
+
+error_free_pool:
+       tp_mempool_destroy();
+end:
+       return ret;
+}
+
+void tp_mempool_destroy(void)
+{
+       int cpu;
+
+       if (!pool) {
+               return;
+       }
+
+       for_each_possible_cpu(cpu) {
+               struct per_cpu_buf *cpu_buf = per_cpu_ptr(pool, cpu);
+               struct tp_buf_entry *entry, *tmp;
+
+               list_for_each_entry_safe(entry, tmp, &cpu_buf->free_list, list) 
{
+                       list_del(&entry->list);
+                       kfree(entry);
+               }
+               if (!list_empty(&cpu_buf->allocated_list)) {
+                       printk(KERN_WARNING "TP allocated memory pool not 
empty");
+                       list_for_each_entry_safe(entry, tmp,
+                                       &cpu_buf->allocated_list, list) {
+                               list_del(&entry->list);
+                               kfree(entry);
+                       }
+               }
+       }
+       free_percpu(pool);
+       pool = NULL;
+}
+
+void *tp_mempool_alloc(size_t size)
+{
+       void *ret;
+       struct tp_buf_entry *entry;
+       struct per_cpu_buf *cpu_buf;
+       int cpu = smp_processor_id();
+
+       if (size > BUF_SIZE) {
+               ret = NULL;
+               goto end;
+       }
+
+       cpu_buf = per_cpu_ptr(pool, cpu);
+       if (list_empty(&cpu_buf->free_list)) {
+               ret = NULL;
+               goto end;
+       }
+
+       entry = list_first_entry(&cpu_buf->free_list, struct tp_buf_entry, 
list);
+       /* Remove the entry from the free list. */
+       list_del(&entry->list);
+       /* Add it to the allocated list for tracking. */
+       list_add_tail(&entry->list, &cpu_buf->allocated_list);
+
+       ret = (void *) entry->buf;
+
+end:
+       return ret;
+}
+
+void tp_mempool_free(void *ptr)
+{
+       struct tp_buf_entry *entry;
+       struct per_cpu_buf *cpu_buf;
+
+       if (!ptr) {
+               goto end;
+       }
+
+       entry = container_of(ptr, struct tp_buf_entry, buf);
+       if (!entry) {
+               goto end;
+       }
+       memset(entry->buf, 0, BUF_SIZE);
+
+       cpu_buf = per_cpu_ptr(pool, entry->cpu);
+       if (!cpu_buf) {
+               goto end;
+       }
+
+       /* Remove the entry from the allocated list. */
+       list_del(&entry->list);
+       /* Add it to the free list. */
+       list_add_tail(&entry->list, &cpu_buf->free_list);
+
+end:
+       return;
+}
diff --git a/tp-mempool.h b/tp-mempool.h
new file mode 100644
index 0000000..93c2239
--- /dev/null
+++ b/tp-mempool.h
@@ -0,0 +1,48 @@
+#ifndef LTTNG_TP_MEMPOOL_H
+#define LTTNG_TP_MEMPOOL_H
+
+/*
+ * tp-mempool.h
+ *
+ * Copyright (C) 2018 Julien Desfossez <jdesfos...@efficios.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; only
+ * version 2.1 of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <linux/percpu.h>
+
+/*
+ * Initialize the pool, only performed once.
+ * Returns 0 on success, a negative value on error.
+ */
+int tp_mempool_init(void);
+
+/*
+ * Destroy the pool and free all the memory allocated.
+ */
+void tp_mempool_destroy(void);
+
+/*
+ * Ask for a buffer on the current cpu.
+ * Return a pointer to a buffer on success, NULL on error.
+ */
+void *tp_mempool_alloc(size_t size);
+
+/*
+ * Release the memory reserved.
+ */
+void tp_mempool_free(void *ptr);
+
+#endif /* LTTNG_TP_MEMPOOL_H */
-- 
2.7.4

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