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mm: mmap_lock: use local locks instead of disabling preemption

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mmap_lock will explicitly disable/enable preemption upon manipulating its
local CPU variables.  This is to be expected, but in this case, it doesn't
play well with PREEMPT_RT.  The preemption disabled code section also
takes a spin-lock.  Spin-locks in RT systems will try to schedule, which
is exactly what we're trying to avoid.

To mitigate this, convert the explicit preemption handling to local_locks.
Which are RT aware, and will disable migration instead of preemption when
PREEMPT_RT=y.

The faulty call trace looks like the following:
    __mmap_lock_do_trace_*()
      preempt_disable()
      get_mm_memcg_path()
        cgroup_path()
          kernfs_path_from_node()
            spin_lock_irqsave() /* Scheduling while atomic! */

Link: https://lkml.kernel.org/r/20210604163506.2103900-1-nsaenzju@redhat.com
Fixes: 2b5067a814 ("mm: mmap_lock: add tracepoints around lock acquisition ")
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Tested-by: Axel Rasmussen <axelrasmussen@google.com>
Reviewed-by: Axel Rasmussen <axelrasmussen@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nicolas Saenz Julienne 2021-06-28 19:35:13 -07:00 committed by Linus Torvalds
parent 65ac1a60a5
commit 832b507253
1 changed files with 22 additions and 11 deletions
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33
mm/mmap_lock.c
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@ -11,6 +11,7 @@
#include <linux/rcupdate.h>
#include <linux/smp.h>
#include <linux/trace_events.h>
#include <linux/local_lock.h>
EXPORT_TRACEPOINT_SYMBOL(mmap_lock_start_locking);
EXPORT_TRACEPOINT_SYMBOL(mmap_lock_acquire_returned);
@ -39,21 +40,30 @@ static int reg_refcount; /* Protected by reg_lock. */
*/
#define CONTEXT_COUNT 4
static DEFINE_PER_CPU(char __rcu *, memcg_path_buf);
struct memcg_path {
local_lock_t lock;
char __rcu *buf;
local_t buf_idx;
};
static DEFINE_PER_CPU(struct memcg_path, memcg_paths) = {
.lock = INIT_LOCAL_LOCK(lock),
.buf_idx = LOCAL_INIT(0),
};
static char **tmp_bufs;
static DEFINE_PER_CPU(int, memcg_path_buf_idx);
/* Called with reg_lock held. */
static void free_memcg_path_bufs(void)
{
struct memcg_path *memcg_path;
int cpu;
char **old = tmp_bufs;
for_each_possible_cpu(cpu) {
*(old++) = rcu_dereference_protected(
per_cpu(memcg_path_buf, cpu),
memcg_path = per_cpu_ptr(&memcg_paths, cpu);
*(old++) = rcu_dereference_protected(memcg_path->buf,
lockdep_is_held(&reg_lock));
rcu_assign_pointer(per_cpu(memcg_path_buf, cpu), NULL);
rcu_assign_pointer(memcg_path->buf, NULL);
}
/* Wait for inflight memcg_path_buf users to finish. */
@ -88,7 +98,7 @@ int trace_mmap_lock_reg(void)
new = kmalloc(MEMCG_PATH_BUF_SIZE * CONTEXT_COUNT, GFP_KERNEL);
if (new == NULL)
goto out_fail_free;
rcu_assign_pointer(per_cpu(memcg_path_buf, cpu), new);
rcu_assign_pointer(per_cpu_ptr(&memcg_paths, cpu)->buf, new);
/* Don't need to wait for inflights, they'd have gotten NULL. */
}
@ -122,23 +132,24 @@ out:
static inline char *get_memcg_path_buf(void)
{
struct memcg_path *memcg_path = this_cpu_ptr(&memcg_paths);
char *buf;
int idx;
rcu_read_lock();
buf = rcu_dereference(*this_cpu_ptr(&memcg_path_buf));
buf = rcu_dereference(memcg_path->buf);
if (buf == NULL) {
rcu_read_unlock();
return NULL;
}
idx = this_cpu_add_return(memcg_path_buf_idx, MEMCG_PATH_BUF_SIZE) -
idx = local_add_return(MEMCG_PATH_BUF_SIZE, &memcg_path->buf_idx) -
MEMCG_PATH_BUF_SIZE;
return &buf[idx];
}
static inline void put_memcg_path_buf(void)
{
this_cpu_sub(memcg_path_buf_idx, MEMCG_PATH_BUF_SIZE);
local_sub(MEMCG_PATH_BUF_SIZE, &this_cpu_ptr(&memcg_paths)->buf_idx);
rcu_read_unlock();
}
@ -179,14 +190,14 @@ out:
#define TRACE_MMAP_LOCK_EVENT(type, mm, ...) \
do { \
const char *memcg_path; \
preempt_disable(); \
local_lock(&memcg_paths.lock); \
memcg_path = get_mm_memcg_path(mm); \
trace_mmap_lock_##type(mm, \
memcg_path != NULL ? memcg_path : "", \
##__VA_ARGS__); \
if (likely(memcg_path != NULL)) \
put_memcg_path_buf(); \
preempt_enable(); \
local_unlock(&memcg_paths.lock); \
} while (0)
#else /* !CONFIG_MEMCG */