perf/core: Implement fast breakpoint modification via _IOC_MODIFY_ATTRIBUTES

Problem and motivation: Once a breakpoint perf event (PERF_TYPE_BREAKPOINT)
is created, there is no flexibility to change the breakpoint type
(bp_type), breakpoint address (bp_addr), or breakpoint length (bp_len). The
only option is to close the perf event and configure a new breakpoint
event. This inflexibility has a significant performance overhead. For
example, sampling-based, lightweight performance profilers (and also
concurrency bug detection tools),  monitor different addresses for a short
duration using PERF_TYPE_BREAKPOINT and change the address (bp_addr) to
another address or change the kind of breakpoint (bp_type) from  "write" to
a "read" or vice-versa or change the length (bp_len) of the address being
monitored. The cost of these modifications is prohibitive since it involves
unmapping the circular buffer associated with the perf event, closing the
perf event, opening another perf event and mmaping another circular buffer.

Solution: The new ioctl flag for perf events,
PERF_EVENT_IOC_MODIFY_ATTRIBUTES, introduced in this patch takes a pointer
to a struct perf_event_attr as an argument to update an old breakpoint
event with new address, type, and size. This facility allows retaining a
previous mmaped perf events ring buffer and avoids having to close and
reopen another perf event.

This patch supports only changing PERF_TYPE_BREAKPOINT event type; future
implementations can extend this feature. The patch replicates some of its
functionality of modify_user_hw_breakpoint() in
kernel/events/hw_breakpoint.c. modify_user_hw_breakpoint cannot be called
directly since perf_event_ctx_lock() is already held in _perf_ioctl().

Evidence: Experiments show that the baseline (not able to modify an already
created breakpoint) costs an order of magnitude (~10x) more than the
suggested optimization (having the ability to dynamically modifying a
configured breakpoint via ioctl). When the breakpoints typically do not
trap, the speedup due to the suggested optimization is ~10x; even when the
breakpoints always trap, the speedup is ~4x due to the suggested
optimization.

Testing: tests posted at
https://github.com/linux-contrib/perf_event_modify_bp demonstrate the
performance significance of this patch. Tests also check the functional
correctness of the patch.

Signed-off-by: Milind Chabbi <chabbi.milind@gmail.com>
[ Using modify_user_hw_breakpoint_check function. ]
[ Reformated PERF_EVENT_IOC_*, so the values are all in one column. ]
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Hari Bathini <hbathini@linux.vnet.ibm.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Oleg Nesterov <onestero@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/20180312134548.31532-8-jolsa@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
index cf04588..6058c38 100644 (file)
--- a/include/linux/hw_breakpoint.h
+++ b/include/linux/hw_breakpoint.h
@@ -53,6 +53,9 @@ register_user_hw_breakpoint(struct perf_event_attr *attr,
 /* FIXME: only change from the attr, and don't unregister */
 extern int
 modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *attr);
+extern int
+modify_user_hw_breakpoint_check(struct perf_event *bp, struct perf_event_attr *attr,
+                               bool check);
 
 /*
  * Kernel breakpoints are not associated with any particular thread.
@@ -97,6 +100,10 @@ register_user_hw_breakpoint(struct perf_event_attr *attr,
 static inline int
 modify_user_hw_breakpoint(struct perf_event *bp,
                          struct perf_event_attr *attr) { return -ENOSYS; }
+static inline int
+modify_user_hw_breakpoint_check(struct perf_event *bp, struct perf_event_attr *attr,
+                               bool check)     { return -ENOSYS; }
+
 static inline struct perf_event *
 register_wide_hw_breakpoint_cpu(struct perf_event_attr *attr,
                                perf_overflow_handler_t  triggered,

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 6f87350..912b85b 100644 (file)
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -448,17 +448,18 @@ struct perf_event_query_bpf {
 /*
  * Ioctls that can be done on a perf event fd:
  */
-#define PERF_EVENT_IOC_ENABLE          _IO ('$', 0)
-#define PERF_EVENT_IOC_DISABLE         _IO ('$', 1)
-#define PERF_EVENT_IOC_REFRESH         _IO ('$', 2)
-#define PERF_EVENT_IOC_RESET           _IO ('$', 3)
-#define PERF_EVENT_IOC_PERIOD          _IOW('$', 4, __u64)
-#define PERF_EVENT_IOC_SET_OUTPUT      _IO ('$', 5)
-#define PERF_EVENT_IOC_SET_FILTER      _IOW('$', 6, char *)
-#define PERF_EVENT_IOC_ID              _IOR('$', 7, __u64 *)
-#define PERF_EVENT_IOC_SET_BPF         _IOW('$', 8, __u32)
-#define PERF_EVENT_IOC_PAUSE_OUTPUT    _IOW('$', 9, __u32)
-#define PERF_EVENT_IOC_QUERY_BPF       _IOWR('$', 10, struct perf_event_query_bpf *)
+#define PERF_EVENT_IOC_ENABLE                  _IO ('$', 0)
+#define PERF_EVENT_IOC_DISABLE                 _IO ('$', 1)
+#define PERF_EVENT_IOC_REFRESH                 _IO ('$', 2)
+#define PERF_EVENT_IOC_RESET                   _IO ('$', 3)
+#define PERF_EVENT_IOC_PERIOD                  _IOW('$', 4, __u64)
+#define PERF_EVENT_IOC_SET_OUTPUT              _IO ('$', 5)
+#define PERF_EVENT_IOC_SET_FILTER              _IOW('$', 6, char *)
+#define PERF_EVENT_IOC_ID                      _IOR('$', 7, __u64 *)
+#define PERF_EVENT_IOC_SET_BPF                 _IOW('$', 8, __u32)
+#define PERF_EVENT_IOC_PAUSE_OUTPUT            _IOW('$', 9, __u32)
+#define PERF_EVENT_IOC_QUERY_BPF               _IOWR('$', 10, struct perf_event_query_bpf *)
+#define PERF_EVENT_IOC_MODIFY_ATTRIBUTES       _IOW('$', 11, struct perf_event_attr *)
 
 enum perf_event_ioc_flags {
        PERF_IOC_FLAG_GROUP             = 1U << 0,

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ee145bd..3b4c779 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2846,6 +2846,41 @@ int perf_event_refresh(struct perf_event *event, int refresh)
 }
 EXPORT_SYMBOL_GPL(perf_event_refresh);
 
+static int perf_event_modify_breakpoint(struct perf_event *bp,
+                                        struct perf_event_attr *attr)
+{
+       int err;
+
+       _perf_event_disable(bp);
+
+       err = modify_user_hw_breakpoint_check(bp, attr, true);
+       if (err) {
+               if (!bp->attr.disabled)
+                       _perf_event_enable(bp);
+
+               return err;
+       }
+
+       if (!attr->disabled)
+               _perf_event_enable(bp);
+       return 0;
+}
+
+static int perf_event_modify_attr(struct perf_event *event,
+                                 struct perf_event_attr *attr)
+{
+       if (event->attr.type != attr->type)
+               return -EINVAL;
+
+       switch (event->attr.type) {
+       case PERF_TYPE_BREAKPOINT:
+               return perf_event_modify_breakpoint(event, attr);
+       default:
+               /* Place holder for future additions. */
+               return -EOPNOTSUPP;
+       }
+}
+
 static void ctx_sched_out(struct perf_event_context *ctx,
                          struct perf_cpu_context *cpuctx,
                          enum event_type_t event_type)
@@ -4952,6 +4987,8 @@ static int perf_event_set_output(struct perf_event *event,
                                 struct perf_event *output_event);
 static int perf_event_set_filter(struct perf_event *event, void __user *arg);
 static int perf_event_set_bpf_prog(struct perf_event *event, u32 prog_fd);
+static int perf_copy_attr(struct perf_event_attr __user *uattr,
+                         struct perf_event_attr *attr);
 
 static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned long arg)
 {
@@ -5024,6 +5061,17 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
 
        case PERF_EVENT_IOC_QUERY_BPF:
                return perf_event_query_prog_array(event, (void __user *)arg);
+
+       case PERF_EVENT_IOC_MODIFY_ATTRIBUTES: {
+               struct perf_event_attr new_attr;
+               int err = perf_copy_attr((struct perf_event_attr __user *)arg,
+                                        &new_attr);
+
+               if (err)
+                       return err;
+
+               return perf_event_modify_attr(event,  &new_attr);
+       }
        default:
                return -ENOTTY;
        }

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 0c82663..6253d55 100644 (file)
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -456,7 +456,7 @@ register_user_hw_breakpoint(struct perf_event_attr *attr,
 }
 EXPORT_SYMBOL_GPL(register_user_hw_breakpoint);
 
-static int
+int
 modify_user_hw_breakpoint_check(struct perf_event *bp, struct perf_event_attr *attr,
                                bool check)
 {

diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
index 6f87350..912b85b 100644 (file)
--- a/tools/include/uapi/linux/perf_event.h
+++ b/tools/include/uapi/linux/perf_event.h
@@ -448,17 +448,18 @@ struct perf_event_query_bpf {
 /*
  * Ioctls that can be done on a perf event fd:
  */
-#define PERF_EVENT_IOC_ENABLE          _IO ('$', 0)
-#define PERF_EVENT_IOC_DISABLE         _IO ('$', 1)
-#define PERF_EVENT_IOC_REFRESH         _IO ('$', 2)
-#define PERF_EVENT_IOC_RESET           _IO ('$', 3)
-#define PERF_EVENT_IOC_PERIOD          _IOW('$', 4, __u64)
-#define PERF_EVENT_IOC_SET_OUTPUT      _IO ('$', 5)
-#define PERF_EVENT_IOC_SET_FILTER      _IOW('$', 6, char *)
-#define PERF_EVENT_IOC_ID              _IOR('$', 7, __u64 *)
-#define PERF_EVENT_IOC_SET_BPF         _IOW('$', 8, __u32)
-#define PERF_EVENT_IOC_PAUSE_OUTPUT    _IOW('$', 9, __u32)
-#define PERF_EVENT_IOC_QUERY_BPF       _IOWR('$', 10, struct perf_event_query_bpf *)
+#define PERF_EVENT_IOC_ENABLE                  _IO ('$', 0)
+#define PERF_EVENT_IOC_DISABLE                 _IO ('$', 1)
+#define PERF_EVENT_IOC_REFRESH                 _IO ('$', 2)
+#define PERF_EVENT_IOC_RESET                   _IO ('$', 3)
+#define PERF_EVENT_IOC_PERIOD                  _IOW('$', 4, __u64)
+#define PERF_EVENT_IOC_SET_OUTPUT              _IO ('$', 5)
+#define PERF_EVENT_IOC_SET_FILTER              _IOW('$', 6, char *)
+#define PERF_EVENT_IOC_ID                      _IOR('$', 7, __u64 *)
+#define PERF_EVENT_IOC_SET_BPF                 _IOW('$', 8, __u32)
+#define PERF_EVENT_IOC_PAUSE_OUTPUT            _IOW('$', 9, __u32)
+#define PERF_EVENT_IOC_QUERY_BPF               _IOWR('$', 10, struct perf_event_query_bpf *)
+#define PERF_EVENT_IOC_MODIFY_ATTRIBUTES       _IOW('$', 11, struct perf_event_attr *)
 
 enum perf_event_ioc_flags {
        PERF_IOC_FLAG_GROUP             = 1U << 0,