The user level rsp register value attached to the sample is crucial
for proper user stack dump and for proper dwarf backtrace post unwind.
But currently, if the event happens within the system call fast path,
we don't store proper rsp register value in the event sample.
The reason is that the syscall fast path stores just minimal set of
registers to the task's struct pt_regs area. The rsp itself is stored
in per cpu variable 'old_rsp'.
This patch fixes this rsp register value based on the:
- 'old_rsp' per cpu variable
(updated within the syscall fast path)
- guess on how we got into the kernel - syscall or interrupt
(via pt_regs::orig_ax value)
We can use 'old_rsp' value only if we are inside the syscall.
Thanks to Oleg who outlined this solution!
Above guess introduces 2 race windows (fully desccribed within the patch
comments), where we might get incorrect user level rsp value stored in
sample. However, in comparison with system call fast path length, we still
get much more precise rsp values than without the patch.
Note, that we use statically allocated pt_regs inside the sample data when
we need to change it. In other cases we still use the pt_regs pointer.
Example of syscall fast path dwarf backtrace unwind:
(perf record -e cycles -g dwarf ls; perf report --stdio)
Before the patch applied:
--23.76%-- preempt_schedule_irq
retint_kernel
tty_ldisc_deref
tty_write
vfs_write
sys_write
system_call_fastpath
__GI___libc_write
0x6
With the patch applied:
--12.37%-- finish_task_switch
__schedule
preempt_schedule
queue_work
schedule_work
tty_flip_buffer_push
pty_write
n_tty_write
tty_write
vfs_write
sys_write
system_call_fastpath
__GI___libc_write
_IO_file_write@@GLIBC_2.2.5
new_do_write
_IO_do_write@@GLIBC_2.2.5
_IO_file_overflow@@GLIBC_2.2.5
print_current_files
main
__libc_start_main
_start
Cc: Frederic Weisbecker <fweis...@gmail.com>
Cc: Ingo Molnar <mi...@elte.hu>
Cc: Paul Mackerras <pau...@samba.org>
Cc: Peter Zijlstra <a.p.zijls...@chello.nl>
Cc: Arnaldo Carvalho de Melo <a...@redhat.com>
Cc: Oleg Nesterov <o...@redhat.com>
Signed-off-by: Jiri Olsa <jo...@redhat.com>
---
arch/x86/kernel/cpu/perf_event.c | 37 +++++++++++++++++++++++++++++++++++++
include/linux/perf_event.h | 1 +
kernel/events/core.c | 10 ++++++++--
3 files changed, 46 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 915b876..834fe96 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -34,6 +34,7 @@
#include <asm/timer.h>
#include <asm/desc.h>
#include <asm/ldt.h>
+#include <asm/syscall.h>
#include "perf_event.h"
@@ -1699,6 +1700,42 @@ void arch_perf_update_userpage(struct
perf_event_mmap_page *userpg, u64 now)
userpg->time_offset = this_cpu_read(cyc2ns_offset) - now;
}
+#ifdef CONFIG_X86_64
+void arch_sample_regs_user_fixup(struct perf_regs_user *uregs, int kernel)
+{
+ /*
+ * If the perf event was triggered within the kernel code
+ * path, then it was either syscall or interrupt. While
+ * interrupt stores almost all user registers, the syscall
+ * fast path does not. At this point we can at least set
+ * rsp register right, which is crucial for dwarf unwind.
+ *
+ * The syscall_get_nr function returns -1 (orig_ax) for
+ * interrupt, and positive value for syscall.
+ *
+ * We have two race windows in here:
+ *
+ * 1) Few instructions from syscall entry until old_rsp is
+ * set.
+ *
+ * 2) In syscall/interrupt path from entry until the orig_ax
+ * is set.
+ *
+ * Above described race windows are fractional opposed to
+ * the syscall fast path, so we get much better results
+ * fixing rsp this way.
+ */
+ if (kernel && (syscall_get_nr(current, uregs->regs) >= 0)) {
+ /* Make a copy and link it to regs pointer. */
+ memcpy(&uregs->regs_copy, uregs->regs, sizeof(*uregs->regs));
+ uregs->regs = &uregs->regs_copy;
+
+ /* And fix the rsp. */
+ uregs->regs->sp = this_cpu_read(old_rsp);
+ }
+}
+#endif
+
/*
* callchain support
*/
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 599afc4..817e192 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -697,6 +697,7 @@ struct perf_branch_stack {
struct perf_regs_user {
__u64 abi;
struct pt_regs *regs;
+ struct pt_regs regs_copy;
};
struct task_struct;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7b9df35..71329d6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3780,10 +3780,15 @@ perf_output_sample_regs(struct perf_output_handle
*handle,
}
}
+__weak void
+arch_sample_regs_user_fixup(struct perf_regs_user *regs_user, int kernel) { }
+
static void perf_sample_regs_user(struct perf_regs_user *regs_user,
- struct pt_regs *regs)
+ struct pt_regs *regs)
{
- if (!user_mode(regs)) {
+ int kernel = !user_mode(regs);
+
+ if (kernel) {
if (current->mm)
regs = task_pt_regs(current);
else
@@ -3793,6 +3798,7 @@ static void perf_sample_regs_user(struct perf_regs_user
*regs_user,
if (regs) {
regs_user->regs = regs;
regs_user->abi = perf_reg_abi(current);
+ arch_sample_regs_user_fixup(regs_user, kernel);
}
}
--
1.7.7.6
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
