This change improves performance by moving the hot path of the
trace_vhost_commit()(or any other trace function) logic to the header file.
Previously, even when the trace event was disabled, the function call chain:-
trace_vhost_commit()(Or any other trace function) →
_nocheck__trace_vhost_commit() → _simple_trace_vhost_commit()
incurred a significant function prologue overhead before checking the
trace state.
Disassembly of _simple_trace_vhost_commit() (from the .c file) showed that 11
out of the first 14 instructions were prologue-related, including:
0x10 stp x29, x30, [sp, #-64]! Prologue: allocates 64-byte frame and
saves old FP (x29) & LR (x30)
0x14 adrp x3, trace_events_enabled_count Prologue: computes page-base of
the trace-enable counter
0x18 adrp x2, __stack_chk_guard Important (maybe prolog don't
know?)(stack-protector): starts up the stack-canary load
0x1c mov x29, sp Prologue: sets new frame pointer
0x20 ldr x3, [x3] Prologue: loads the actual trace-enabled count
0x24 stp x19, x20, [sp, #16] Prologue: spills callee-saved regs used by this
function (x19, x20)
0x28 and w20, w0, #0xff Tracepoint setup: extracts the low-8 bits of
arg0 as the “event boolean”
0x2c ldr x2, [x2] Prologue (cont’d): completes loading of the
stack-canary value
0x30 and w19, w1, #0xff Tracepoint setup: extracts low-8 bits of arg1
0x34 ldr w0, [x3] Important: loads the current trace-enabled flag from
memory
0x38 ldr x1, [x2] Prologue (cont’d): reads the canary
0x3c str x1, [sp, #56] Prologue (cont’d): writes the canary into the
new frame
0x40 mov x1, #0 Prologue (cont’d): zeroes out x1 for the upcoming
branch test
0x44 cbnz w0, 0x88 Important: if tracing is disabled (w0==0) skip the
heavy path entirely
The trace-enabled check happens after the prologue. This is wasteful when
tracing is disabled, which is often the case in production.
To optimize this:
_nocheck__trace_vhost_commit() is now fully inlined in the .h file with
the hot path.It checks trace_event_get_state() before calling into
_simple_trace_vhost_commit(), which remains in .c.
This avoids calling into the .c function altogether when the tracepoint is
disabled, thereby skipping unnecessary prologue instructions.
This results in better performance by removing redundant instructions in the
tracing fast path.
Signed-off-by: Tanish Desai <tanishdesa...@gmail.com>
---
scripts/tracetool/backend/simple.py | 23 ++++++++++-------------
1 file changed, 10 insertions(+), 13 deletions(-)
diff --git a/scripts/tracetool/backend/simple.py
b/scripts/tracetool/backend/simple.py
index a74d61fcd6..2688d4b64b 100644
--- a/scripts/tracetool/backend/simple.py
+++ b/scripts/tracetool/backend/simple.py
@@ -36,8 +36,17 @@ def generate_h_begin(events, group):
def generate_h(event, group):
- out(' _simple_%(api)s(%(args)s);',
+ event_id = 'TRACE_' + event.name.upper()
+ if "vcpu" in event.properties:
+ # already checked on the generic format code
+ cond = "true"
+ else:
+ cond = "trace_event_get_state(%s)" % event_id
+ out(' if (%(cond)s) {',
+ ' _simple_%(api)s(%(args)s);',
+ ' }',
api=event.api(),
+ cond=cond,
args=", ".join(event.args.names()))
@@ -72,22 +81,10 @@ def generate_c(event, group):
if len(event.args) == 0:
sizestr = '0'
- event_id = 'TRACE_' + event.name.upper()
- if "vcpu" in event.properties:
- # already checked on the generic format code
- cond = "true"
- else:
- cond = "trace_event_get_state(%s)" % event_id
-
out('',
- ' if (!%(cond)s) {',
- ' return;',
- ' }',
- '',
' if (trace_record_start(&rec, %(event_obj)s.id, %(size_str)s)) {',
' return; /* Trace Buffer Full, Event Dropped ! */',
' }',
- cond=cond,
event_obj=event.api(event.QEMU_EVENT),
size_str=sizestr)
--
2.34.1
