When livepatching is enabled, this function is used all the time. Really do
check the fastpath first, and annotate it likely() as this is the right answer
100% of the time (to many significant figures).
This cuts out 3 pointer dereferences in the "nothing to do path", and it seems
the optimiser has an easier time too. Bloat-o-meter reports:
add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-57 (-57)
Function old new delta
check_for_livepatch_work.cold 1201 1183 -18
check_for_livepatch_work 1021 982 -39
which isn't too shabby for no logical change.
Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
---
CC: Konrad Rzeszutek Wilk <konrad.w...@oracle.com>
CC: Ross Lagerwall <ross.lagerw...@citrix.com>
CC: Jan Beulich <jbeul...@suse.com>
CC: Roger Pau Monné <roger....@citrix.com>
CC: Wei Liu <w...@xen.org>
I'm still a little disappointed with the code generation. GCC still chooses
to set up the full stack frame (6 regs, +3 more slots) intermixed with the
per-cpu calculations.
In isolation, GCC can check the boolean without creating a stack frame:
<work_to_to>:
48 89 e2 mov %rsp,%rdx
48 8d 05 de e1 37 00 lea 0x37e1de(%rip),%rax #
ffff82d0405b6068 <per_cpu__work_to_do>
48 81 ca ff 7f 00 00 or $0x7fff,%rdx
8b 4a c1 mov -0x3f(%rdx),%ecx
48 8d 15 45 aa 39 00 lea 0x39aa45(%rip),%rdx #
ffff82d0405d28e0 <__per_cpu_offset>
48 8b 14 ca mov (%rdx,%rcx,8),%rdx
0f b6 04 02 movzbl (%rdx,%rax,1),%eax
c3 retq
but I can't find a way to convince GCC that it would be worth not setting up a
stack frame in in the common case, and having a few extra mov reg/reg's later
in the uncommon case.
I haven't tried manually splitting the function into a check() and a do()
function. Views on whether that might be acceptable? At a guess, do() would
need to be a static noinline to avoid it turning back into what it currently
is.
---
xen/common/livepatch.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/xen/common/livepatch.c b/xen/common/livepatch.c
index 1209fea2566c..b6275339f663 100644
--- a/xen/common/livepatch.c
+++ b/xen/common/livepatch.c
@@ -1706,15 +1706,15 @@ void check_for_livepatch_work(void)
s_time_t timeout;
unsigned long flags;
+ /* Fast path: no work to do. */
+ if ( likely(!per_cpu(work_to_do, cpu)) )
+ return;
+
/* Only do any work when invoked in truly idle state. */
if ( system_state != SYS_STATE_active ||
!is_idle_domain(current->sched_unit->domain) )
return;
- /* Fast path: no work to do. */
- if ( !per_cpu(work_to_do, cpu ) )
- return;
-
smp_rmb();
/* In case we aborted, other CPUs can skip right away. */
if ( !livepatch_work.do_work )
base-commit: 49818cde637b5ec20383e46b71f93b2e7d867686
--
2.30.2
