[PATCH 1/3] audit: cache ctx->major in audit_filter_syscall()
ctx->major contains the current syscall number. This is, of course, a
constant for the duration of the syscall. Unfortunately, GCC's alias
analysis cannot prove that it is not modified via a pointer in the
audit_filter_syscall() loop, and so always loads it from memory.
In and of itself the load isn't very expensive (ops dependent on the
ctx->major load are only used to determine the direction of control flow
and have short dependence chains and, in any case the related branches
get predicted perfectly in the fastpath) but still cache ctx->major
in a local for two reasons:
* ctx->major is in the first cacheline of struct audit_context and has
similar alignment as audit_entry::list audit_entry. For cases
with a lot of audit rules, doing this reduces one source of contention
from a potentially busy cache-set.
* audit_in_mask() (called in the hot loop in audit_filter_syscall())
does cast manipulation and error checking on ctx->major:
audit_in_mask(const struct audit_krule *rule, unsigned long val):
if (val > 0xffffffff)
return false;
word = AUDIT_WORD(val);
if (word >= AUDIT_BITMASK_SIZE)
return false;
bit = AUDIT_BIT(val);
return rule->mask[word] & bit;
The clauses related to the rule need to be evaluated in the loop, but
the rest is unnecessarily re-evaluated for every loop iteration.
(Note, however, that most of these are cheap ALU ops and the branches
are perfectly predicted. However, see discussion on cycles
improvement below for more on why it is still worth hoisting.)
On a Skylakex system change in getpid() latency (aggregated over
12 boot cycles):
Min Mean Median Max pstdev
(ns) (ns) (ns) (ns)
- 201.30 216.14 216.22 228.46 (+- 1.45%)
+ 196.63 207.86 206.60 230.98 (+- 3.92%)
Performance counter stats for 'bin/getpid' (3 runs) go from:
cycles 836.89 ( +- .80% )
instructions 2000.19 ( +- .03% )
IPC 2.39 ( +- .83% )
branches 430.14 ( +- .03% )
branch-misses 1.48 ( +- 3.37% )
L1-dcache-loads 471.11 ( +- .05% )
L1-dcache-load-misses 7.62 ( +- 46.98% )
to:
cycles 805.58 ( +- 4.11% )
instructions 1654.11 ( +- .05% )
IPC 2.06 ( +- 3.39% )
branches 430.02 ( +- .05% )
branch-misses 1.55 ( +- 7.09% )
L1-dcache-loads 440.01 ( +- .09% )
L1-dcache-load-misses 9.05 ( +- 74.03% )
(Both aggregated over 12 boot cycles.)
instructions: we reduce around 8 instructions/iteration because some of
the computation is now hoisted out of the loop (branch count does not
change because GCC, for reasons unclear, only hoists the computations
while keeping the basic-blocks.)
cycles: improve by about 5% (in aggregate and looking at individual run
numbers.) This is likely because we now waste fewer pipeline resources
on unnecessary instructions which allows the control flow to
speculatively execute further ahead shortening the execution of the loop
a little. The final gating factor on the performance of this loop
remains the long dependence chain due to the linked-list load.
Signed-off-by: Ankur Arora <ankur.a.arora(a)oracle.com>
---
kernel/auditsc.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 79a5da1bc5bb..533b087c3c02 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -843,13 +843,14 @@ static void audit_filter_syscall(struct task_struct *tsk,
{
struct audit_entry *e;
enum audit_state state;
+ unsigned long major = ctx->major;
if (auditd_test_task(tsk))
return;
rcu_read_lock();
list_for_each_entry_rcu(e, &audit_filter_list[AUDIT_FILTER_EXIT], list) {
- if (audit_in_mask(&e->rule, ctx->major) &&
+ if (audit_in_mask(&e->rule, major) &&
audit_filter_rules(tsk, &e->rule, ctx, NULL,
&state, false)) {
rcu_read_unlock();
--
2.31.1