Re: [PATCH 1/3] audit: cache ctx->major in audit_filter_syscall()

Hello, Thanks for the detailed notes on this investigation. It really is a lot of good information backing this up. However, there will come a day when someone sees this "major = ctx->major" and they will send a patch to "fix" this unnecessary assignment. If you are sending a V2 of this set, I would suggest adding some comment in the code that this is for a performance improvement and to see the commit message for additional info.

Thanks, -Steve On Tuesday, September 27, 2022 6:59:42 PM EDT Ankur Arora wrote: > 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&gt; > --- > 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();