Re: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads

On Mon, Jan 30, 2023 at 01:40:18PM +0100, Peter Zijlstra wrote:

On Fri, Jan 27, 2023 at 02:11:31PM -0800, Josh Poimboeuf wrote:

quoted

@@ -8500,8 +8502,10 @@ EXPORT_STATIC_CALL_TRAMP(might_resched);
 static DEFINE_STATIC_KEY_FALSE(sk_dynamic_cond_resched);
 int __sched dynamic_cond_resched(void)
 {
-	if (!static_branch_unlikely(&sk_dynamic_cond_resched))
+	if (!static_branch_unlikely(&sk_dynamic_cond_resched)) {
+		klp_sched_try_switch();
 		return 0;
+	}
 	return __cond_resched();
 }
 EXPORT_SYMBOL(dynamic_cond_resched);

I would make the klp_sched_try_switch() not depend on
sk_dynamic_cond_resched, because __cond_resched() is not a guaranteed
pass through __schedule().

But you'll probably want to check with Mark here, this all might
generate crap code on arm64.

IIUC here klp_sched_try_switch() is a static call, so on arm64 this'll generate
at least a load, a conditional branch, and an indirect branch. That's not
ideal, but I'd have to benchmark it to find out whether it's a significant
overhead relative to the baseline of PREEMPT_DYNAMIC.

For arm64 it'd be a bit nicer to have another static key check, and a call to
__klp_sched_try_switch(). That way the static key check gets turned into a NOP
in the common case, and the call to __klp_sched_try_switch() can be a direct
call (potentially a tail-call if we made it return 0).

Thanks,
Mark.