Re: [patch 00/17] CFS Bandwidth Control v7.1

[Peter Zijlstra]

On Thu, 2011-07-07 at 13:23 +0200, Ingo Molnar wrote:
> 
> The +1.5% increase in vanilla kernel context switching performance is 
> unfortunate - where does that overhead come from?

Looking at the asm output, I think its partly because things like:

@@ -602,6 +618,8 @@ static void update_curr(struct cfs_rq *c
                cpuacct_charge(curtask, delta_exec);
                account_group_exec_runtime(curtask, delta_exec);
        }
+
+       account_cfs_rq_runtime(cfs_rq, delta_exec);
 }


+static void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
+               unsigned long delta_exec)
+{
+       if (!cfs_rq->runtime_enabled)
+               return;
+
+       cfs_rq->runtime_remaining -= delta_exec;
+       if (cfs_rq->runtime_remaining > 0)
+               return;
+
+       assign_cfs_rq_runtime(cfs_rq);
+}

generate a call, only to then take the first branch out, marking that
function __always_inline would cure the call problem. Going beyond that
would be using static_branch() to track if there is any bandwidth
tracking required at all.
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