Messages in this thread Patch in this message | | | Date | Thu, 6 Nov 2008 11:40:28 -0800 | Subject | [patch] restore sched_exec load balance heuristics | From | Ken Chen <> |
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We've seen long standing performance regression on sys_execve for several upstream kernels, largely on workload that does heavy execve. The main reason for the regression was due to a change in sched_exec load balance heuristics. For example, on 2.6.11 kernel, the "exec" task will run on the same cpu if that is the only task running. However, 2.6.13 and onward kernels will go around the sched-domain looking for most idle CPU (which doesn't treat task exec'ing as an idle CPU). Thus bouncing the exec'ing task all over the place which leads to poor CPU cache and numa locality. (The workload happens to share common data between subsequent exec program).
This execve heuristic was removed in upstream kernel by this git commit:
commit 68767a0ae428801649d510d9a65bb71feed44dd1 Author: Nick Piggin <nickpiggin@yahoo.com.au> Date: Sat Jun 25 14:57:20 2005 -0700
[PATCH] sched: schedstats update for balance on fork Add SCHEDSTAT statistics for sched-balance-fork.
From the commit description, it appears that deleting the heuristics was an accident, as the commit is supposedly just for schedstats.
So, restore the sched-exec load balancing if exec'ing task is the only task running on that specific CPU. The logic make sense: newly exec program should continue to run on current CPU as it doesn't change any load imbalance nor does it help anything by bouncing to another idle CPU. By keeping on the same CPU, it preserves cache and numa locality.
Signed-off-by: Ken Chen <kenchen@google.com>
diff --git a/kernel/sched.c b/kernel/sched.c index e8819bc..4ad1907 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -2873,7 +2873,12 @@ out: */ void sched_exec(void) { - int new_cpu, this_cpu = get_cpu(); + int new_cpu, this_cpu; + + if (this_rq()->nr_running <= 1) + return; + + this_cpu = get_cpu(); new_cpu = sched_balance_self(this_cpu, SD_BALANCE_EXEC); put_cpu(); if (new_cpu != this_cpu)
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