On Sun, May 08, 2005 at 03:31:00PM +0200, Andi Kleen wrote:
> I think the best way is to let other CPUs handle the load balancing
> for idle CPUs. Basically when a CPU goes fully idle then you mark
> this in some global data structure,
nohz_cpu_mask already exists for this purpose.
> and CPUs doing load balancing after doing their own thing look for others
> that need to be balanced too and handle them too.
This is precisely what I had proposed in my watchdog implementation.
> When no CPU is left non idle then nothing needs to be load balanced anyways.
> When a idle CPU gets a task it just gets an reschedule IPI as usual, that
> wakes it up.
True.
>
> I call this the "scoreboard".
>
> The trick is to evenly load balance the work over the remaining CPUs.
> Something simple like never doing work for more than 1/idlecpus is
> probably enough.
Well, there is this imbalance_pct which acts as a trigger threshold before
which load balance won't happen. I do take this into account before
waking up the sleeping idle cpu (the same imbalance_pct logic would
have been followed by the idle CPU if it were to continue taking timer
ticks).
So I guess your 1/idlecpus and the imbalance_pct may act on parallel lines.
> In theory one could even use machine NUMA topology
> information for this, but that would be probably overkill for the
> first implementation.
>
> With the scoreboard implementation CPus could be virtually idle
> forever, which I think is best for virtualization.
>
> BTW we need a very similar thing for RCU too.
RCU is taken care of already, except it is broken. There is a small
race which is not fixed. Following patch (which I wrote aainst 2.6.10 kernel
maybe) should fix that race. I intend to post this patch after test
agaist more recent kernel.
--- kernel/rcupdate.c.org 2005-02-11 11:38:47.000000000 +0530
+++ kernel/rcupdate.c 2005-02-11 11:44:08.000000000 +0530
@@ -199,8 +199,11 @@ static void rcu_start_batch(struct rcu_c
*/
static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp, struct rcu_state *rsp)
{
+ cpumask_t tmpmask;
+
cpu_clear(cpu, rsp->cpumask);
- if (cpus_empty(rsp->cpumask)) {
+ cpus_andnot(tmpmask, rsp->cpumask, nohz_cpu_mask);
+ if (cpus_empty(tmpmask)) {
/* batch completed ! */
rcp->completed = rcp->cur;
rcu_start_batch(rcp, rsp, 0);
--
Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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