Re: too many timer retries happen when do local timer swtich withbroadcast timer

From: Santosh Shilimkar
Date: Fri Feb 22 2013 - 06:01:45 EST

On Friday 22 February 2013 04:01 PM, Lorenzo Pieralisi wrote:
On Fri, Feb 22, 2013 at 10:24:00AM +0000, Thomas Gleixner wrote:
On Fri, 22 Feb 2013, Santosh Shilimkar wrote:
BTW, Lorenzo off-list mentioned to me about warning in boot-up
which I missed while testing your patch. It will take bit more
time for me to look into it and hence thought of reporting it.

[ 2.186126] ------------[ cut here ]------------
[ 2.190979] WARNING: at kernel/time/tick-broadcast.c:501

Which one is that? tick_broadcast_pending or tick_force_broadcast_mask ?

It is the tick_force_broadcast_mask and I think that's because on all
systems we are testing, the broadcast timer IRQ is a thundering herd,
all CPUs get out of idle at once and try to get out of broadcast mode
at more or less the same time.

So the issue comes ups only when the idle state used where CPU wakeup
more or less at same time as Lorenzo mentioned. I have two platforms
where I could test the patch and see the issue only with one platform.

Yesterday I didn't notice the warning because it wasn't seen on that
platform :-) OMAP4 idle entry and exit in deep state is staggered
between CPUs and hence the warning isn't seen. On OMAP5 though,
there is an additional C-state where idle entry/exit for CPU
isn't staggered and I see the issue in that case.

Actually the broad-cast code doesn't expect such a behavior
from CPUs since only the broad-cast affine CPU should wake
up and rest of the CPU should be woken up by the broad-cast


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