On 08/14/2014 04:10 PM, Liu, Chuansheng wrote:
-----Original Message-----
From: Peter Zijlstra [mailto:[email protected]]
Sent: Thursday, August 14, 2014 9:13 PM
To: Liu, Chuansheng
Cc: Daniel Lezcano; Rafael J. Wysocki; [email protected]; LKML; Liu,
Changcheng; Wang, Xiaoming; Chakravarty, Souvik K
Subject: Re: [PATCH] cpuidle: Fix the CPU stuck at C0 for 2-3s after PM_QOS
back to DEFAULT
On Thu, Aug 14, 2014 at 11:24:06AM +0000, Liu, Chuansheng wrote:
If inspecting the polling flag, we can not fix the race between poll_idle and
smp_callback,
since in poll_idle(), before set polling flag, if the smp_callback come in, then
no resched bit set,
after that, poll_idle() will do the polling action, without reselection
immediately, it will bring power
regression here.
-ENOPARSE. Is there a question there?
Lezcano suggest to inspect the polling flag, then code is like below:
smp_callback() {
if (polling_flag)
set_resched_bit;
}
And the poll_idle code is like below:
static int poll_idle(struct cpuidle_device *dev,
struct cpuidle_driver *drv, int index)
{
local_irq_enable();
if (!current_set_polling_and_test()) {
while (!need_resched())
Or alternatively, something like:
while (!need_resched() || kickme) {
...
}
smp_callback()
{
kickme = 1;
}
kickme is a percpu variable and set to zero when exiting the 'enter'
callback.
So we don't mess with the polling flag, which is already a bit tricky.
This patch is very straightforward to illustrate the idea.
cpu_relax();
}
current_clr_polling();
return index;
}
The race is:
Idle task:
poll_idle
local_irq_enable()
<== IPI interrupt coming, check the polling flag is not set yet, do nothing;
Come back to poll_idle, it will stay in the poll loop for a while, instead break
it immediately to let governor reselect the right C-state.
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