Re: [PATCH 01/10] tick/nohz: Prevent tick_nohz_get_sleep_length() from returning negative value
From: Rafael J. Wysocki
Date: Tue Mar 16 2021 - 11:27:25 EST
On 3/16/2021 3:53 PM, Frederic Weisbecker wrote:
On Tue, Mar 16, 2021 at 03:35:37PM +0100, Peter Zijlstra wrote:
On Tue, Mar 16, 2021 at 02:37:03PM +0100, Frederic Weisbecker wrote:
On Tue, Mar 16, 2021 at 01:21:29PM +0100, Peter Zijlstra wrote:
On Thu, Mar 11, 2021 at 01:36:59PM +0100, Frederic Weisbecker wrote:
From: "Zhou Ti (x2019cwm)" <x2019cwm@xxxxxxx>
If the hardware clock happens to fire its interrupts late, two possible
issues can happen while calling tick_nohz_get_sleep_length(). Either:
1) The next clockevent device event is due past the last idle entry time.
or:
2) The last timekeeping update happened before the last idle entry time
and the next timer callback expires before the last idle entry time.
Make sure that both cases are handled to avoid returning a negative
duration to the cpuidle governors.
Why? ... and wouldn't it be cheaper the fix the caller to
check negative once, instead of adding two branches here?
There are already two callers and potentially two return values to check
for each because the function returns two values.
I'd rather make the API more robust instead of fixing each callers and worrying
about future ones.
But what's the actual problem? The Changelog doesn't say why returning a
negative value is a problem, and in fact the return value is explicitly
signed.
Anyway, I don't terribly mind the patch, I was just confused by the lack
of actual justification.
And you're right, the motivation is pure FUD: I don't know exactly
how the cpuidle governors may react to such negative values and so this
is just to prevent from potential accident.
Rafael, does that look harmless to you?
No, this is a problem now. Both governors using this assign the return
value of it to a u64 var and so negative values confuse them.
That said I think it's better to deal with the issue in the callers.
I can send a patch for that if needed.