Re: [RFC 0/7] hrtimer: drop active hrtimer checks after adding it

From: Thomas Gleixner
Date: Wed Jul 09 2014 - 17:31:08 EST

On Wed, 9 Jul 2014, Viresh Kumar wrote:

So your patch series drops active hrtimer checks after adding it,
according to your subject line.

Quite useeul to drop something after adding it, right?

> hrtimer_start*() family never fails to enqueue a hrtimer to a clock-base. The
> only special case is when the hrtimer was in past. If it is getting enqueued to
> local CPUs's clock-base, we raise a softirq and exit, else we handle that on
> next interrupt on remote CPU.
> At several places in the kernel, we try to make sure if hrtimer was added
> properly or not by calling hrtimer_active(), like:
> hrtimer_start(timer, expires, mode);
> if (hrtimer_active(timer)) {
> /* Added successfully */
> } else {
> /* Was added in the past */
> }
> As hrtimer_start*() never fails, hrtimer_active() is guaranteed to return '1'.
> So, there is no point calling hrtimer_active().

Wrong as usual.

It's a common pattern that short timeouts are given which lead to
immediate expiry so the extra round through schedule is even more
pointless than the extra check.

Aside of that it's a long discussed issue that we really should tell
the caller right away that the timer was setup in the past and not
enqueued at all.

That requires to fixup a few call sites, but that'd far more valuable
than removing a few assumed to be pointless checks.


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